Internal resistance. Car batteries

The impedance of a lead-acid battery is the sum of polarization resistance and ohmic resistance. Ohmic resistance is the sum of the resistances of the battery separators, electrodes, positive and negative terminals, connections between cells and electrolyte.

The resistance of the electrodes is influenced by their design, porosity, geometry, lattice design, state of the active substance, the presence of alloying components, and the quality of the electrical contact of the lattice and coating. The resistance values ​​of the negative electrode arrays and the sponge lead (Pb) on them are approximately the same. At the same time, the resistance of lead peroxide (PbO2), which is applied to the positive electrode grid, is 10 thousand times greater.

During the discharge of a lead-acid battery, lead sulfate (PbSO4) is released on the surface of the electrodes. This is a poor conductor, which significantly increases the resistance of the electrode plates. In addition, lead sulfate is deposited in the pores of the plate coating and significantly reduces the diffusion of sulfuric acid from the electrolyte into them. As a result, by the end of the discharge cycle of a lead-acid battery, its resistance increases by 2-3 times. During the charging process, lead sulfate dissolves and the battery resistance returns to its original value.

The resistance of the lead-acid battery has a significant impact on the resistance of the electrolyte. This value, in turn, strongly depends on the concentration and temperature of the electrolyte. As the temperature decreases, the resistance of the electrolyte increases and reaches infinity when it freezes.

With an electrolyte density of 1.225 g/cm3 and a temperature of +15 C, it has a minimum resistance value. As the density decreases or increases, the resistance increases, which means the internal resistance of the battery also increases.

The resistance of separators changes depending on changes in their thickness and porosity. The amount of current that flows through the battery affects the polarization resistance. A few words about polarization and the reasons why it occurs. The first reason is that the electrode potentials change in the electrolyte and on the surface of the electrodes (electric double layer). The second reason is that when current passes, the electrolyte concentration changes in the immediate vicinity of the electrodes. This leads to a change in the electrode potentials. When the circuit opens and the current disappears, the electrode potentials return to their original values.

One of the features of lead-acid batteries is their low internal resistance compared to other types of batteries. Thanks to this, they can deliver high current (up to 2 thousand amperes) in a short time. Therefore, their main area of ​​application is starter batteries in vehicles with internal combustion engines.

It is also worth noting that the internal resistance of the battery at alternating or direct current strongly depends on its frequency. There are a number of studies whose authors observed the internal resistance of a lead-acid battery at a current frequency of several hundred hertz.

How can you estimate the internal resistance of a battery?

As an example, consider a 55 Ah car lead-acid battery with a nominal voltage of 12 volts. A fully charged battery has a voltage of 12.6-12.9 volts. Let's assume that a resistor with a resistance of 1 ohm is connected to the battery. Let the voltage of the open battery be 12.9 volts. Then the current should theoretically be 12.9 V / 1 Ohm = 12.9 amperes. But in reality it will be below 12.5 volts. Why is this happening? This is explained by the fact that in an electrolyte the rate of diffusion of ions is not infinitely large.

The image shows the battery as a 2-pole power source. It has an electromotive force (EMF), which corresponds to the open circuit voltage, and internal resistance. In the diagram they are designated E and Rin. When the circuit is closed, the emf of the battery partially drops across the resistor, as well as through the internal resistance itself. That is, what happens in the circuit can be described by the following formula.

E = (R + Rin) * I.

In the images below you can see the values ​​of the EMF of a car battery in an open circuit and the voltage when connecting a load in the form of two car light bulbs connected in parallel.

The voltage of a car battery is the potential difference across the pole terminals. For greater accuracy, it is recommended to measure the voltage when the transients caused by the charging or discharging current have ended. Their duration can be several hours, and the voltage change can reach 0.6-1.8 Volts. Although it is generally accepted that car starter batteries have a nominal voltage of 12 Volts, in reality the voltage of a new charged battery is in the range of 12.7-13.3 Volts.

The capacity of the battery is characterized by the amount of electricity, measured in ampere-hours, received from the battery when it is discharged to a set final voltage of 10.5 Volts and a temperature of 20 degrees. During normal operation, it is not recommended to discharge a car battery below its final voltage. Otherwise, its service life is sharply reduced.

