AA batteries (Ni-MH, Ni-Cd) and Proper charging, or praise of Maha and LaCrosse (TechnoLine). Methods for charging Ni-Cd and Ni-MH batteries Nickel magnesium battery

The scope of application of electric batteries is quite wide. Small batteries are equipped with household appliances that are familiar to everyone, slightly larger batteries are equipped with cars, and very large and capacitive batteries are mounted in industrial stations loaded with work. It would seem that in addition to the user purpose, different types of batteries can have something in common? However, in fact, such batteries have more than enough similarities. Perhaps one of the main among the possible similarities of batteries is the principle of organizing their work. In today's material, our resource decided to consider just one of those. To be more precise, below we will talk about the functioning and operating rules of nickel-metal hydride batteries.

The history of the appearance of nickel-metal hydride batteries

The creation of nickel-metal hydride batteries began to arouse considerable interest among engineering representatives more than 60 years ago, that is, in the 50s of the 20th century. Scientists specializing in the study of the physical and chemical properties of batteries seriously thought about how to overcome the shortcomings of nickel-cadmium batteries popular at that time. Perhaps one of the main goals of scientists was to create such a battery that could speed up and simplify the process of all reactions associated with the electrolytic transfer of hydrogen.

As a result, only by the end of the 70s did specialists manage to first design, and then create and fully test more or less high-quality nickel-metal hydride batteries. The main difference between the new type of battery and its predecessors was that it had strictly defined places for the accumulation of the bulk of hydrogen. More precisely, the accumulation of matter occurred in alloys of several metals located on the electrodes of the battery. The composition of the alloys had such a structure that one or more metals accumulated hydrogen (sometimes several thousand times their volume), while other metals acted as catalysts for electrolytic reactions, ensuring the transition of the hydrogen substance into the metal electrode grid.

The made battery, which has a hydrogen-metal hydride anode and a nickel cathode, received the abbreviation "Ni-MH" (from the name of conductive, accumulating substances). Such batteries work on an alkaline electrolyte and provide an excellent charge-discharge cycle - up to 2,000 thousand for one full-fledged battery. Despite this, the path to the design of Ni-MH batteries was not easy, and currently existing designs are still being upgraded. The main modernization vector is aimed at increasing the energy density of batteries.

Note that today nickel-metal hydride batteries are mostly produced on the basis of the LaNi5 metal alloy. The first sample of such batteries was patented in 1975 and began to be actively used in the general industry. Modern nickel-metal hydride batteries have a high energy density and consist of completely non-toxic raw materials, which makes them easy to dispose of. Perhaps it is precisely because of these advantages that they have become very popular in many areas where long-term storage of an electric charge is required.

The device and principle of operation of the nickel-metal hydride battery

Nickel-metal hydride batteries of all dimensions, capacities and purposes are produced in two main types of shapes - prismatic and cylindrical. Regardless of the form, such batteries consist of the following mandatory elements:

• metal hydride and nickel electrodes (cathodes and anodes), which form a galvanic element of the grid structure, which is responsible for the movement and accumulation of electric charge;
• separator areas separating the electrodes and also participating in the process of electrolytic reactions;
• output contacts that give off the accumulated charge to the external environment;
• covers with a valve built into it, necessary to relieve excess pressure from the accumulator cavities (pressure over 2-4 megapascals);
• a thermally protective and strong case containing the battery cells described above.

The design of nickel-metal hydride batteries, like many other types of this device, is quite simple and does not present any particular difficulties in consideration. This is clearly shown in the following battery design diagrams:

The principles of operation of the considered batteries, in contrast to their general design scheme, look slightly more complicated. To understand their essence, let's pay attention to the phased functioning of nickel-metal hydride batteries. In a typical embodiment, the stages of operation for these batteries are as follows:

1. Positive electrode - anode, carries out an oxidative reaction with the absorption of hydrogen;
2. The negative electrode, the cathode, implements a reduction reaction in the disabsorption of hydrogen.

talking plain language, the electrode grid organizes the ordered movement of particles (electrodes and ions) through specific chemical reactions. At the same time, the electrolyte does not directly participate in the main reaction of electricity generation, but is included in the work only under certain circumstances of the operation of Ni-MH batteries (for example, when recharging, realizing the oxygen circulation reaction). We will not consider in more detail the principles of operation of nickel-metal hydride batteries, since this requires special chemical knowledge, which many readers of our resource do not have. If you want to learn about the principles of battery operation in greater detail, you should refer to the technical literature, which covers as much as possible the course of each reaction at the ends of the electrodes, both when the batteries are charged and when they are discharged.

The specifications of a standard Ni-MH battery can be seen in the following table (middle column):

Operating rules

Any battery is a relatively unpretentious device in maintenance and operation. Despite this, its cost is often high, so every owner of a particular battery is interested in increasing its service life. With regard to the batteries of the Ni-MH formation, it is not so difficult to extend the operational period. For this it is enough:

• First, follow the rules for charging the battery;
• Secondly, it is correct to operate and store it when idle.

We will talk about the first aspect of battery maintenance a little later, but now let's pay attention to the main list of rules for operating nickel-metal hydride batteries. The template list of these rules is as follows:

• Storage of nickel-metal hydride batteries should only be carried out in their charged state at a level of 30-50%;
• It is strictly forbidden to overheat the Ni-MH batteries, since compared to the same nickel-cadmium batteries, the ones we are considering are much more sensitive to heat. Work overload has a negative effect on all processes occurring in the cavities and at the outputs of the battery. The current output is especially affected;
• Never recharge nickel-metal hydride batteries. Always follow the charging rules described in this article or reflected in the technical documentation for the battery;
• In the process of weak operation or long-term storage, "train" the battery. Often, a periodically conducted “charge-discharge” cycle (about 3-6 times) is enough. It is also desirable to subject new Ni-MH batteries to such a "training";
• Nickel-metal hydride batteries must be stored at room temperature. The optimum temperature is 15-23 degrees Celsius;
• Try not to discharge the battery to the minimum limits - a voltage less than 0.9 volts for each cathode-anode pair. Of course, nickel-metal hydride batteries can be restored, but it is advisable not to bring them to a “dead” state (we will also talk about how to restore the battery below);
• Keep track of the structural quality of the battery. Serious defects, lack of electrolyte and the like are not allowed. The recommended frequency of battery checks is 2-4 weeks;
• In the case of using large, stationary batteries, it is also important to follow the rules:
  • their current repair (at least once a year):
  • capital restoration (at least once every 3 years);
  • reliable fastening of the battery at the place of use;
  • the presence of lighting;
  • using the correct chargers;
  • and compliance with safety regulations for the use of such batteries.

It is important to adhere to the described rules not only because such an approach to the operation of nickel-metal hydride batteries will significantly extend their service life. They also guarantee a safe and generally hassle-free use of the battery.

Charging Rules

It was noted earlier that operating rules are far from the only thing required to achieve the maximum operating life of nickel-metal hydride batteries. In addition to proper use, it is extremely important to properly charge such batteries. In general, answering the question - “How to properly charge a Ni-MH battery?” Is quite difficult. The fact is that each type of alloy used on battery electrodes requires certain rules for this process.

