Battery voltage and capacity. We will change earthly life! Difference between nominal and actual battery capacity

6. What determines the battery capacity?

Discharge current

ABOUT Typically, the manufacturer assigns the nominal capacity of a lead-acid battery for long-term (10, 20 or 100 hours) discharges. The battery capacity at such discharges is designated as C 10, C 20 or C 100. We can calculate the current flowing through the load during a 20-hour (for example) discharge - I 20:

I 20 [A] = E 20 [A*hour] / 20[hour]

Z Does this mean that with a 15-minute (1/4 hour) discharge the current will be equal to E 20 x 4? No, that's not true. With a 15-minute discharge, the capacity of a lead-acid battery is typically just under half its rated capacity. Therefore, the current I 0.25 does not exceed E 20 x 2. That is The discharge current and discharge time of a lead battery are not proportional to each other.

Z The dependence of the discharge time on the discharge current is close to a power law. In particular, Peukert's formula (law) is widespread - named after the German scientist Peukert. Peukert found that:

I p * T = const

Here p is the Peukert number - an exponent that is constant for a given battery or type of battery. Peukert's formula also applies to modern sealed lead acid batteries.

D For lead-acid batteries, the Peukert number typically ranges from 1.15 to 1.35. The value of the constant on the right side of the equation can be determined from the nominal capacity of the battery. Then, after several transformations, we obtain a formula for the battery capacity E at an arbitrary discharge current I:

E = E n * (I n /I) p-1

Here E n is the nominal capacity of the battery, and I n is the discharge current at which the nominal capacity is set (usually a 20-hour or 10-hour discharge current).

Final discharge voltage

P As the battery discharges, the voltage on the battery drops. When the final discharge voltage is reached, the battery is disconnected. The lower the final discharge voltage, the greater the battery capacity. The battery manufacturer sets the minimum permissible final discharge voltage (it depends on the discharge current). If the battery voltage drops below this value ( deep discharge), the battery may be damaged.

Temperature

P When the temperature rises from 20 to 40 degrees Celsius, the capacity of a lead battery increases by about 5%. When the temperature decreases from 20 to 0 degrees Celsius, the battery capacity decreases by approximately 15%. When the temperature decreases by another 20 degrees, the battery capacity drops by another 25%.

Battery wear

E The capacity of a lead-acid battery as delivered may be slightly more or slightly less than the nominal capacity. After several discharge-charge cycles or several weeks of being under a “floating” charge (in a buffer), the battery capacity increases. During further use or battery storage The battery capacity decreases - the battery wears out, getting old and must eventually be replaced with a new battery. To replace the battery on time, it is better to monitor the battery wear using a modern battery capacity tester - Lead-acid battery capacity indicator "Pendant"

7. How to check the capacity of a lead-acid battery?

TO classical method battery check is the control digit. The battery is charged and then discharged with a constant current, recording the time to the final discharge voltage. Next, determine the residual capacity of the battery using the formula:

E [A*hour]= I [A] * T [hour]

T The discharge time is usually chosen so that the discharge time is approximately 10 or 20 hours (depending on the discharge time for which the nominal battery capacity is indicated). Now you can compare the remaining battery capacity with the nominal capacity. If the residual capacity is less than 70-80% of the nominal capacity, the battery is taken out of service, because with such wear, further battery aging will happen very quickly.

N The disadvantages of the traditional method of monitoring battery capacity are obvious:

complexity and labor intensity;

removing the battery from use for a long period of time.

For quick battery test Now there are special devices that allow you to check the battery capacity in a few seconds.

The concept of battery capacity

Capacity battery is one of its most important technical characteristics. This term is understood as the amount of time that a source of autonomous energy is capable of powering the electrical consumers connected to it. In other words, this is the maximum amount of electricity accumulated by the battery during a full charging cycle. The unit of capacity is Ah (ampere-hour), for small batteries it is mAh (milliamp-hour).

