Most UPS systems are designed to recharge their batteries to 80% within a 24 hour period. This is achieved through the UPS having an internal charging system. External battery packs may also be fitted with their own individual AC powered battery charger. From 80% the batteries are trickled charged as the typical charging curve is non-linear. The simplest way as a rule of thumb to calculate the recharge time is to take the Ampere-hour (Ah) of the battery set used and divide by the charging current:
Recharge time = Ampere-hour / recharge current