Summarizing and averaging them, we can distinguish the following fundamental principles of charging nickel-metal hydride batteries:

• First, you need to observe the correct charging time. For most Ni-MH batteries, it is either 15 hours at a charging current of about 0.1 C, or 1-5 hours at a charging current in the range of 0.1-1 C for batteries with high activity electrodes. Exceptions are rechargeable batteries, which can take more than 30 hours to charge;
• Secondly, it is important to monitor the temperature of the battery during the charging process. Many manufacturers do not recommend exceeding a temperature maximum of 50-60 degrees Celsius;
• And thirdly, the order of charging should be taken into account directly. This approach is considered optimal when the battery is discharged with a rated current to a voltage at the outputs of 0.9-1 Volt, after which it is charged by 75-80% of its maximum capacity. At the same time, it is important to take into account that during fast charging (the supplied current is more than 0.1), it is important to organize pre-charging with a high current supply to the battery for about 8-10 minutes. After that, the charging process should be organized with a smooth increase in the voltage supplied to the battery to 1.6-1.8 Volts. By the way, during normal recharging of a nickel-metal hydride battery, the voltage often does not change and is normally 0.3-1 volts.

Note! The battery charging rules noted above are of an average nature. Keep in mind that for a particular brand of nickel-metal hydride battery, they may differ slightly.

Battery recovery

Along with the high cost and rapid self-discharge, Ni-MH batteries have another drawback - a pronounced "memory effect". Its essence lies in the fact that with the systematic charging of an incompletely discharged battery, it seems to remember this and, over time, significantly loses its capacity. To neutralize such risks, the owners of such batteries need to charge the most discharged batteries, as well as periodically “train” them through the recovery process.

To restore nickel-metal hydride batteries during "training" or when they are strongly discharged, it is necessary as follows:

1. First of all, you need to prepare. Recovery will require:
   • high-quality and, preferably, smart charger;
   • tools for measuring voltage and current;
   • any device capable of drawing power from a battery.
2. After preparation, you can already wonder how to restore the battery. First, it is necessary to charge the battery in accordance with all the rules, and then discharge it according to the voltage at the battery outputs of 0.8-1 Volt;
3. Then the recovery begins directly, which, again, must be carried out in accordance with all the rules for charging nickel-metal hydride batteries. The standard recovery process can be carried out in two ways:
   • The first is if the battery shows signs of "life" (as a rule, when it is discharged at a level of 0.8-1 Volt). Charging takes place with a constant increase in the supplied voltage from 0.3 to 1 Volt with a current of 0.1 C for 30-60 minutes, after which the voltage remains unchanged, and the current increases to 0.3-0.5 C;
   • The second - if the battery does not show signs of "life" (with a discharge of less than 0.8 volts). In this case, charging is carried out with a 10-minute high-current pre-charge for 10-15 minutes. After that, the above steps are carried out.

It should be understood that the restoration of nickel-metal hydride batteries is a procedure that must be periodically carried out for absolutely all batteries (both “live” and “non-live”). Only such an approach to the operation of this type of battery will help to “squeeze” the maximum out of them.

Perhaps, this story on today's topic can be completed. We hope that the material presented above was useful for you and gave answers to your questions.

If you have any questions - leave them in the comments below the article. We or our visitors will be happy to answer them.

This article about Nickel-metal hydride (Ni-MH) batteries has long been a classic on the Russian Internet. I recommend checking out…

Nickel-metal hydride (Ni-MH) batteries are analogous in design to nickel-cadmium (Ni-Cd) batteries, and in electrochemical processes - nickel-hydrogen batteries. The specific energy of a Ni-MH battery is significantly higher than the specific energy of Ni-Cd and hydrogen batteries (Ni-H2)

VIDEO: Nickel Metal Hydride Batteries (NiMH)

Comparative characteristics of batteries

*** A large spread of some parameters in the table is caused by different purpose (designs) of batteries. In addition, the table does not take into account data on modern batteries with low self-discharge.

History of the Ni-MH battery

Development of Nickel Metal Hydride (Ni-MH) batteries began in the 1950s and 1970s. The result was a new way to store hydrogen in nickel-hydrogen batteries that were used in spacecraft. In the new element, hydrogen accumulated in alloys of certain metals. Alloys absorbing 1,000 times their own volume of hydrogen were discovered in the 1960s. These alloys are composed of two or more metals, one of which absorbs hydrogen and the other is a catalyst that promotes the diffusion of hydrogen atoms into the metal lattice. The number of possible combinations of metals used is practically unlimited, which makes it possible to optimize the properties of the alloy. To create Ni-MH batteries, it was necessary to create alloys that can work at low hydrogen pressure and room temperature. Currently, work on the creation of new alloys and technologies for their processing continues throughout the world. Alloys of nickel with metals of the rare earth group can provide up to 2000 charge-discharge cycles of the battery with a decrease in the capacity of the negative electrode by no more than 30%. The first Ni-MH battery, using LaNi5 alloy as the main active material of the metal hydride electrode, was patented by Bill in 1975. In early experiments with metal hydride alloys, nickel-metal hydride batteries were unstable, and the required battery capacity could not be achieved. Therefore, the industrial use of Ni-MH batteries began only in the mid-80s after the creation of the La-Ni-Co alloy, which allows electrochemically reversible absorption of hydrogen for more than 100 cycles. Since then, the design of Ni-MH batteries has been continuously improved in the direction of increasing their energy density. The replacement of the negative electrode made it possible to increase the load of active masses of the positive electrode by 1.3-2 times, which determines the capacity of the battery. Therefore, Ni-MH batteries have significantly higher specific energy characteristics compared to Ni-Cd batteries. The success of the distribution of nickel-metal hydride batteries was ensured by the high energy density and non-toxicity of the materials used in their production.

Basic processes of Ni-MH batteries

Ni-MH batteries use a nickel-oxide electrode as the positive electrode, like a nickel-cadmium battery, and a hydrogen-absorbing nickel-rare-earth alloy electrode instead of the negative cadmium electrode. On the positive nickel oxide electrode of the Ni-MH battery, the reaction proceeds:

Ni(OH) 2 + OH- → NiOOH + H 2 O + e - (charge) NiOOH + H 2 O + e - → Ni(OH) 2 + OH - (discharge)

At the negative electrode, the metal with absorbed hydrogen is converted into a metal hydride:

M + H 2 O + e - → MH + OH- (charge) MH + OH - → M + H 2 O + e - (discharge)

The overall reaction in a Ni-MH battery is written as follows:

Ni(OH) 2 + M → NiOOH + MH (charge) NiOOH + MH → Ni(OH) 2 + M (discharge)

The electrolyte does not participate in the main current-forming reaction. After reporting 70-80% of the capacity and during recharging, oxygen begins to be released on the oxide-nickel electrode,

2OH- → 1/2O 2 + H2O + 2e - (recharge)

which is restored at the negative electrode:

1/2O 2 + H 2 O + 2e - → 2OH - (recharge)

The last two reactions provide a closed oxygen cycle. When oxygen is reduced, an additional increase in the capacitance of the metal hydride electrode is also provided due to the formation of the OH - group.