An example of calculating the required capacity

As you know, the calculation of power consumption is made in W, and the battery capacity for a UPS is in Ah. To calculate the required battery capacity to power a particular equipment, it is necessary to make some recalculation. For a better understanding, let's look at a specific example. Let's say there is a 500 W critical load that requires backup for 3 hours. Since the amount of accumulated energy depends not only on the battery capacity, but also on its voltage, to calculate we divide the total power of the redundant equipment by their operating voltage (often confused with voltage idle move fully charged battery). For a standard 12V battery, the required battery capacity will be:

Q= (P t) / V k

where Q is the required battery capacity, Ah;

V – voltage of each battery, V;

t – reservation time, h;

k is the coefficient of battery capacity utilization (the amount of electrical energy allowed for use by consumers).

The need to introduce a coefficient is due to the possibility of an incomplete charge of the battery. In addition to this, a strong (deep) discharge following a small number of charge and discharge cycles leads to premature wear and failure of the battery. For example, if new battery discharge by 30% of its total capacity, and then immediately charge it, it can withstand about 1000 such cycles. If the discharge value decreases to 70%, the number of these cycles will decrease by approximately 200.

In total, we find that to power this load for the specified period of time it will be necessary:

Q= 500·3/ 12·0.7 = 178.6 Ah.

This is the minimum required battery capacity for the case under consideration. Ideally, it is better to take an energy source with a small reserve (about 20%) so as not to completely discharge it each time - this will help preserve the battery performance for as long as possible.

Q = 178.6 1.2 = 214.3 Ah.

This means that to solve this problem it is necessary to purchase batteries with a total capacity of at least 215 Ah. When using a UPS in conjunction with a generator, it is recommended to reduce the capacitance correction factor to 0.4, since in such a combination batteries are most often used to maintain continuous power supply until the power plant turns on and the entire load is switched to it. Moreover, if the coefficient value of 0.4 includes the loss of battery capacity during its aging, due to the peculiarity pulse converter and others, then on average the battery discharge can reach 50% of its nominal capacity.

In the case when several batteries are used to back up the load, the amount of energy accumulated in them is absolutely independent of the type of their connection - parallel, serial, or mixed. Considering this feature It is necessary to substitute the voltage of one battery into the formula for determining the total capacity of batteries, but in this case it is allowed to use only batteries with the same technical characteristics.

Indicators of batteries, with which the concept of capacity is inextricably linked

1. Dependence of battery capacity on its discharge current.

This dependence is based on the following fact: when the protected load is connected to the battery without using a converter, the amount of current consumed by the battery remains unchanged. In this case, the operating time of connected electrical consumers will be determined as the ratio of the selected capacity to the consumed current. In a more familiar form, this formula is written as follows:

where Q is the battery capacity, Ah (mAh);

T – battery discharge time, hours.

If we are dealing with large amounts of current consumption, then real indicators power is often lower than the nominal specified in the passport.

1. Dependence of battery capacity on energy

Today, it is quite common among users that the capacity of a battery is a value that fully characterizes its electrical energy, accumulated by the battery when it is 100% charged. This statement is not entirely correct. Here it is also necessary to make a reservation that the battery’s ability to accumulate energy directly depends on its voltage and the higher it is, the more energy the battery can accumulate. In fact, electrical energy is defined as the product of the charging current, battery voltage and the flow time of this current:

where W is the energy accumulated by the battery, J;

U – battery voltage, V;

I – D.C. battery discharge, A;

T – battery discharge time, hours.

Based on the fact that the product of current and charging time gives us the battery capacity (as discussed above), it turns out that the electrical energy of the battery is found by multiplying the rated voltage of the battery and its capacity:

where W is the energy accumulated by the battery, Wh;

Q – battery capacity, Ah;

U – battery voltage, V.

When connecting several batteries of the same capacity in series, general indicator of a given bundle is equal to the sum of the capacities of all batteries included in its composition. In this case, the energy of the resulting battery pack will be determined as the product of the electricity of one battery and their number.

