Voltage set points on modified APC UPS units

I have noticed now that I have modified about 8 of these units that when they are attached to a battery, they float at different voltages depending on whether the main power switch is on or off.

Typically, the powered ON voltage is adjusted for about 13.8V (a good voltage to float most VRLA batteries). However, the powered OFF voltage jumps by up to 0.5V. This is a little high to float at for lengthy periods, but shouldn't hurt anything if left for a day or so. In fact, it may be helpful to let a battery float for a little while at this voltage to help equalize the individual cells. A fully charged 18Ah battery that had been floating at 13.8V drew only 60mA or so for a couple of minutes before dropping to less than 10mA when it reached 14.3V.

Float voltage adjustment

It's also interesting that the charge rates are different depending on the power switch. With the power switch ON, these units will charge a nearly dead battery at about 350mA. With the power switch OFF it charges at around 400mA. While this isn't exactly a fast charge for an 18Ah battery, it is plenty to maintain a bank of batteries in an always ready state. If you intend to charge batteries with one of these units, I would only count on about 5Ah a day.