Construction of Ni-MH battery electrodes

Metal hydrogen electrode

The main material that determines the performance of a Ni-MH battery is a hydrogen-absorbing alloy that can absorb up to 1000 times its own volume of hydrogen. The most widely used alloys are LaNi5, in which part of the nickel is replaced by manganese, cobalt and aluminum to increase the stability and activity of the alloy. To reduce the cost, some manufacturers use misch metal instead of lanthanum (Mm, which is a mixture of rare earth elements, their ratio in the mixture is close to the ratio in natural ores), which, in addition to lanthanum, also includes cerium, praseodymium and neodymium. During charge-discharge cycling, there is an expansion and contraction of 15-25% of the crystal lattice of hydrogen-absorbing alloys due to the absorption and desorption of hydrogen. Such changes lead to the formation of cracks in the alloy due to an increase in internal stress. The formation of cracks causes an increase in the surface area, which is corroded when interacting with an alkaline electrolyte. For these reasons, the discharge capacity of the negative electrode gradually decreases. In a battery with a limited amount of electrolyte, this causes electrolyte redistribution problems. Corrosion of the alloy leads to chemical passivity of the surface due to the formation of corrosion-resistant oxides and hydroxides, which increase the overvoltage of the main current-generating reaction of the metal hydride electrode. The formation of corrosion products occurs with the consumption of oxygen and hydrogen from the electrolyte solution, which, in turn, causes a decrease in the amount of electrolyte in the battery and an increase in its internal resistance. To slow down the undesirable processes of dispersion and corrosion of alloys, which determine the service life of Ni-MH batteries, two main methods are used (in addition to optimizing the composition and production mode of the alloy). The first method is microencapsulation of alloy particles, i.e. in covering their surface with a thin porous layer (5-10%) - by weight of nickel or copper. The second method, which has found the widest application at present, consists in treating the surface of alloy particles in alkaline solutions with the formation of protective films permeable to hydrogen.

Nickel oxide electrode

Oxide-nickel electrodes in mass production are manufactured in the following design modifications: lamella, lamellaless sintered (metal-ceramic) and pressed, including pellets. In recent years, lamellaless felt and polymer foam electrodes have begun to be used.

Lamellar electrodes

Lamellar electrodes are a set of interconnected perforated boxes (lamellae) made of thin (0.1 mm thick) nickel-plated steel tape.

Sintered (cermet) electrodes

electrodes of this type consist of a porous (with a porosity of at least 70%) cermet base, in the pores of which the active mass is located. The base is made from carbonyl nickel fine powder, which, mixed with ammonium carbonate or carbamide (60-65% nickel, the rest is filler), is pressed, rolled or sprayed onto a steel or nickel mesh. Then the grid with the powder is subjected to heat treatment in a reducing atmosphere (usually in a hydrogen atmosphere) at a temperature of 800-960 ° C, while the ammonium carbonate or carbamide decomposes and volatilizes, and the nickel is sintered. The substrates thus obtained have a thickness of 1-2.3 mm, a porosity of 80-85% and a pore radius of 5-20 µm. The base is alternately impregnated with a concentrated solution of nickel nitrate or nickel sulfate and an alkali solution heated to 60-90 ° C, which induces the precipitation of nickel oxides and hydroxides. Currently, the electrochemical impregnation method is also used, in which the electrode is subjected to cathodic treatment in a nickel nitrate solution. Due to the formation of hydrogen, the solution in the pores of the plate is alkalized, which leads to the deposition of oxides and hydroxides of nickel in the pores of the plate. Foil electrodes are classified as varieties of sintered electrodes. The electrodes are produced by applying on a thin (0.05 mm) perforated nickel tape on both sides, by spraying, an alcohol emulsion of nickel carbonyl powder containing binders, sintering and further chemical or electrochemical impregnation with reagents. The thickness of the electrode is 0.4-0.6 mm.

Pressed electrodes

Pressed electrodes are made by pressing under a pressure of 35-60 MPa of the active mass onto a mesh or a steel perforated tape. The active mass consists of nickel hydroxide, cobalt hydroxide, graphite and a binder.

Metal felt electrodes

Metal felt electrodes have a highly porous base made of nickel or carbon fibers. The porosity of these foundations is 95% or more. The felt electrode is made on the basis of nickel-plated polymer or graphite felt. The thickness of the electrode, depending on its purpose, is in the range of 0.8-10 mm. The active mass is introduced into the felt by different methods, depending on its density. Can be used instead of felt nickel foam obtained by nickel-plating polyurethane foam followed by annealing in a reducing environment. A paste containing nickel hydroxide and a binder are usually introduced into a highly porous medium by spreading. After that, the base with the paste is dried and rolled. Felt and foam polymer electrodes are characterized by high specific capacity and long service life.

Construction of Ni-MH batteries

Cylindrical Ni-MH batteries

The positive and negative electrodes, separated by a separator, are rolled up in the form of a roll, which is inserted into the housing and closed with a sealing cap with a gasket (Figure 1). The cover has a safety valve that operates at a pressure of 2-4 MPa in the event of a failure in the operation of the battery.

Fig.1. The design of the nickel-metal hydride (Ni-MH) battery: 1-body, 2-cap, 3-valve cap, 4-valve, 5-positive electrode collector, 6-insulating ring, 7-negative electrode, 8-separator, 9- positive electrode, 10-insulator.

Ni-MH Prismatic Batteries

In prismatic Ni-MH batteries, positive and negative electrodes are placed alternately, and a separator is placed between them. The block of electrodes is inserted into a metal or plastic case and closed with a sealing cover. A valve or pressure sensor is usually installed on the cover (Figure 2).

Fig.2. Ni-MH battery structure: 1-body, 2-cap, 3-valve cap, 4-valve, 5-insulating gasket, 6-insulator, 7-negative electrode, 8-separator, 9-positive electrode.

Ni-MH batteries use an alkaline electrolyte consisting of KOH with the addition of LiOH. As a separator in Ni-MH batteries, non-woven polypropylene and polyamide 0.12-0.25 mm thick, treated with a wetting agent, are used.

positive electrode

Ni-MH batteries use positive nickel oxide electrodes, similar to those used in Ni-Cd batteries. In Ni-MH batteries, ceramic-metal electrodes are mainly used, and in recent years, felt and polymer foam electrodes (see above).

Negative electrode

Five designs of a negative metal hydride electrode (see above) have found practical application in Ni-MH batteries: - lamellar, when the powder of a hydrogen-absorbing alloy with or without a binder is pressed into a nickel mesh; - nickel foam, when a paste with an alloy and a binder is introduced into the pores of the nickel foam base, and then dried and pressed (rolled); - foil, when a paste with an alloy and a binder is applied to perforated nickel or nickel-plated steel foil, and then dried and pressed; - rolled, when the powder of the active mass, consisting of an alloy and a binder, is applied by rolling (rolling) on ​​an tensile nickel grid or copper grid; - sintered, when the alloy powder is pressed onto a nickel grid and then sintered in a hydrogen atmosphere. The specific capacitances of metal hydride electrodes of different designs are close in value and are determined mainly by the capacitance of the alloy used.

