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Basics You Should Know About Battery Capacity
What is battery capacity? Battery capacity refers to the energy extracted from the battery under certain condition. For instance, 2800Ah means if the battery is fully charged, it can provide a constant current of 100Amps for 28 hours before it is completely discharged, but these are ideal theoretical values.

The actual available capacity varies with many factors such as discharge current, the history of the battery, temperature etc. Think twice before purchasing a new battery because capacity ratings are overstated by some manufacturers. They tend to claim a much higher capacity, which lasts much less than it actually runs.

Besides, battery ages after it goes through a couple of charge cycles (a cycle means to fully charge a battery then discharge it to terminal voltage) and the capacity decreases gradually as well. Therefore, capacity can also be an indicator of battery status.

How to measure battery capacity? As pouring out the water to know bottle capacity, you can measure battery capacity by discharge the energy, which is the most commonly used method. Firstly, take a fully charged battery and discharge it with a constant current. Do remember to check the time during the process. Stop the test when the battery reaches its cut-off voltage (the cut-off voltage varies on batteries).

For example, you set the discharge current 0.5A and the test runs for 8.6 hours in total, the capacity should be 0.5 * 8.6 = 43Ah. But as I stated above, battery capacity depends on many variations. Like if you use a lower current, you will get slightly higher capacity. And sometimes in a warmer environment, the capacity may be higher. We need to take everything into consideration to come to an objective conclusion.