Ran into something interesting when fixing 2 UPS power supplies.
Both had the same problem: Bad Battery indicator light on.
They both use 2 12v hobby batteries (7ah) and both had the same problem: 1 good battery and 1 bad battery. (I had previously changed the batteries, so each UPS should have had a matched set.)
Now the interesting find. I put the 2 good batteries in 1 UPS. The UPS charges them up and I get 27.42 volts (<10ma) across the 2 batteries or 13.71 each.
Except it's not 13.71 volts each........ 1 battery is 14.17, the other is 13.25. So you'd think of the 2 good batteries, 1 is better than the other.
But, I can make the voltages flip-flop. On the higher voltage battery I connect a test light for a couple minutes to drain it, then let the UPS charge things up again. And the UPS settles down to 27.42 volts, with 14.17 and 13.25 (give or take a few .01) but with the high and low voltage batteries reversed.
So since no batteries are ever exactly alike, it seems that eventually the UPS will kill a battery because of low voltage.
I'm developing a little circuit to add a small load to the battery with the higher voltage to keep them in sync.
Both had the same problem: Bad Battery indicator light on.
They both use 2 12v hobby batteries (7ah) and both had the same problem: 1 good battery and 1 bad battery. (I had previously changed the batteries, so each UPS should have had a matched set.)
Now the interesting find. I put the 2 good batteries in 1 UPS. The UPS charges them up and I get 27.42 volts (<10ma) across the 2 batteries or 13.71 each.
Except it's not 13.71 volts each........ 1 battery is 14.17, the other is 13.25. So you'd think of the 2 good batteries, 1 is better than the other.
But, I can make the voltages flip-flop. On the higher voltage battery I connect a test light for a couple minutes to drain it, then let the UPS charge things up again. And the UPS settles down to 27.42 volts, with 14.17 and 13.25 (give or take a few .01) but with the high and low voltage batteries reversed.
So since no batteries are ever exactly alike, it seems that eventually the UPS will kill a battery because of low voltage.
I'm developing a little circuit to add a small load to the battery with the higher voltage to keep them in sync.