Characteristics of Ni-MH batteries. Electrical characteristics

Open circuit voltage

Open circuit voltage value Ur.c. Ni-MH systems are difficult to accurately determine due to the dependence of the equilibrium potential of the nickel oxide electrode on the degree of nickel oxidation, as well as the dependence of the equilibrium potential of the metal hydride electrode on the degree of hydrogen saturation. 24 hours after the battery is charged, the open circuit voltage of the charged Ni-MH battery is in the range of 1.30-1.35V.

Rated discharge voltage

Ur at a normalized discharge current Ir = 0.1-0.2C (C is the nominal capacity of the battery) at 25 ° C is 1.2-1.25V, the usual final voltage is 1V. Voltage decreases with increasing load (see figure 3)

Fig.3. Discharge characteristics of a Ni-MH battery at a temperature of 20°C and different normalized load currents: 1-0.2C; 2-1C; 3-2C; 4-3C

Battery capacity

With an increase in load (decrease in the discharge time) and with a decrease in temperature, the capacity of a Ni-MH battery decreases (Figure 4). The effect of temperature reduction on the capacitance is especially noticeable at high discharge rates and at temperatures below 0°C.

Fig.4. The dependence of the discharge capacity of Ni-MH battery on temperature at different discharge currents: 1-0.2C; 2-1C; 3-3C

Safety and service life of Ni-MH batteries

During storage, the Ni-MH battery self-discharges. After a month at room temperature, the loss of capacity is 20-30%, and with further storage, the loss decreases to 3-7% per month. The self-discharge rate increases with increasing temperature (see figure 5).

Fig.5. The dependence of the discharge capacity of the Ni-MH battery on the storage time at different temperatures: 1-0°С; 2-20°C; 3-40°С

Charging a Ni-MH battery

The operating time (number of discharge-charge cycles) and service life of a Ni-MH battery are largely determined by operating conditions. The operating time decreases with an increase in the depth and speed of the discharge. The operating time depends on the speed of the charge and the method of controlling its completion. Depending on the type of Ni-MH batteries, operating mode and operating conditions, the batteries provide from 500 to 1800 discharge-charge cycles at a depth of discharge of 80% and have a service life (on average) from 3 to 5 years.

To ensure reliable operation of the Ni-MH battery during the guaranteed period, you must follow the manufacturer's recommendations and instructions. The greatest attention should be paid to the temperature regime. It is desirable to avoid overdischarges (below 1V) and short circuits. It is recommended to use Ni-MH batteries for their intended purpose, avoid mixing used and unused batteries, and do not solder wires or other parts directly to the battery. Ni-MH batteries are more sensitive to overcharging than Ni-Cd. Overcharging can lead to thermal runaway. Charging is usually carried out with a current of Iz \u003d 0.1C for 15 hours. Compensation charging is carried out with a current Iz = 0.01-0.03C for 30 hours or more. Accelerated (in 4 - 5 hours) and fast (in 1 hour) charges are possible for Ni-MH batteries with highly active electrodes. With such charges, the process is controlled by changes in temperature ΔТ and voltage ΔU and other parameters. Quick charge is used, for example, for Ni-MH batteries that power laptops, cell phones, and power tools, although laptops and cell phones now mostly use lithium-ion and lithium-polymer batteries. A three-stage charge method is also recommended: the first stage of a fast charge (1C and above), a charge at a rate of 0.1C for 0.5-1 h for the final recharge, and a charge at a rate of 0.05-0.02C as a compensation charge. Information on how to charge Ni-MH batteries is usually contained in the manufacturer's instructions, and the recommended charging current is indicated on the battery case. The charging voltage Uz at Iz=0.3-1C lies in the range of 1.4-1.5V. Due to the release of oxygen at the positive electrode, the amount of electricity delivered during charging (Qz) is greater than the discharge capacity (Cp). At the same time, the return on capacity (100 Ср/Qз) is 75-80% and 85-90%, respectively, for disk and cylindrical Ni-MH batteries.

Charge and discharge control

To prevent overcharging of Ni-MH batteries, the following charge control methods can be used with appropriate sensors installed in batteries or chargers:

• charge termination method by absolute temperature Tmax. The battery temperature is constantly monitored during the charging process, and when the maximum value is reached fast charge interrupted;
• charge termination method by temperature change rate ΔT/Δt. With this method, the slope of the battery temperature curve is constantly monitored during the charging process, and when this parameter rises above a certain set value, the charge is interrupted;
• charge termination method by negative voltage delta -ΔU. At the end of the battery charge, during the oxygen cycle, its temperature begins to rise, leading to a decrease in voltage;
• charge termination method according to the maximum charge time t;
• method of termination of the charge by the maximum pressure Pmax. It is usually used in prismatic batteries of large sizes and capacities. The level of allowable pressure in a prismatic accumulator depends on its design and lies in the range of 0.05-0.8 MPa;
• method of termination of the charge by the maximum voltage Umax. It is used to disconnect the charge of batteries with high internal resistance, which appears at the end of the service life due to lack of electrolyte or at low temperature.

When using the Tmax method, the battery may be overcharged if the ambient temperature drops, or the battery may not be sufficiently charged if the ambient temperature rises significantly. The ΔT/Δt method can be used very effectively to terminate the charge at low ambient temperatures. But if only this method is used at higher temperatures, the batteries inside the batteries will be exposed to undesirably high temperatures before the ΔT/Δt value for shutdown can be reached. For a certain value of ΔT/Δt, a larger input capacitance can be obtained at a lower ambient temperature than at a higher temperature. At the beginning of a battery charge (as well as at the end of a charge), there is a rapid rise in temperature, which can lead to premature charge shutdown when using the ΔT/Δt method. To eliminate this, charger developers use timers for the initial sensor response delay with the ΔT / Δt method. The -ΔU method is effective for terminating the charge at low ambient temperatures rather than at elevated temperatures. In this sense, the method is similar to the ΔT/Δt method. In order to ensure that the charge is terminated in cases where unforeseen circumstances prevent the normal interruption of the charge, it is also recommended to use a timer control that regulates the duration of the charge operation (method t). Thus, to quickly charge batteries with rated currents of 0.5-1C at temperatures of 0-50 °C, it is advisable to simultaneously apply the Tmax methods (with a shutdown temperature of 50-60 °C, depending on the design of the batteries and batteries), -ΔU (5- 15 mV per battery), t (usually to get 120% nominal capacity) and Umax (1.6-1.8 V per battery). Instead of the -ΔU method, the ΔT/Δt method (1-2 °C/min) with an initial delay timer (5-10 min) can be used. For charge control, also see the corresponding article. After a quick charge of the battery, chargers provide for switching them to recharge with a rated current of 0.1C - 0.2C for a certain time. Constant voltage charging is not recommended for Ni-MH batteries as "thermal failure" of the batteries can occur. This is because at the end of the charge there is an increase in current, which is proportional to the difference between the power supply voltage and the battery voltage, and the battery voltage at the end of the charge decreases due to the increase in temperature. At low temperatures, the charge rate should be reduced. Otherwise, oxygen will not have time to recombine, which will lead to an increase in pressure in the accumulator. For operation in such conditions, Ni-MH batteries with highly porous electrodes are recommended.