The value of the battery capacity allows you to calculate the approximate time it delivers (or operates) the average current to the load. The capacity depends on the strength of the discharge current, so during testing the discharge conditions are standardized. The discharge current is set to 0.05 Cp for a 20-hour discharge mode and 0.1 Cp for 10 hours. For a battery with a capacity of 60 Ah, it is, respectively, 3 Amperes and 6 Amperes. At such currents, the capacity of the new one corresponds to the nominal value. And for a discharge current of 25 Amps, the typical capacity of this battery is 40 Ah. This capacity will provide power to electrical equipment for 96 minutes.

40 Ah x 60 minutes / 25 Ampcr = 96 minutes.

The current value of 25 A was not adopted in the tests by chance. It is believed that this is the current consumption of the electrical equipment of a typical passenger car. With starter currents, the capacity of a car battery can drop 5 times relative to the nominal value. So, for a 6ST-55A battery with a starter current of 250 A and a temperature of minus 18 degrees, the capacity is only 10 Ah instead of 55 Ah. And yet this value will provide a total starter cranking time of 2.4 minutes.

10 Ah x 60 minutes / 250 Amps = 2.4 minutes.

The capacity of a car battery decreases very sharply at negative temperatures and already at minus 20 degrees it decreases to 40-50%

Reducing the cold cranking current and the capacity of the 6ST-55 battery as the temperature drops.

With a larger capacity, a car battery also produces a higher cold cranking current. For example, a 55 Ah capacity provides a current of 420-480 Ampere according to the EN standard and 250-290 Ampere according to DIN, a battery with a capacity of 62 Ah provides a current of 510 Ampere according to the EN standard and 340 Ampere according to DIN, and a 77 Ah battery already provides 600 Ampere according to EN and 360 Amps according to DIN.

Cold start current (Cold Cranking Ampere - CCA) of a car battery, requirements of DIN 43539 T2, EN 60095-1, SAE, IEC 95-1 (IEC 95-1).

The cold start current of a car battery determines its maximum starting capacity, that is, how much current the battery can deliver at a temperature of minus 18 degrees at the end of a given time interval, until the battery voltage drops to the required minimum level. The DIN and EN standards provide for two checks on the process of discharging a car battery to a voltage of 6 Volts.

The first check is carried out 30 seconds from the start of the discharge, and it measures the voltage U30 of the battery, which for the DIN standard must be greater than 9 Volts, and for the EN standard - greater than 7.5 Volts. The second check consists of measuring the duration of the T6v discharge until the battery voltage reaches 6 Volts, which should be at least 150 seconds.

There are four standards, DIN 43539 T2, EN 60095-1, SAE, IEC 95-1, which define the duration of the test interval and the permissible minimum voltage of a car battery, the requirements for which are indicated in the table below

The SAE and IEC standards only define the limiting voltage value U30. For ease of comparison, the cold cranking current values ​​of a car battery can be converted from one standard to another. Currents are recalculated using the following formulas.

Isae = 1.5Idin + 40 (A)
Iiec = Idin/0.85 (A)
Ien = Idin/0.6 (A)
Idin = 0.6Ien (A)

Values ​​in the EN standard are rounded.

— At a current of less than 200 A in 10 A increments.
— At a current of 200-300 A in steps of 20 A (220, 240, 260, 280 A).
- At a current of 300-600 A in steps of 30 A (330, 360, 390 A, etc.).

For example, a VARTA battery with a capacity of 55 Ah has a DIN current of 255 Ampere. Using the above formulas, we get for Isae = 422.5 Ampere, Iiec = 300 Ampere, Ien = 425 Ampere, rounding - 420 A.

Typically, the cold start current of a car battery is 6.5-7.5 times higher than the nominal capacity. The number of possible engine starts over the entire service life of a car battery ranges from 4,000 for low-maintenance batteries and up to 12,000 for specially designed batteries, such as the Optima battery, according to the manufacturer.

It is believed that in one year, during operation of moderate intensity, from 1,000 to 2,000 engine starts are made. Thus, the service life of a car battery can be from 4 to 2 years. We note in view of the importance that the cold start current CCA, in accordance with the standards, is standardized by each car battery manufacturer only for a temperature of minus 18 degrees. The manufacturer does not provide data for lower temperatures.

