How to test which battery is bad (internal or external?) on a Smart-Ups XL model
All legacy (SU, SUA, SUM, SURT) Smart-UPS models with an "XL" suffix, All Serial Numbers
When a legacy Smart-UPS indicates "replace battery", it is referring to the full battery string. The UPS does not have the ability to test specific modules or external battery frames individually. When the UPS tests the batteries, it is looking for the rate that voltage drops over time for a given load. There is not a way to easily replicate this using a voltmeter. You should defer to the results of the UPS self test with a fully charged battery and the normal load attached. The self test will run once every two weeks by default.
Furthermore, it is important to never mix mix old and new battery cartridges within the same Smart-UPS system. When batteries are replaced, you should replace all the battery modules (internal and external) at the same time. In addition, if you add more external batteries to an existing unit, you should replace the existing batteries at the same time if they are more than 1 year old. For more information on this, refer to FAQ #FA317828 "Can I mix old and new batteries in a Smart-UPS?"
Batteries typically last 3-5 years in an appropriately cooled (68°F-77°F / 20°C-25°C) environment.