Advantages and disadvantages of Ni-MH batteries

A significant increase in specific energy parameters is not the only advantage of Ni-MH batteries over Ni-Cd batteries. Moving away from cadmium also means moving towards cleaner production. The problem of recycling failed batteries is also easier to solve. These advantages of Ni-MH batteries determined the faster growth of their production volumes in all the world's leading battery companies compared to Ni-Cd batteries.

Ni-MH batteries do not have the "memory effect" that Ni-Cd batteries have due to the formation of nickelate in the negative cadmium electrode. However, the effects associated with the overcharging of the nickel oxide electrode remain. The decrease in the discharge voltage, observed with frequent and long recharges in the same way as with Ni-Cd batteries, can be eliminated by periodically performing several discharges up to 1V - 0.9V. It is enough to carry out such discharges once a month. However, nickel-metal hydride batteries are inferior to nickel-cadmium batteries, which they are designed to replace, in some performance characteristics:

• Ni-MH batteries operate effectively in a narrower range of operating currents, which is associated with limited desorption of hydrogen from the metal hydride electrode at very high discharge rates;
• Ni-MH batteries have a narrower operating temperature range: most of them are inoperable at temperatures below -10 °C and above +40 °C, although in some battery series the adjustment of the recipes has provided an expansion of temperature limits;
• during the charge of Ni-MH batteries, more heat is released than when charging Ni-Cd batteries, therefore, in order to prevent overheating of the battery from Ni-MH batteries during fast charging and / or significant overcharging, thermal fuses or thermal relays are installed in them, which are located on the wall of one of the batteries in the central part of the battery (this applies to industrial battery assemblies);
• Ni-MH batteries have an increased self-discharge, which is determined by the inevitability of the reaction of hydrogen dissolved in the electrolyte with a positive oxide-nickel electrode (but, thanks to the use of special negative electrode alloys, it was possible to achieve a decrease in the self-discharge rate to values ​​close to those for Ni-Cd batteries );
• the risk of overheating when charging one of the Ni-MH batteries of the battery, as well as reversal of the battery with a lower capacity when the battery is discharged, increases with the mismatch of the battery parameters as a result of long cycling, so the creation of batteries from more than 10 batteries is not recommended by all manufacturers;
• the loss of capacity of the negative electrode that occurs in a Ni-MH battery when discharging below 0 V is irreversible, which puts forward more stringent requirements for the selection of batteries in the battery and the control of the discharge process than in the case of using Ni-Cd batteries, as a rule, discharge to 1 V/ac in low voltage batteries and up to 1.1 V/ac in a battery of 7-10 batteries.

As noted earlier, the degradation of Ni-MH batteries is determined primarily by a decrease in the sorption capacity of the negative electrode during cycling. In the charge-discharge cycle, the volume of the crystal lattice of the alloy changes, which leads to the formation of cracks and subsequent corrosion upon reaction with the electrolyte. The formation of corrosion products occurs with the absorption of oxygen and hydrogen, as a result of which the total amount of electrolyte decreases and the internal resistance of the battery increases. It should be noted that the characteristics of Ni-MH batteries significantly depend on the alloy of the negative electrode and the processing technology of the alloy to improve the stability of its composition and structure. This forces battery manufacturers to be careful in the choice of alloy suppliers, and battery consumers to be careful in choosing a manufacturing company.

Based on the materials of the sites powerinfo.ru, "Chip and Dip"

IntroductionDespite the widespread use of lithium-ion batteries in small-sized devices - players, mobile phones, expensive wireless mice - conventional AA batteries are not going to give up positions yet. They are cheap, you can buy them at any kiosk, finally, by making them powered by standard batteries, the device manufacturer can shift the care of changing them (or, in the case of batteries, charging) to the user and thereby save a few more dollars.

AA batteries are used in most inexpensive wireless mice, in almost all wireless keyboards, in remote controls, in inexpensive cameras and expensive professional flashlights, in flashlights and children's toys ... in general, you can list for a long time.

And more and more often these batteries are being replaced by batteries, usually nickel-metal hydride, with a nameplate capacity of 2500 to 2700 mAh and an operating voltage of 1.2 V. The dimensions and close voltage of the batteries make it possible to install them without problems in almost any device, originally designed for batteries. The benefit is obvious: not only does one battery withstand several hundred recharge cycles, but also its capacity, with at least some serious load, turns out to be significantly higher than batteries. So, you will not only save money, but also get a more “long-playing” device.

In today's article, we will look at - and test in practice - 16 batteries from different manufacturers and with different parameters in order to decide which ones are worth buying. In particular, batteries with a reduced self-discharge current that have recently appeared on sale, capable of lying in a charged state for months and remaining ready for use at any moment, will not be left without attention.

We remind our readers that the device and the basic features of various types of batteries, as well as the issues of choosing chargers for Ni-MH batteries, we already described before.

Test Methodology

Ansmann Energy Digital (2700 mAh)

Ansmann Energy Digital (2850 mAh)

Ansmann Energy Max-E (2100 mAh)

Camelion High Energy NH-AA2600 (2500 mAh)

Duracell (2650 mAh)

Energizer (2650 mAh)

GP "2700 Series" 270AAHC (2600 mAh)

GP ReCyko+ 210AAHCB (2050 mAh)

NEXcell (2300 mAh)

NEXcell (2600 mAh)

Philips MultiLife (2600 mAh)

Philips MultiLife (2700 mAh)

Sanyo HR-3U (2700 mAh)

Varta Power Accu (2700 mAh)

Varta Professional (2700 mAh)

Varta Ready2Use (2100 mAh)

Load tests

Self-discharge of batteries in 1 week

Conclusion

The actual battery capacity, as our measurements showed, depends more on their manufacturer than on the numbers on the label - Sanyo (2650 mAh) and Varta (2700 mAh) confidently overtook Ansmann (2850 mAh).
Do not chase after a large passport capacity. Batteries with a higher capacity often have a higher self-discharge current, which means that if you use them not immediately after charging, but for several days, then batteries with a lower nameplate capacity may be more efficient.
When buying, pay attention to the dimensions of the battery. Three of the models we tested - two Philips batteries and one Ansmann - had an increased case size, which is why they did not work in all devices.
Estimate in advance how intensively you will use the batteries. If you plan to charge them at least once a week, then you should pay attention to models with a passport capacity of about 2700 mAh. If the batteries have to be charged for a long time (much longer than a week) “just in case” or used in devices with low consumption, for example, remote controls or watches, then preference should be given to models with a reduced self-discharge current, despite their lower nameplate capacity.