For a fully charged and new battery with a capacity of 50-60 Ah, the cold cranking current is in the range of 300-500 Amperes. If the starting current of a typical 6ST-55 battery at a temperature of plus 25 degrees is 400 Amperes, then at a temperature of minus 30 degrees it will drop to 200 A. With each new attempt at an unsuccessful start, its value will be less and less. Although battery production technologies are improving, these changes have had almost no effect on the degree to which their starting current decreases at subzero temperatures.

Reserve capacity (RC - residual capacity) of a car battery.

The reserve capacity or residual capacity of a car battery is rarely indicated in the battery data sheet, but it is important for the consumer because it shows the time during which the battery will provide operation of the car if the car fails. At the same time, current consumption by all vehicle systems is normalized to 25 Amperes.

The reserve capacity of a car battery is defined as the period of time in minutes during which the battery can maintain a discharge current of 25 Amps until the voltage drops to 10.5 Volts. The standards do not establish a requirement for the amount of reserve capacity. For many batteries with a capacity of 55 Ah, the reserve capacity reaches 100
minutes, which is a good indicator.

Internal resistance of a car battery.

Typical internal resistance values ​​for a new car battery are 0.005 ohms at room temperature. It consists of the resistance between the electrodes and the electrolyte and the resistance of the internal connections. Towards the end of its service life, the internal resistance of a car battery increases many times, which leads to the fact that the battery cannot crank.

Based on materials from the book “Tutorial for installing car theft protection systems.”
Naiman V. S., Tikheev V. Yu.

If you take a new lithium-ion battery, say size 18650, with a nominal capacity of 2500mAh, bring its voltage to exactly 3.7 volts, and then connect it to an active load in the form of a 10-watt resistor with a nominal value of R = 1 Ohm, then what is the value of the constant is the current we expect to measure through this resistor?

What will happen there at the very first moment, until the battery practically begins to discharge? In accordance with Ohm's law, it would seem that there should be 3.7A, since i=U/R=3.7/1 = 3.7[A]. In fact, the current will be slightly less, namely in the region of I = 3.6A. Why will this happen?

The reason is that not only the resistor, but also the battery itself has a certain internal resistance, since chemical processes inside it cannot occur instantly. If you imagine a battery in the form of a real two-terminal network, then 3.7V will be its EMF, in addition to which there will also be an internal resistance r, equal, for our example, to approximately 0.028 Ohm.

Indeed, if you measure the voltage across a resistor of R = 1 Ohm attached to the battery, it will turn out to be approximately 3.6 V, and 0.1 V will therefore drop at the internal resistance r of the battery. This means that if a resistor has a resistance of 1 ohm, the voltage measured across it was 3.6 V, therefore the current through the resistor is equal to I = 3.6A. Then, if u = 0.1V went to the battery, and our circuit is closed, in series, then the current through the battery is I = 3.6A, therefore, according to Ohm’s law, its internal resistance will be equal to r = u / I = 0.1/3.6 = 0.0277 Ohm.

What determines the internal resistance of a battery?

In reality, the internal resistance of batteries of different types is not constant all the time. It is dynamic and depends on several parameters: on the load current, on the battery capacity, on the degree of charge of the battery, as well as on the temperature of the electrolyte inside the battery.

The higher the load current, the lower, as a rule, the internal resistance of the battery, since the charge transfer processes inside the electrolyte are more intense in this case, more ions are involved in the process, and the ions move more actively in the electrolyte from electrode to electrode. If the load is relatively small, then the intensity of chemical processes on the electrodes and in the battery electrolyte will also be less, which means the internal resistance will seem greater.

Batteries with a larger capacity have a larger electrode area, which means the area of ​​interaction between the electrodes and the electrolyte is larger. Therefore, more ions are involved in the charge transfer process, more ions create a current. A similar principle is demonstrated - the larger the capacitance, the more charge can be used in the vicinity of a given voltage. So, the higher the battery capacity, the lower its internal resistance.

Now let's talk about temperature. Each battery has its own safe operating temperature range, within which the following applies. The higher the temperature of the battery, the faster the diffusion of ions inside the electrolyte occurs, therefore, at a higher operating temperature, the internal resistance of the battery will be lower.

