How do manufacturers test battery life?

Lead acid batteries are life tested by various methods dependent on their application. If the battery is designed to deliver low rate capacity like a trolling motor or golf car, the battery is subjected to continuous cycles that are counted until the capacity falls below an acceptable level. This is compared to rating.

There are many types of tests for the life testing of automotive batteries. It would be too expensive to continuously test starting batteries at hundreds of amps, so most tests perform short discharges and charges at a current that is typical of the background discharge level of a vehicle followed by periodic discharges at high rates until a failure voltage is observed. Generally the tests are accelerated by increasing the temperature and charging voltage, but they can still take a long time. The amount of life cycles the battery achieves is compared to a rating for the battery.

There are also accelerated life tests for standby batteries that generally put the batteries on a higher float voltage charge at a high temperature until the battery fails to deliver satisfactory capacity determined by periodic discharges.

Accelerated life tests rarely mimic actual usage, and the failure modes during life testing often do not match that of actual field use. The correlation to actual field tests is usually low. Often the life of a battery is deteriorated during stand periods when not in use rather than actual charging and discharging operations that life tests focus on.