Other materials on this topic

AA battery testing
Battery Testing Method

It's no secret that at any time you can find yourself in such conditions when it becomes necessary to recharge "dead" batteries. For example, Ni-MH batteries widely used in everyday life and in production - how to charge them correctly? Of course, you can use the simplest charger that comes with any household appliance. However, their strength is very low, so such a charge will “hold” for a very short time. The use of more complex type of chargers helps to ensure that the battery not only works "on full power”, but also used all its possible resources. Also, batteries are different. Their names directly depend on what composition they are made of.

Common types of nickel batteries, their similarities and differences

There are many, which include various chemical compounds. In domestic consumption, it is optimal to use nickel-metal hydride, cadmium and nickel-zinc elements. Of course, any battery needs some care, so it is always important to follow the rules of operation and charging.

Ni-MH

Nickel-metal hydride batteries are secondary chemical current sources with a much higher capacity than their predecessors - but their service life is shorter. One of the popular applications for nickel cells is model building (except for aviation, due to the fact that the battery is quite heavy in weight).

The first development of these cells began in the 70s of the twentieth century with the aim of improving Cd batteries. After 10 years, in the late 80s, it was possible to ensure that the chemical compounds used to create Ni-MH batteries became more stable. In addition, they are much less susceptible to the “memory effect” than Ni-Cd: they do not immediately “remember” the charge current remaining inside if the element was not completely discharged before use. Therefore, they do not need a full discharge so often.

Ni-Cd

Despite the fact that Ni-MH has a number of obvious advantages over Ni-Cd, it is worth noting that the latter do not lose their popularity. Mainly because they do not heat up so much when charging due to the greater conservation of energy inside the cell. As you know, there are various types of chemical processes occurring between substances.

If you charge Ni-MH, the reactions will be exothermic, and if the cadmium batteries - endothermic, which provides a higher efficiency. Thus, Cd can be charged with a higher current without fear of overheating.

Ni-Zn

Recently, much attention has been paid to discussion on the Internet of batteries, which include zinc. They are not as well known to consumers as the previous ones, but are ideal for use as batteries for digital cameras.

Their main feature is high voltage and resistance, due to which even by the end of the charge-discharge cycle there is no sharp drop in voltage, like a Ni charge. If there are metal hydride batteries in the camera, it will turn off even if the battery is not completely discharged, and Ni-Zn does not have this even at the end of the discharge.

Due to the nature of these batteries, they may require an individual charger, or they can be charged on any universal smart charger, such as the ImaxB6. Ni-Zn batteries are also great for use in electric children's toys and blood pressure monitors.

Rapid charging of NiMH batteries and other power sources

It is better to charge the battery using more complex models of the corresponding devices. Their current algorithms have a more complex sequence. Of course, doing this is a bit more complicated than simply inserting the battery into the basic charger included in the package. But the quality of charging when using a "smart" device will be an order of magnitude higher. So how do you charge Ni-MH batteries?

First, the current is turned on and the voltage at the battery terminals is checked (the current parameters are 0.1 of the battery capacity, or C). If the voltage exceeds 1.8 V, this means either the battery is missing or the battery is damaged. In this case, the process cannot be started. You need to either replace the damaged element with a whole one, or insert a new one into the device.

After checking the voltage, the initial discharge of the battery is evaluated. If U is less than 0.8 V, then you can not immediately proceed to fast charging, and if U = 0.8 V or more, then you can. This is the so-called "pre-charge phase", used to prepare cells that are very heavily discharged. The current value here is 0.1-0.3 C, and the duration in time is half an hour, no less. It should immediately be noted that at all stages it is important to constantly control the temperature . Especially when it comes to what current and how to properly charge a Ni-MH battery. Such batteries heat up much faster, especially towards the end of the process. Their temperature should not exceed 50°C.

Fast charging is only carried out if the previous checks have been carried out correctly. How to charge the battery correctly? So, the initial voltage is 0.8 V or a little more. The power supply starts. It is carried out smoothly and carefully for 2-4 minutes - until the desired level is reached. Optimal current level for Ni-MH and Ni-Cd batteries - 0.5-1.0 C, but sometimes it is recommended not to exceed 0.75.

It is important to determine the moment when the fast phase ends in time to avoid damaging the battery. The most reliable, in this case, is the dv method, which is used differently when charging nickel-cadmium and Ni-MH batteries. For Ni-Cd, the voltage becomes larger and drops towards the end of charging, so the signal for its completion is the moment when U drops to 30 mV.

Since the drop in U of the charged cells is much less pronounced for Ni-MH, in this case, the dv=0 method is used. A period of 10 minutes is recorded during which U of the battery remains stable - that is, with a voltage fluctuation threshold set to zero.

In conclusion, a small recharging phase follows. Current - within 0.1-0.3 C, duration - up to half an hour. This is necessary to ensure that the battery is fully charged, as well as to equalize the charge potential in it.

An important point (this also applies to charging Ni-Cd batteries): if it is carried out immediately after a fast one, you should definitely cool the battery for several minutes: the heated element is unable to take charge properly.

In addition to fast charging, there is also drip charging, which is produced by small currents. Some believe that it "prolongs the life" of the batteries, but this is not so. In fact, drip charging is no different from the effect of a standard charger without “serious” adjustment of current indicators. Any battery, if it is not used, sooner or later loses the accumulated energy, and it will still need a full-fledged charging process, regardless of its duration and "labor intensity". Such a charging process is also attractive for many because the current indicators here can not be fixed due to their smallness. However, only a serious approach to the use of "smart" chargers can "extend the life" of batteries. As well as their proper storage, taking into account the characteristics of a particular type of battery.

Temperature factor and storage conditions

Modern chargers are equipped with a special system for "evaluating" environmental conditions, including temperature factors. Such a “charger” can determine for itself whether to charge under certain conditions or not. It has already been mentioned that the level of efficiency inside the battery is the highest precisely at the beginning of the process, when the hydride plan batteries do not heat up so much. At the end of the charging process or closer to it, the efficiency drops sharply, and all the energy, turning into heat due to exothermic chemical reactions, is released outside. It is important to stop charging the Ni-MH battery in time. And, if possible, get the latest charger that will accurately control this process.

Currently, all chargers, including Cd batteries, can be charged with a current of up to 1C with the establishment of air cooling standards. The optimum temperature of the room in which charging is carried out is 20 ° C. It is not recommended to start the process at temperatures below +5 and above 50°C.

Ni-Cd is unique in that it is the only type of cell that will not be damaged if stored completely discharged, unlike Ni-MH. For better current output, it is recommended to charge nickel-cadmium batteries immediately before use. Also, after long-term storage, they need a "buildup": you should fully charge and discharge the Ni-Cd battery in a day for optimal performance.

Nickel-metal hydride cells, unlike their predecessors, can easily fail when deeply discharged. Therefore, you need to store them only charged. At the same time, the voltage should be checked regularly every two months. Its minimum level should always remain 1 V, and if it drops, recharging is necessary.