1. The concept of battery energy capacity

An equally useful indicator for the consumer of rechargeable batteries is their energy capacity, measured in units such as W/cell. This concept characterizes the battery’s ability for a certain short period of time, which is most often no more than 15 minutes, in constant power mode. This indicator is most widespread in the United States, but has recently been gaining popularity among consumers in many other countries. To approximate the calculation of the battery capacity, measured in Ah based on its energy capacity in W/cell for a period of 15 minutes, use the formula:

W – energy capacity of the battery, W/cell.

1. The concept of battery reserve capacity

For car batteries, another characteristic is distinguished - reserve capacity, which indicates the battery’s ability to power the electrical equipment of a moving car when the standard generator vehicle does not work. This parameter also better known in the USA and called “reserve capacity”. It is measured in minutes of battery discharge with a current value of 25 A. To approximate the nominal capacity of the battery based on its reserve capacity indicator, indicated in minutes, you must use the formula:

where Q is the battery capacity, Ah;

T – battery reserve capacity, min.

Battery capacity and charge (charge)

Another fairly popular misconception is the identification of the concepts of battery capacity and its charge (charge). Let's dot the i's. Capacity refers to the maximum potential of a battery, that is, the amount of energy that it can accumulate in a fully charged state. The charge, in turn, represents this energy necessary to power the load in autonomous mode. Hence the conclusion is that the amount of charge of the same battery can be different depending on the charging time of the battery, and the amount of its capacity in the discharged and charged state is the same. Here we can draw an analogy with a glass into which water is poured. The volume of the device will represent the capacity - this is a value that does not depend on whether the glass is full or empty, and the water that is poured is the charge.

What other factors does the battery capacity depend on?

Discharge current

The battery capacity indicators that can be found in their technical documentation and on the product case are indicated by the manufacturer based on the results of test measurements made according to the above formula (Q = I T) at a standard discharge duration (10, 20, 100 hours, etc.). d.). The capacitance is designated accordingly - Q10, Q20 and Q100, as well as the discharge current - I10, I20 I100. In this case, the amount of current flowing through the load with a discharge time of 20 hours will be determined by the formula:

Following this logic, we can assume that with a discharge lasting a quarter of an hour (15 minutes), the current will be equal to Q20 x 4. However, this is not the case, as practice shows; in the case of a 15-minute discharge, the capacity of the standard lead battery will be no more than half of its nominal capacity. Accordingly, the value of the parameter I0.25 will be slightly less than Q20 x 2. From here we can conclude that characteristics such as time and discharge current are not proportional to each other.

Final discharge voltage

Each time the battery is discharged, the voltage on it gradually drops, and when the so-called final discharge voltage is reached, it is imperative to disconnect the battery. Moreover, the lower this characteristic, the correspondingly higher will be the actual battery capacity. As a rule, manufacturers indicate on their own batteries the minimum value of the final discharge voltage, which in turn depends on the current used to discharge the battery. There are situations when the voltage of the energy source drops below this value (they forgot to turn off the battery in time or this could not be done because it was impossible to de-energize the load for a long period). Then a phenomenon called deep discharge of the battery occurs. If the battery is often allowed to be deeply discharged, it can quickly fail.

Battery wear

It is generally accepted that a new battery has rated capacity(the one specified by the manufacturer). However, the actual value of this indicator may differ slightly - it may be less than declared due to long-term storage in a warehouse, or after several full charge and discharge cycles and short-term operation in buffer mode, it may increase slightly. Further use of the battery, as well as its storage, invariably leads to physical wear and tear of the energy source, its aging and gradual failure.

Temperature

Such important factor, How ambient temperature in the place where the battery is used greatly affects the capacity of the latter. If the temperature rises from 20°C to 40°C, the battery capacity increases by 5%, and when it drops to 0°C, it decreases by an average of 15%. A further decrease in air temperature leads to a drop in this parameter by another 25% relative to the nominal value.

How to check battery capacity?

