T i m wrote:

We have several (mainly APC) UPS's here and my normal test to their
efficiency (outside plugging them in my power meter socket thing) is
how warm they sit when idle. Most of them are 'just warm' but I would
be interested to learn if there was a specific procedure to lower the
risk of overcharging the battery

Probably only applies to Smart-UPS rather than Back-UPS, but have a look

http://www.jjoseph.org/notes/apc_smartups_battery_float_voltage