What does lead acid battery temperature indicate?

UPS systems and generator start motors use standby lead acid batteries which degrade over time. Monitoring their temperature when charging or discharging can be used to indicate their overall health and ability to provide backup power. As lead acid batteries age, their internal resistance increase, leading to heat rise on discharge and a faster rate of discharge which can impact service life. Monitoring individual batteries and comparing readings taken over a time period can help to identify early failures which could lower the overall performance of the battery set.