Very often, the owner of a used battery is faced with the task of determining its residual capacity. The classic and, to our credit, the most reliable and effective way to check the actual capacity of a battery is considered to be a test discharge. This term refers to the following procedure. The battery is first fully charged, after which it is discharged with direct current, and the time during which it is completely discharged is measured. After this, the battery capacity is calculated using the already known formula:

For greater calculation accuracy, it is better to select the value of the constant discharge current so that the discharge time is about 10 or 20 hours (this depends on the discharge time at which the nominal battery capacity was calculated by the manufacturer). Then the obtained data is compared with the passport data, and if the residual capacity is 70-80% less than the nominal capacity, the battery must be replaced, since this is a clear sign of severe wear of the battery and its further wear will occur at an accelerated pace.

The main disadvantages of this method are the complexity and labor-intensive implementation, as well as the need to remove batteries from service for a fairly long period of time. Today, most devices that use rechargeable batteries for their operation have a self-diagnosis function - a quick (in just a couple of seconds) check of the condition and performance of energy sources, but the accuracy of such measurements is not always high.

Battery capacity- This technical specifications, showing the period of time during which the battery will supply power to the load connected to it. The unit of measurement for battery capacity is ampere-hour, but if the battery is small, then milliamp-hour. The battery capacity is determined by the formula, which is the product of the constant discharge current of the battery (in amperes or milliamps) and the charging time (in hours):

E [A * hour] = I [A] x T [hour]

What is the battery capacity?

What is the battery capacity- this is a concept that makes it clear how long the battery can power the load connected to it. Battery energy (which can accumulate in a fully charged battery) is not fully characterized Battery capacity. Therefore, the higher the battery voltage, the greater the energy accumulated in it. Electrical energy is equal to the product of voltage times current and the time the current flows:

[J]= I [A] x U [V] x T [s]

And the battery energy for a UPS is equal to the product of the battery capacity and its rated voltage:

W [W*hour]= E [A*hour] x U [V]

This indicator indicates the capabilities of this battery. Often Battery capacity confused with its charge (charge). Capacity shows only the potential of the battery, that is, the time it is capable of powering the load when it is fully charged.

As clear example you can take a glass of water. Depending on whether the glass is full or empty, its capacity (volume) does not change. The situation is similar with the battery - the capacity is the same both in the charged and in the discharged state of the battery.

Battery energy capacity[W/cell] is a characteristic of a battery showing its ability to discharge in constant power mode over a certain short period of time (usually 15 minutes). For battery capacity measurements in ampere-hours according to its energy in W/el (15 min) there is a formula:

E [Ah] = W [W/el] / 4

Battery reserve capacity is a characteristic car battery, which shows its ability to provide power to the electrical system of a moving car when the car's generator is not working. The unit of measurement is minutes of battery charge with a current of 25 A. Battery capacity(ampere-hour) from its reserve capacity (minutes) can be approximately estimated using the formula:

E [Ah] = T [min] / 2

Values ​​on which the battery capacity depends:

1. Charge current.

Manufacturers usually designate the nominal capacity of a lead-acid battery for UPS for long-term discharges (10, 20, 100 hours). With such discharges, the battery capacity is designated: C10, C20 and C100. The current flowing through the load, for example, with a 20-hour charge - I20 can be calculated using the formula:

I20 [A] = E20 [A*hour] / 20[hour]

2. Battery wear

As delivered lead battery capacity may be slightly less or more than its rated capacity. When performing discharge-charge cycles several times or after several weeks in a buffer (under a “floating” charge), the battery capacity will increase. However, further use or storage of the battery leads to a decrease in the battery capacity, and the battery itself ages, wears out, and therefore requires replacing the battery with a new one.

This process is most often performed using the control discharge method. First, it is charged, and then discharged with a direct current, while the time to the final discharge voltage is recorded. Then the residual capacity of the battery is determined using the formula:

E [A*hour]= I [A] * T [hour]

How and why is battery capacity measured?

Charge Q, as the amount of electricity, is measured in coulombs (C), the electrical capacity of capacitors C is in farads, microfarads (μF), but for some reason it is measured not in farads, but in ampere-hours (milliamp-hours).

