A battery is a reusable current source that operates due to reversible internal chemical processes. Rechargeable batteries are used for autonomous power supply of various equipment. When choosing a battery for a car, other equipment or electronics, first of all, pay attention to the battery capacity - the key parameter of the device. It is important not to confuse it with charge or charge.
What is battery capacity and how is it measured
Expressed in ampere-hours (Ah), the capacity of a battery indicates the amount of time it can supply autonomous energy to equipment connected to it within a single charge. For small batteries that power electronics, a different unit is used to display capacity - mAh (milliamp hour). In other words, battery capacity is the maximum amount of energy that it can store in one full charge cycle.
The capacity of a battery is what its potential is measured in, but not the charge. You can make a comparison with a water bottle - regardless of whether it is filled with liquid or not, its volume does not change.In this case, the capacity is correct to compare with the volume: it does not change regardless of whether the battery is fully charged or discharged. This figure is in most cases indicated on the battery, for example, on a car battery sticker it is written next to the starting current.
Example: A 60Ah car battery says it can run for an hour with a load of 60Amps and a nominal voltage of 12.7V (the classic voltage for most car batteries).
If you need a source of autonomous energy for some equipment, then how to accurately calculate the capacity of a battery suitable for this task yourself? To do this, you need to know a few variables:
- critical load, measured in Watts (designation - P);
- the time that the battery must power the electrical equipment (t);
- voltage of each battery (V, measured in Volts)
- battery capacity utilization ratio: 1 - 100% usage, 0.5 - 50% usage, etc. (symbol - k).
The letter Q indicates the required capacity. To calculate it, use the formula:
Q = (P t) / V k
Intuitive: Using a standard 12V battery, required 5 hours of operation, 500W critical load, and maximum battery discharge of up to 80%
Q \u003d (500 5) / (12 0.8) \u003d 260.4 Ah
This is the minimum battery capacity for the task, as well as the total capacity of 12-volt batteries. But, in any case, it is better to purchase an energy source with a small margin of capacity, for example, 20% more. Then it will be less likely to be discharged to zero and the battery will “live” longer.
We figured out what the battery capacity is and how to calculate it, but sometimes, the inscription on it may lie or be missing. Or you need to compare passport data and the real picture. How to measure the battery capacity in this case? Ideally, this would require a test discharge procedure. The bottom line is simple: you need to replenish the battery charge by 100% using a charger with suitable characteristics, and then completely discharge it with direct current, measuring the time spent on discharging. Next, the formula is used:
Q = I T
where I is the continuous discharge current measured in Amps and T is the discharge time in hours. For example, measuring the capacity of a fully charged battery that has been running for 22 and a half hours with a constant current of 3.6 A gives a figure of 81 Ah. But this does not mean that the same battery will last more than 2 hours with a current of 36 A: an increase in current leads to a decrease in the discharge time. It is also influenced by other factors, such as the temperature of the electrolyte.
It is important to note that at the end of the discharge cycle, the minimum voltage at the terminals should not be lower than the final discharge voltage (often 10.8 volts). This minimum allowable value is set by the manufacturer - when it is reached, the battery must be disconnected. If you frequently discharge the battery below this value, it may fail.
Over time, the capacity of the battery in any case falls due to inevitable degradation. If after the measurement it turned out that the capacity is less than the nominal by 70-80%, then it is time to change the battery.
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