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.
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.
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 will last for 10 years, assuming perfect operating conditions. However, it’s performance will naturally reduce over time to a degree where it isn’t safe to carry on using it.
However, there is no such thing as a 'perfect' installation. It would be technically impractical and cost too much.
There are several factors that can influence battery service life, such as operational and storage temperatures, the frequency and depth of discharge, and the battery maintenance regime.
But even before such influences are taken into account, the operational capacity of a 10-year design life battery will drop below 100% capacity by year 6 and will continue to fall to around 80% over the next 4 years. You can see this deterioration process outlined in the image below.
When Should UPS Batteries Be Replaced?
Taking into account the EUROBAT recommendation that batteries should be swapped out before capacity falls below 80%, it’s clear why it is accepted industry best practice to replace 10-year design life batteries in year 7 or 8.
Doing so takes into account all the external factors that will impact service life whilst providing a safe enough margin for potential failure without compromising the load.
Similar performance drop-offs occur in 5-year design life batteries too. This is why it’s advisable to replace them in either year 3 or 4 of service life.