A load/discharge test is the only definitive way to assess a battery’s capacity. When used regularly, discharge testing can be used for tracking the battery’s health and actual capacity and estimating the remaining life of the battery. During the test, we measure how much capacity the battery can deliver (i.e., current multiplied by time, given in Amp-hours (Ah)) before its terminal voltage drops to a level signifying that the discharge is complete. This terminal end voltage equals the battery cells’ end of discharge voltage x the number of cells. For example, suppose the end cell voltage is 1.75 V, and the battery has 60 cells. In that case, the test stops when the terminal voltage reaches 60 x 1.75 V = 105 V. Throughout the test, the current is maintained at a constant value. If the battery reaches the end of discharge voltage at the same time as the specified test time ends, then the battery’s measured capacity is 100 % of the rated capacity. Conversely, if the battery reaches the end of discharge before 80 % of the specified test time has passed (e.g., within less than 8 hours of a 10-hour specified time), you should replace the battery.
At least a couple of times during the discharge test, you must measure the individual cell voltages. The most critical time for these voltage measurements is at the end of the discharge test to find the weak cells. It is also imperative that the time, or the current, during a discharge test is adjusted for the battery’s temperature. A cold battery will give less Ah than a warm one. Temperature correction factors and methods are described in the IEEE standards.
Batteries can also be tested in a shorter time than their duty cycle, for instance, in 1 hour. If you elect a reduced test time, the current rate has to be increased. The advantage of this approach is that a battery’s remaining capacity is greater at the test’s conclusion than that for a full-length test. A battery with less capacity is inconvenient and possibly very expensive to rectify in time, resources, and money.