Depending on the application, the batteries in an uninterruptible power supply can consist of a single block or thousands of cells. But without a well-maintained, properly functioning battery system that performs when required, a UPS is practically worthless.
Design life is how long a UPS battery will last for assuming perfect operating conditions, while service life is the length of time before the battery needs replacing. No installation can ever be perfect though. It would be technically impractical and cost too much.
All UPS batteries have a stipulated design life. Standard design life tends to be 5 years, although 10, 15, and even 20-year design life batteries are available too. Even though some batteries have a ’10-year design life’, that doesn’t mean they’ll work effectively for that long.
Under internationally-recognised EUROBAT (Association of European Automotive and Industrial Battery Manufacturers) guidelines, a UPS battery is considered at the end of its service life when capacity falls 80% below the original value.
So in effect, a 10-year design life battery should last for 10 years, assuming perfect operating conditions. However, it’s performance will reduce over time to a degree where it isn’t safe to carry on using it.
Several factors can influence battery service life, such as operational and storage temperatures, the frequency and depth of discharge, and the battery maintenance regime.
Before such influences are even taken into account, the operational capacity of a 10-year design life battery will drop below 100% capacity by year 5-6 and will continue to fall to 80% over the next 4-5 years.
This helps explain why it’s become accepted best practice to replace 10-year design life batteries in years 7 or 8.
This takes into account all the external factors that can reduce battery life and provides a safe enough margin for potential failure, without compromising the risk to the protected load. Comparable drop-offs in performance over time are also seen in 5-year design life batteries, which are typically replaced in either year 3 or 4.