Shared Knowledge
"APC RS900 - Testing/Charging Individual Batteries of Battery Pack"
Summary: there are (2) separate 12 volt direct current (VDC) sealed batteries in a RS900 battery pack (RBC32). If you have a bad battery pack, it is possible that only 1 of the (2) batteries needs replacing or perhaps both batteries only need a full battery charge. It is certainly do-it-yourself (DIY) to test and charge these batteries.
Background:
The battery pack used in the APC RS900 (RBC32) consists of (2) readily available 12 VDC, 7200 milliamp hour (Mah) batteries.
If you are getting a "Replace Battery" status indicator light, it is possible that only 1 of the 2 batteries in the battery pack is bad and needs to be replaced.
Note: in addition to the RS900 I also have 2 other different model APC battery backup units and I have found all to be "touchy" when it comes to batteries. Seems like they will "fail" a battery that tests fine and will charge!??
A new complete battery pack from APC is $80+.
A new single 12VDC battery, 7200 milliamp hour (Mah), like used in the battery pack, can be had for $20 or so or both batteries off eBay for $45 or so. Home Depot also stocks the battery used in the APC battery pack but is more expensive at $36+.
There is nothing magical about an APC battery pack. What I mean by this is there is no embedded chip in the pack that tells the RS900 you are using an official APC battery pack.
Unfortunately, I have found that the power-up self test of the RS900 does not 100% confirm the battery pack is good.
Unfortunately, batteries of the size used in the APC battery pack can read 12VDC on a volt meter but not be able to supply any current.
All above means that no matter where you buy a battery pack or individual batteries, you must test and recharge them before installing in the RS900.
Testing:
Batteries have a finite life and often fail for no obvious reason.
If you buy a battery pack directly from APC or elsewhere or you buy individual batteries and assemble a battery pack yourself, you really must test each battery before using it to make sure it will supply current and that it will recharge 100%.
To test, you need some sort of 12VDC device such as a tire inflator. Something that will draw significant current from the battery.
Note here that I did try a 100amp battery tester from a local auto parts supply company but either tester was bad or it just would not work on smaller 12VDC batteries.
If you buy a preassembled battery pack, cut the tape holding the 2 batteries together and then remove each battery for testing.
Once you are sure the battery will power whatever load you place on it for several minutes, then you must recharge using a car battery charger.
Note here, that simply using a voltmeter to test a battery pack or even individual batteries will prove nothing. Bad batteries can show 12VDC and yet have no capacity to provide current to any load.