How is battery capacity measured?
How is battery capacity measured?
Batteries are measured in milliampere-hours (mAh) or ampere-hours (Ah) and indicate how much electrical charge a cell is able to give off at a given load before it discharges.
What does the unit mAh mean?
Capacity, expressed in mAh (milliampere-hours), tells you how long a battery can power a device drawing a specific current. For example, a battery with a capacity of 2000 mAh theoretically can power a consumer drawing 2000 mA (i.e. 2 A) for one hour - or 200 mA for 10 hours. This is a theoretical value, which in reality depends on many factors, including temperature, cell type and even mounting position.
How does the measurement work?
Capacity is measured by discharging the battery with a controlled current until the final voltage is reached, which is the threshold below which the cell is considered discharged. The total amount of energy discharged during this time allows the capacity to be calculated. In practice, electronic discharge testers or specialised meters are used which count the value of the total current (charge) flow over time. The measurement method must take into account the specific discharge current, as most Batteries have different capacities depending on the load. A lithium cell, for example, may have a nominal capacity of 3,000 mAh, but will give back less at a high current draw than at a slow discharge.
Note the test conditions
Batteries are quoted by manufacturers as having capacity under optimal conditions, i.e. moderate temperature, constant load and slow discharge. Under actual conditions, e.g. low temperature or pulsed power consumption, Batteries may show a lower usable capacity, even if the catalogue value is high. Therefore, when comparing battery capacity, it is worth considering not only the numbers, but also the testing method and the requirements of the specific device.
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