Jerry G. wrote:

Your UPS uses gel type batteries. DO NOT use liquid acid batteries, such
as car batteries for your UPS. The charger for the UPS was designed for
a particular type of battery with a specific charge current range. The
charging circuits sense the battery load during charge.

With a liquid acid battery, there is a chance of it being over charged,
and thus can cause boiling!

In my experience, the charge current limit for the native batteries is
far lower than the larger automotive or marine deep cycle types folks
sometimes use as replacements and undercharging is the problem.
The VRLA charge profile from the UPS is usually 0.1C to 0.3C during
charge below Vmax followed by Vmax constant as trickle charge. The charge
controller may shut down when it sees that the battery is not following
the VRLA profile if using wet cells also.

Regarding paralleling of batteries, it may be wise to isolate each battery
(group of cells) with diodes (of appropriate current capacity) when making parallel
connections between batteries of varying age and capacities to prevent internal
current loops. Heat sink the diodes if dissipation ratings require.

Using marine deep-cycle batteries with an external charger is sometimes a
good strategy for a UPS, especially if the UPS is robust enough for extended
runtimes such a strategy would permit.

Regards,

Michael