Can I extend the runtime of a UPS with external batteries?

Expansion is done with additional batteries in a separate enclosure, connected through the connector the manufacturer provides. The voltage of the external battery string must match the UPS battery voltage, and the batteries should ideally be of the same type and age as the internal ones.

Matching the voltage is not enough, though. The built-in charger of many UPS units is small and sized only for the internal batteries. If a large external capacity is added, recharging after an outage can take a very long time, and a second outage will find the batteries half-empty. That is why UPS units intended for long backup times usually have a stronger charger or the option of an additional charger.

In most UPS units, you also need to enter the number of external batteries in the settings so that the remaining runtime is calculated correctly. Backup time does not always increase in proportion to capacity, because it also depends on the load, so ask for a calculation based on your actual load.

A common mistake is adding new batteries alongside old ones. The most worn battery determines the performance of the whole string.