A new Ni-MH battery must be fully charged and discharged three times before use, then immediately put on the "base" for 8-12 hours. Later, there will be no need to keep it on charge for a long time - remove it immediately after indicating a special indicator on the charger.

Although all these batteries have long been replaced by more capacious ones, based on lithium, they are actively used now. It's more familiar and much cheaper. In addition, lithium batteries perform much worse at low temperatures.

Nickel-metal hydride batteries are gradually spreading in the market, and their production technology is being improved. Many manufacturers are gradually improving their characteristics. In particular, the number of charge-discharge cycles increases and the self-discharge of Ni-MH batteries decreases. This type of battery was produced to replace Ni-Cd batteries and little by little they are pushing them out of the market. But there remain some areas of use where NiMH batteries cannot replace cadmium batteries. Especially where high discharge currents are required. Both types of batteries require competent charging. We have already talked about charging nickel-cadmium batteries, and now it's the turn to charge Ni-MH batteries.

In the process of charging, a battery undergoes a series of chemical reactions, to which part of the supplied energy goes. The rest of the energy is converted into heat. The efficiency of the charging process is that part of the supplied energy that remains in the “reserve” of the battery. The efficiency value may vary depending on the charging conditions, but is never 100 percent. It should be noted that the efficiency when charging Ni-Cd batteries is higher than in the case of nickel-metal hydride. The process of charging Ni-MH batteries occurs with a large heat release, which imposes its own limitations and features. For more information, read the article at the link provided.

Charging speed is most dependent on the amount of current supplied. What currents to charge Ni-MH batteries is determined by the selected type of charge. In this case, the current is measured in fractions of the capacity (C) of Ni-MH batteries. For example, with a capacity of 1500 mAh, a current of 0.5C will be 750 mA. Depending on the charge rate of nickel-metal hydride batteries, there are three types of charging:

• Drip (charge current 0.1C);
• Fast (0.3C);
• Accelerated (0.5-1C).

By and large, there are only two types of charging: drip and accelerated. Fast and accelerated are practically the same thing. They differ only in the method of stopping the charge process.

In general, any charging of Ni-MH batteries with a current greater than 0.1C is fast and requires monitoring of some process termination criteria. Drip charging does not require this and can continue indefinitely.

Types of charging nickel-metal hydride batteries

Now, let's look at the features of different types of charging in more detail.

Drip charging of Ni-MH batteries

It is worth mentioning here that this type of charging does not increase the life of Ni-MH batteries. Since trickle charging does not turn off even after a full charge, the current is chosen very small. This is done so that the batteries do not overheat during prolonged charging. In the case of Ni-MH batteries, the current value can even be reduced to 0.05C. For nickel-cadmium, 0.1C is suitable.

With drip charging, there is no characteristic maximum voltage and only time can act as a limitation of this type of charging. To estimate the required time, you will need to know the capacity and initial charge of the battery. To calculate the charging time more accurately, you need to discharge the battery. This will eliminate the influence of the initial charge. The efficiency of drip charging Ni-MH batteries is at the level of 70 percent, which is lower than other types. Many NiMH battery manufacturers do not recommend trickle charging. Although recently there is more and more information that modern models of Ni-MH batteries do not degrade in the process of drip charging.

Fast Charging NiMH Batteries

Manufacturers of Ni-MH batteries in their recommendations give characteristics for charging with a current value in the range of 0.75-1C. Be guided by these values ​​when choosing what current to charge Ni-MH batteries. Charging currents above these values ​​are not recommended as this may cause the safety valve to open to relieve pressure. Fast charging of nickel-metal hydride batteries is recommended at a temperature of 0-40 degrees Celsius and a voltage of 0.8-.8 volts.

The efficiency of the fast charging process is much greater than that of drip charging. It is about 90 percent. However, by the end of the process, the efficiency drops sharply, and the energy is converted into heat. Inside the battery, the temperature and pressure rise sharply. have an emergency valve that can open when pressure increases. In this case, the properties of the battery will be irretrievably lost. And the high temperature itself has a detrimental effect on the structure of the battery electrodes. Therefore, clear criteria are needed by which the charging process will stop.

The requirements for the charger (charger) for Ni-MH batteries will be presented below. For now, we note that such chargers charge according to a certain algorithm. The stages of this algorithm in general view the following:

• determining the presence of a battery;
• battery qualification;
• pre-charging;
• transition to fast charging;
• fast charging;
• recharging;
• support charging.

Let's consider these stages in more detail.

At this stage, a current of 0.1C is applied and a voltage test is performed at the poles. To start the charging process, the voltage should be no more than 1.8 volts. Otherwise, the process will not start.

It is worth noting that the check for the presence of the battery is carried out at other stages. This is necessary in case the battery is removed from the charger.

If the memory logic determines that the voltage value is greater than 1.8 volts, then this is perceived as the absence of a battery or its damage.

Battery Qualification

Here, an approximate estimate of the battery charge is determined. If the voltage is less than 0.8 volts, then the fast charge of the battery cannot be started. In this case, the charger will turn on the pre-charge mode. Under normal use, Ni-MH batteries rarely discharge below 1 volt. Therefore, pre-charging is only activated in case of deep discharges and after long storage of the batteries.

Pre-charge

As mentioned above, pre-charging is activated when the Ni-MH batteries are deeply discharged. The current at this stage is set at 0.1-0.3C. This stage is limited in time and is somewhere around 30 minutes. If during this time the battery does not restore the voltage of 0.8 volts, then the charge is interrupted. In this case, the battery is most likely damaged.

Transition to fast charging

At this stage, there is a gradual increase in the charging current. The increase in current occurs smoothly within 2-5 minutes. In this case, as in other stages, the temperature is controlled and the charge is turned off at critical values.

The charge current at this stage is in the range of 0.5-1C. The most important thing at the stage of fast charging is the timely shutdown of the current. To do this, when charging Ni-MH batteries, control is used according to several different criteria.

For those who are not in the know, when charging, the voltage delta control method is used. In the process of charging, it constantly grows, and at the end of the process it begins to fall. Typically, the end of the charge is determined by a voltage drop of 30 mV. But this method of control with NiMH batteries does not work very well. In this case, the voltage drop is not as pronounced as in the case of Ni-Cd. Therefore, to trigger a trip, you need to increase the sensitivity. And with increased sensitivity, the likelihood of false alarms due to battery noise increases. In addition, when charging several batteries, the operation occurs at different times and the whole process is smeared.

But still, stopping charging due to a voltage drop is the main one. When charging with a current of 1C, the voltage drop to turn off is 2.5-12 mV. Sometimes manufacturers set detection not by a drop, but by the absence of a voltage change at the end of a charge.

At the same time, during the first 5-10 minutes of charging, the voltage delta control is turned off. This is due to the fact that when fast charging is started, the battery voltage can vary greatly as a result of the fluctuation process. Therefore, at the initial stage, control is turned off to eliminate false positives.

Due to the not too high reliability of charging off by voltage delta, control is also used according to other criteria.