The first lithium batteries, which did not have protection against overheating, even exploded because of this, since the oxygen formed due to the rapid disintegration of the anode (as a result of a rapid reaction on it) was released too actively. One way or another, batteries are characterized by an almost linear dependence of internal resistance on temperature in the range of acceptable operating temperatures.

As the battery discharges, its active capacity decreases, since the amount of active substance in the plates that can still participate in creating current becomes less and less. Therefore, the current becomes less and less, and accordingly the internal resistance increases. The more charged the battery, the lower its internal resistance. This means that as the battery discharges, its internal resistance becomes greater.

An essential characteristic for a battery - internal resistance - is designated by the letter “R”. It affects a lot, and its measurement is one of the main stages of battery diagnostics. This parameter is divided into several types. Perhaps the most significant is the internal resistance of the battery. It's helpful to understand what it means and how it is measured.

Parameter description

To begin with, it is worth saying that there is a total resistance of the battery. This is the sum of ohmic R and R polarization. At the same time, ohmic is the sum of the electrolyte resistances, connections between battery elements, negative and positive terminals, electrodes, separators.

Battery internal resistance- such R, which turns out to be a battery for the current flowing inside it. It does not matter whether the current is charging or discharging. However, it will vary in different battery elements. The elements will have their own indicator:

• electrode arrays;
• electrolyte;
• separators.

The indicator in these cells is influenced by several factors, due to which it can vary greatly between different batteries. That's why it wouldn't hurt to measure the battery's resistance.

Related factors

There is practically no difference between the performance of sponge lead and the negative electrode grid. However, the resistance of lead peroxide is 10,000 times greater than that of the positive electrode array on which it is applied.

The electrodes of the device themselves can be made in different ways, which causes differences in performance. May vary, including:

• quality of electrical contact of coating and gratings;
• electrode design;
• lattice design;
• the presence of alloying components in the battery.

The R of separators is affected by changes in porosity and thickness. For an electrolyte, it depends on its temperature and concentration. If the electrolyte freezes, the indicator will reach infinity.

It must be said that whatever the internal resistance of the battery, it will depend on the frequency.

Resistance measurement

This value is conditional. It changes depending on the degree of charge of the battery, the load size, and temperatures. That is why, when making accurate calculations regarding the battery, it is customary to use not the value of internal resistance, but the so-called discharge curves.

However, there are situations when you need to find out the internal resistance of a car battery. For these purposes, you can use an incandescent lamp from a headlight.

This option will give a completely accurate result. For example, this could be a halogen lamp with a power of 60 watts.

A parallel connection is made to the voltmeter battery and the above lamp. Next you need to remember the voltage value. Then the lamp turns off. Naturally, after this the tension will increase. If the latter increased by no more than 0.02 volts, then the battery is in satisfactory condition. That is, internal R is no more than 0.01 Ohm.

Finding out this parameter yourself is not difficult at all. The main thing is- do not use LED lamps. The entire procedure will take a few minutes.

Experience of car enthusiasts

I never do this on my own. And in general, I rarely take care of the battery the way I should. Therefore, difficulties with ignition often arise. You have to go to car repair shops to get rid of them. I pay money, but I don’t waste my time and energy.

Igor Slabkin

Of course, you need to measure. But do not rely on absolute indicators taken from the Internet. It is much more relevant to compare new results with old ones, because they will greatly depend not only on the model, but also on natural conditions. Of course, certain frameworks and standards still exist, but they should only be taken from the official specifications presented on the device body or in its original packaging.

Kirill Semenov

I measure this parameter regularly. One day it got too big. It took me a long time to figure out the reason, and then I realized that something had happened to the coating. I didn’t understand why, but I quickly corrected it. I just replaced the element. The ignition is still fine, so you can do this.

Alexander Rasskazov

I constantly take care of my car’s battery, because I’m afraid that it won’t start in the most inappropriate situation. I measure all parameters, including this one. This is the only way to fully understand the situation and track changes. This is important for diagnosing possible problems and malfunctions.

Victor Kuznetsov

Previously, I didn’t understand how to find out what the internal resistance of a battery is. As it turned out, the procedure is very simple - certainly no more difficult than measuring the full capacity. The procedure takes only a few minutes. Only the lamps you need are not LED, but ordinary ones.