Measurement of battery discharge capacity

The discharge method is generally used in battery testing. The test interval is usually once a year to check the capacity of the battery pack. General steps and precautions for discharge testing mainly include:

1. Configure the dummy load and set the appropriate discharge rate

Practical difficulties: Dummy loads are bulky and not available at every site. For large capacity battery packs, the power requirements of the dummy load are higher

Workaround: Use real load discharge (some companies will also use real load for checking discharge in the first 3-5 years of battery use, releasing 30%-40% of the nominal capacity)

Risk: the real load is not controllable, can not achieve constant current discharge; The power is also relatively small, can not achieve enough discharge rate

2. Disconnect the charger

Practical difficulties: The load is actually in an unprotected or inadequately protected state

Flexible approach: shorten the discharge time, or in a special maintenance window

Risk: If the battery capacity is seriously degraded, it is likely to power off at the moment of disconnecting the charger; If the discharge time is shortened within a limited maintenance window, a complete discharge test cannot be completed, and the actual capacity of each battery cannot be calculated

3. Monitor whether the discharge current is constant

Practical difficulties: To shorten the discharge time, it is necessary to increase the current, usually up to several hundred amps

Risk: need to choose a special cable, otherwise it is likely to burn; The heat dissipation of the dummy load must be ensured, otherwise accidents may occur

4. Periodically monitor the voltage of each battery. Once the voltage of a battery drops to the cut-off voltage, the discharge is suspended. Calculate the capacity of the battery

Practical difficulty: not knowing when the cut-off voltage will be reached

Flexible approach: frequent test voltage; Shorten the discharge time to avoid the cut-off voltage; At the end of the discharge, the voltage of each battery is compared horizontally, and the lower voltage is the backward battery

Risk: Once a battery quickly reaches the cut-off voltage without being found, it is likely to cause overdischarge and even combustion; If the cut-off voltage is not included, the capacity cannot be accurately calculated; The transverse comparison voltage can only be qualitative judgment

5. Remove the battery from the battery pack, continue discharging, and Step 4 until all batteries reach the cut-off voltage, calculate the capacity of each battery

Risk: Stop discharging when a battery reaches the cut-off voltage, at best accurately calculate the capacity of the worst battery

6. Perform a discharge test at the next maintenance period

Practical difficulties: The most frequent test cycle is once a year

Risks: The health status of the battery is unknown during the year-long testing interval; Too frequent discharge tests will consume the cycle life of the battery (generally 200-500 times).