At the end of the Ni-MH battery charging process, its temperature begins to rise. According to this parameter, the charge is turned off. To exclude the OS temperature value, monitoring is carried out not by absolute value, but by delta. Usually, a temperature increase of more than 1 degree per minute is taken as a criterion for terminating the charge. But this method may not work at charge currents less than 0.5C, when the temperature rises rather slowly. And in this case, it is possible to recharge the Ni-MH battery.

There is also a method for controlling the charging process by analyzing the derivative of the voltage. In this case, it is not the voltage delta that is monitored, but the rate of its maximum growth. The method allows you to stop fast charging a little earlier than the completion of the charge. But such control is associated with a number of difficulties, in particular, a more accurate voltage measurement.

Some chargers for Ni-MH batteries do not use direct current for charging, but pulsed current. It is served with a duration of 1 second at intervals of 20-30 milliseconds. As the advantages of such a charge, experts call a more uniform distribution active substances by battery volume and reducing the formation of large crystals. In addition, more accurate voltage measurement is reported in the intervals between current applications. As an extension of this method, Reflex Charging has been proposed. In this case, when a pulsed current is applied, the charge (1 second) and discharge (5 seconds) alternate. The discharge current is 1-2.5 times lower than the charge. As advantages, one can single out a lower temperature during charging and the elimination of large crystalline formations.

When charging nickel-metal hydride batteries, it is very important to control the end of the charging process by various parameters. There must be ways to abort the charge. For this, the absolute value of the temperature can be used. Often this value is 45-50 degrees Celsius. In this case, the charge must be interrupted and resumed after cooling. The ability to accept a charge in Ni-MH batteries at this temperature is reduced.

It is important to set a charge time limit. It can be estimated by the capacity of the battery, the magnitude of the charging current and the efficiency of the process. The limit is set at the estimated time plus 5-10 percent. In this case, if none of the previous control methods work, the charge will turn off at the set time.

Recharge stage

At this stage, the charging current is set to 0.1-0.3C. Duration about 30 minutes. Longer recharging is not recommended as it shortens battery life. The recharging stage helps to equalize the charge of the cells in the battery. It is best if, after a quick charge, the batteries cool down to room temperature, and then recharging starts. Then the battery will restore its full capacity.

Chargers for Ni-Cd batteries often put the batteries into drip charging mode after the charging process is completed. For Ni-MH batteries, this will only be useful if a very small current is supplied (about 0.005C). This will be enough to compensate for the self-discharge of the battery.

Ideally, charging should have the function of switching on the maintenance charge when the battery voltage drops. Backup charging only makes sense if a sufficiently long time elapses between charging the batteries and using them.

Ultra-fast charging of Ni-MH batteries

And it is worth mentioning the ultra-fast battery charge. It is known that when charged to 70 percent of its capacity, a nickel-metal hydride battery has a charging efficiency close to 100 percent. Therefore, at this stage it makes sense to increase the current for its accelerated passage. Currents in such cases are limited to 10C. The main problem here is determining those very 70 percent of the charge at which the current should be reduced to a normal fast charge. This is highly dependent on the degree of discharge from which the battery charging began. High current can easily lead to overheating of the battery and destruction of the structure of its electrodes. Therefore, the use of ultra-fast charge is recommended only if you have the appropriate skills and experience.

General requirements for chargers for nickel-metal hydride batteries

It is not advisable to disassemble any individual models for charging Ni-MH batteries within the framework of this article. It is enough to note that these can be narrowly focused chargers for charging nickel-metal hydride batteries. They have a wired charging algorithm (or several) and constantly work on it. And there are universal devices that allow you to fine-tune the charging parameters. For example, . Such devices can be used to charge various batteries. Including, and for, if there is a power adapter of the appropriate power.

It is necessary to say a few words about what characteristics and functionality a charger for Ni-MH batteries should have. The device must be able to adjust the charging current or its automatic installation depending on the type of batteries. Why is it important?

Now there are many models of nickel-metal hydride batteries, and many batteries of the same form factor may differ in capacity. Accordingly, the charging current must be different. If you charge with a current above the norm, there will be heating. If it is below the norm, then the charging process will take longer than expected. In most cases, the currents on the chargers are made in the form of "presets" for typical batteries. In general, when charging, manufacturers of Ni-MH batteries do not recommend setting a current of more than 1.3-1.5 amperes for type AA, regardless of capacity. If for some reason you need to increase this value, then you need to take care of forced cooling of the batteries.

Another problem is related to the charger power being cut off during the charging process. In this case, when the power is turned on, it will start again from the battery detection stage. The moment when fast charging ends is not determined by time, but by a number of other criteria. Therefore, if it passed, then it will be skipped when turned on. But the stage of recharging will take place again, if it has already been. As a result, the battery receives unwanted overcharging and excessive heating. Among other requirements for Ni-MH battery chargers is a low discharge when the charger is turned off. The discharge current in a de-energized charger should not exceed 1 mA.

It is worth noting the presence of another important function in the charger. It must recognize primary current sources. Simply put, manganese-zinc and alkaline batteries.

When installing and charging such batteries in the charger, they may well explode, since they do not have an emergency valve to relieve pressure. The charger is required to be able to recognize such primary current sources and not start charging.

Although it is worth noting here that the definition of batteries and primary current sources has a number of difficulties. Therefore, memory manufacturers do not always equip their models with similar functions.

A Few Tips for Operating Nickel-Metal Hydride Batteries

As you understand, the basic rules for operating Ni-MH batteries are to prevent overheating and overcharging. The following are additional tips for using NiMH batteries to help extend their life:

• If you leave Ni-MH batteries for a long time, then the charge in them should be 30-50 percent of the nominal capacity;
• Nickel-metal hydride batteries are much more sensitive to overcharging and heat than nickel-cadmium batteries. These things negatively affect their life and current output of batteries. Please note that a Ni-MH battery charger can be used to charge Ni-Cd, but not vice versa.;
• Nickel-metal hydride can, but does not have to be subjected to training cycles. A high-quality charger in several charges allows the battery to gain capacity lost during storage in a warehouse and transportation. For products from different manufacturers, the number of cycles for a set of containers varies. For some batteries, 3-4 cycles will be enough, while for others, even fifty may not be enough;
• After the end of the charge or discharge cycle, allow the battery to cool down. Charging at temperatures below 5 and above 50 degrees Celsius should not be carried out. This shortens the life of Ni-MH batteries;
• Avoid discharging the Ni-MH battery below 0.9 volts. In such cases, many inexpensive chargers simply will not be able to start charging. When charging cannot recognize such a discharged battery, you can connect the battery to an external power source (current 90-160 mA) and bring the voltage to 0.9 volts;
• When using the same battery of cells in the recharge mode, it is recommended to discharge the battery to 0.9 volts and then fully charge it in the charger. This process is desirable to repeat once every ten times recharging Ni-MH batteries.

Need information about? Then read the article at the link.

Charging parameters for the most common Ni-MH batteries

In conclusion, we give the parameters for charging the most common types of nickel-metal hydride batteries. Specifications are based on fully discharged batteries. They are summarized in the table below.