Charging AGM Batteries

Pooh BearJune 18, 2008

I have a couple of big AGM batteries sitting here

next to my computer using them for backup power.

I have heard that AGM batteries have to be charged very

slowly. At a rate around one amp. I have heard that charging

them at a higher rate will kill them sooner. Is this true.

I bought a battery charger/maintainer to keep them charged.

I charge them seperately, never together.

My computer is on a UPS and when the power goes out I have

time to unplug the UPS and plug it into an inverter that

I have hooked up to one of the batteries.

Most of the time the batteries just site here un-used.

They were practically new when pulled from service.

The batteries came out of a power wheelchair.

And if I am reading the label correctly they are each 12V 73AH.

So, do they have to be charged slowly, and will the charger

I bought do the job or do I need something different.


Pooh Bear

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I don't know anything about your batteries; Maybe you could google and find their specifications.

I would guess, since these batteries were for wheelchair service, those would be similar to deep discharge, marine duty batteries for trolling motors.

For comparison purposes, a 1.5 amp charge rate is about right for a 12 v motorcycle battery having 14 amp-hour capactiy with a taper-back in the charge rate as the battery approaches full charge voltage.

    Bookmark   June 18, 2008 at 4:01PM
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Pooh Bear

I'm talking about Absorbed Glass Mat (AGM) batteries.
They are used in a variety of different applications.
Some of the high end automotive batteries are AGM types.
The batteries I have look very much like marine batteries.

Pooh Bear

    Bookmark   June 18, 2008 at 5:32PM
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Don't understand why you are doing what you do. My computers and my entire T.V. , dish, sound system, and V.C.R. are on my U.P.S. and only my U.P.S. I have 12 volts at 28 amp.hours connected in series. My U.P.S. is 24 volts.
I can run my system for 4 hours without hydro. I dought very much you have a 73 amp battery. That thing would weigh
about 65 lbs. if not more and be the size of car battery and a half. My 28 amp batteries weigh 25 lbs. Those are gell cells and use a special charger and made to be on charge more than they are off charge. You probably have 37 amp batteries. Why are you not just using your U.P.S. ? Connect your A.G.M. batteries to it. The U.P.S. has it's own proper built in charger that will go into float when the batteries are full.

    Bookmark   June 18, 2008 at 5:34PM
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Pooh Bear

My UPS only gives me a few minutes before it shuts down.
Just enough time to unplug it from the wall and plug it
into a power inverter. The wheelchair batteries do weigh
about 60 pounds each. These batteries are 8.5T x 6.5W x 10L.

I bought a UPS because we have a lot of power "blinks" here.
And being a rural area, when the power goes out completely,
it stays out for quite some time. 12 hours is not uncommon.
If I'm sitting here trying to work I just can't have that.
I know there are better ways of going about this, (like a whole
house backup generator) but this is what is in the budget for now.

Oh, and I do have a smaller AGM battery out of a 4 wheeler
that I am going to wire into my UPS for more run time.

Pooh Bear

    Bookmark   June 18, 2008 at 10:22PM
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I was curious about this because I have an old trailer I want to outfit with a battery pack like this for "boondock" camping, so I looked around online and probably ran across a lot of the same stuff you already know. The consensus seems to be that you want a rapid charge to about 80 percent capacity and then a slow charge to 100 percent. AGM batteries are said to be more resilient to different types of charging than most. I can't see any reason why the charger you have would not be fine.

    Bookmark   June 18, 2008 at 10:40PM
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The first thing i have to know is what is the K.V.A. of your U.P.S. You can run most of your stuff with 1000 K.V.A.
with externally mounted batteries for hours. You can't do
much with 500 K.V.A. I have my system setup that way. I took out my 18 amp batteries because they were dead. I externally mounted my 28 amp batteries because they were too big to fit in the case. As i said before i can get up to 4 hours on the U.P.S. alone. What you can do is find yourself an old car alternator with a little gas motor.
Use the car alternator to keep your U.P.S. batteries up. You can go forever or until you run out of gas. You have to do a little wiring because most larger U.P.S.'s are 24
volts on the battery side. Works just fine. I loose hydro
sometimes for 2 days. Always between 2 to 24 hours in the
summer storms. An active U.P.S. also cleans up hydro spikes
and brown outs. Give it a try and impress your friends with
your wiring project. Oh yah. Depends on the age and size of
the U.P.S. Some you had to use your computer to get into the rom chip and flash it telling it it has larger batteries or it will shut down on the old information of the smaller battery. I had to do that on my commercial U.P.S. when i put 1000 amp batteries in it or it would have
shut down in 17 minuets under full load with the old info.
It still thought it had 28 amp batteries in it.

    Bookmark   June 19, 2008 at 11:31AM
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