Lead-acid batteries are commonly used as a source of backup power in uninterruptible power supply (UPS) and similar standby applications, where they simply sit around with a maintenance charge applied for the vast bulk of their service lives, awaiting an instant when they are called upon to deliver power.
These batteries do not have an infinite life and degrade in their ability to deliver power over time, so it’s important to replace or at least test them periodically; there’s little point to bothering with a battery backup supply if it’s got old batteries that will cough, wheeze, and give up at the very moment when they’re actually needed. How often should they be tested or replaced? That depends, among other things, on temperature and how much loss of capacity can be tolerated.
Better manufacturers of batteries designed for standby applications will provide a service life guesstimate, that should at least come with a temperature condition and (hopefully) a declaration of the end-of-life condition on which the figure is based.
If not indicated, it’s reasonable to guesstimate that end-of-life is regarded as a 50% reduction in original capacity and/or doubling of internal impedance. In terms of UPS applications, this loosely translates to a halving both of maximum output power and low-load run time. In other words, if you’re depending on a UPS to deliver a significant portion of its rated power for a significant portion of its advertised hold-up time, you’ll probably want to replace the batteries before their estimated lifetime has elapsed. On the other hand, if you only need to help a non-critical system ride through the loss of a few AC cycles during routine transfers between utility and generator power, you might be able to stretch things a bit.
Unfortunately, temperature also plays a role in the rate at which batteries degrade; as a first approximation, the battery manufacturer’s estimated lifetime figure should be halved for every 10°C (18°F) increase in battery temperature over the value at which the lifetime estimate was given. This is very relevant for small UPS applications, which often locate their batteries in an enclosure with a bunch of electronics that give off heat. Situations differ, but it’s likely that a fair number of UPS batteries live in an environment that’s closer to 35°C (95°F), where a battery said to hold up for 3-5 years @ 25°C (77°F) can only be expected to last 18-30 months.
One final factor worth considering is the amount of usage a battery is being subjected to. Repeated charge/discharge cycles result in a loss of capacity, to an extent that becomes more profound as the depth of discharge on each cycle is increased. The chart below excerpted from this resource suggests that fully discharging the referenced product series can shorten battery cycle life by a factor of roughly 6.
In summary, a 3-year replacement interval is suggested for lead-acid batteries used in standby/backup applications when no other information about the application is available. System reliability and/or maintenance costs will be significantly improved however, by taking the time to evaluate the actual conditions of each application (noting battery temperature at a minimum) and adjusting this figure accordingly. For devices that use multiple cells/batteries, this post contains info relevant to maximizing the service life of the replacements.