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Batteries in series........

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emsvitil	Batteries in series........	Lead	[-]
Posts: 15 (03/10/09 22:00:43) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	Tags : ups, series 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.
NavyTrained		#1	[-]
Posts: 77 (03/14/09 21:50:37) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	What you're experiencing with the two batteries is very common. They're both in need of extended desulfation to restore lost capacity and equalization. Do you have any idea how old the batteries are? If they're several years old it may be necessary to expose the vent/filler caps in order to inject some distilled water into them carefully with a syringe to replenish water loss. The battery charger in the UPS isn't capable of 'equalization' so it must be done by the addition of a desulfator circuit across the batteries which will bring them both up to a fully restored and fully charged condition. A low power desulfator made to operate from 24 Volts will do the job and may be left connected across the batteries for prolonged periods of time (months or years.) Adding the small load that you propose will not solve the equalization problem and it will, in time, contribute to further sulfation in the batteries and further loss of capacity. You've got a situation which is of great interest to the group. Please keep us informed of your progress in dealing with it!
emsvitil		#2	[-]
Posts: 17 (03/15/09 18:54:56) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	6 batteries total 2 pairs at 3.5 years 1 pair 5 years 2 good batteries (1 3.5 years 1 5 years) 2 so-so batteries (both 3.5 years) 1 not so good (5 years) 1 probably shot (3.5 years) The 2 good are in comparison to everything else, don't know how they'd compare to new batteries..... The equalization circuit has gotten the batteries to be 13.7 Volts each. So each battery is equal, but who knows about the cells. It's 1 led per battery that draws about 15ma per led. So it's really not a load on the battery, but an alternate path that shunts current around the higher voltage battery. I've noticed that the leds will slowly alternate, and then end up both on when the batteries are fully charged (or as charged as they're going to get at 13.7 volts).

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