What would that mean? One ampere is a coulomb in one second; we know from a physics course that if an electric charge equal to 1 coulomb passes through a conductor in 1 second, then a current of 1 ampere flows through the conductor.

So what is an ampere hour then? Ampere-hour (Ah) is the battery capacity at which, based on a reduced current of 1 ampere, the battery will be discharged in 1 hour to the minimum permissible voltage.

1 ampere hour is 3600 coulombs. Suppose we want to obtain a bank of capacitors that is equivalent in discharge characteristics, albeit over a short section, to a 12-volt battery with a capacity of 55 ampere-hours. 55 amperes for an hour is 55 * 3600 coulombs.

Let us assume a voltage change from 13 to 11 volts, then since Q = C(U1-U2), then C = 55 * 3600/2 = 99000 F. Almost 100 kilofarads is the equivalent electrical capacity of a car battery, if its discharge characteristics were the same as at the capacitor.

There is a video on the Internet where six supercapacitors of 3000 F, 2.7 V each, connected in series, replace the starter battery of a car. It turns out 500 F at about 16 V.

Let's estimate what current and for how long such an assembly can produce. Let the operating range be taken again from 13 to 11 volts. For how long can you count on a current of 200 A (with a margin)? I = C(U1-U2)/t, then t = C(U1-U2)/I = 500*2/200 = 5 seconds. Enough to start the engine.

As often happens in our imperfect world, the generally accepted unit for measuring battery capacity has become a unit that cannot accurately reflect the capacity - milliamp-hours (mAh, mAh, mAh). Many manufacturers have tried to “instill” in the population the “correct” unit of measurement - watt-hours (Wh, Wh, Wh), but for some reason it has not yet taken root.

Let me explain why watt-hours are the “correct unit” and milliamp-hours (or ampere-hours) are the “wrong” ones. Batteries and battery assemblies come in different nominal voltages, for example 1.2, 3.6, 3.7, 7.4, 11.1, 14.8 V. However, a 7.4 V 2000 mAh battery has twice the capacity of a 3.7 V 2000 mAh, with watt-hours of such confusion it won’t - the first battery has a capacity of 14.8 Wh, the second 7.4 Wh. In this case, to get the watt-hours, I simply multiplied the rated voltage of the battery by the charge in ampere-hours (1Ah=1000mAh).

But that is not all. Let's see how it discharges Li-ion battery from the Cubot S200 smartphone.

During the discharge process, the voltage on the battery changes. Our lithium ion battery it drops from 4.291 V to 3.0 V.

At the same time, the battery characteristics indicate an average voltage of 3.7 V and a charge in milliamp-hours for this voltage. The real amount of energy that the battery will produce can only be calculated in watt-hours by multiplying the current voltage by the current current at each time and obtaining the final capacity value from the sum of these values, dividing it by the number of such calculations per hour.

The analyzer discharged the battery in 36694 seconds, maintaining a constant discharge current of 301 mA. If we simply multiply 301 by 36694 and divide by 3600 (the number of seconds in an hour) we get 3068 mAh. Let's multiply this value by the nominal battery voltage of 3.7 V and divide by 1000. We get 11.35 Wh.

But what really?

The analyzer measures voltage values ​​10 times per second. By multiplying each voltage value by the discharge current, we obtain the power during each measurement. Let's add up the power values ​​of all 366,913 measurements and divide by the number of measurements per hour (36,000).

With your permission, I will not provide screenshots of 366893 intermediate lines. :)

The resulting value is 11.78 Wh - the real amount of energy that the battery provided. If we divide this value by 3.7V we get a calculated charge of 3184 mAh.

The discrepancy between the actual amount of energy supplied by the battery differs from the calculated one by 3.8%; this is exactly the error that will result if you measure not watt-hours, but milliamp-hours produced by the battery.

In fairness, it must be said that regular batteries this discrepancy is usually about one percent.

That is why all devices that measure battery capacity in milliamp-hours give only approximate results, because the voltage changes during the discharge process, and this is not taken into account.

Accurate results can only be given in watt-hours, provided that many measurements are taken during the discharge process.