Monday, May 3, 2010

Replacing the Battery in your Uninterruptable Power Supply

I decided several years ago (after a brief power outage) that I had better things to worry about than if I had saved my open files recently enough when the power flickers.  So I picked up an APC Back-UPS 650VA power supply to keep my computer running through a power outage.


Fast-forward about four years, and I’m sitting at my PC working, when my UPS beeps and the screen goes black – exactly what I had been trying to avoid.


The most common cause of UPS problems is a failing battery.  The first step in diagnosing the problem is to open up the case to reveal the battery.  Usually the batteries are quite easy to get to – either the front of the box will pull off, revealing a couple of screws, or there will be a couple of obvious screws on the bottom of the unit.  For my Back-UPS 650, it’s the latter.


Pull the battery and disconnect the positive and negative battery leads.  These are normally just spade connectors on the smaller UPS’s, and no tools are required.  This is your first chance to inspect the physical condition of the battery.  Many of the batteries that I’ve had go bad actually produced enough heat internally to cause the sides to bulge.  If the battery has been deformed, it must be replaced.  Sometimes, the battery will actually start leaking, and you’ll see a white powder crusted around the chassis or battery.  This is also a dead-giveaway that your battery is shot.

A standard DC volt meter is often enough to diagnose a battery beyond simple physical symptoms.  Normally, a battery that is actively being charged will read about 13.8V.  Once you disconnect the charger, you should still see about 12.5V.  If you read anything less than about 12.0V, then you are missing a cell (or more) in the battery and it must be replaced.

Once in a while, a battery will hold its voltage when it’s sitting idle, but as soon as a load is placed on it, it will drop.  For a test load, I’d recommend something that will load it down with about 1 amp, such as a 10 ohm, 20W resistor.  Another option would be 12V car brake or dome light if you happen to have one handy.  Measure the voltage before you attach the load across the positive and negative lead, and again while you have the load attached.  It’s normal to see a drop of a couple tenths of a volt, but if the battery drops below 12.0V, it’s probably shot


While it’s possible for other parts of the UPS to fail or become damaged, 95% of the UPS’s I’ve worked on are because of an aged and/or abused battery.  The easiest way to find a replacement is to measure the physical dimensions and look for a replacement on a site such as  (I say that this is the easiest, because usually the original battery that comes from APC does not have any kind of Amp-Hour rating on it – they do that to discourage you from buying your own replacements…)

Search for “Sealed Lead Acid batteries”, and begin comparing the physical dimensions of the batteries.  Normally, you will be looking for a 12V battery, usually in the 7AH to 28AH range.  The physical dimensions are pretty well standardized across the industry, just be sure that it will fit inside the chassis of your UPS, and that the battery posts are compatible with the leads on the UPS.


Once you receive the new battery, installation is exactly opposite from the disassembly.  Often times there is a small spark when you attach the second lead to the battery.  Don’t be alarmed, this is normal.

Put the screws back in, and the UPS is again ready for operation.

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