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post #14 of Old 03-20-2013
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Re: AGM batteries

Originally Posted by Letrappes View Post
Thanks for all the replies. I do like the lack of maintenance with the AGM batteries but I'm uncertain of how much longer they will last. I would rather go to the flooded style in the future when they die. Is there any way I can determine the potential life left in these batteries?


Easiest way to get a reliable estimate of remaining capacity is to find someone with a sophisticated inductance/capacitance tester, like the Midtronics series. These are too costly to buy for your needs (about $600), but they'll tell you the remaining capacity pretty quickly.

Failing that, there's only one reliable way to test remaining capacity, and that's by doing a controlled C/20 load test. This means you fully charge the batteries, then attach a resistive load calculated to be approximately 1/20th the battery capacity, and let it go for upwards of 20 hours or until the voltage drops to 10.5VDC.

A group 27 AGM has between 90-100AH capacity. Say 100AH to make the calculation easy. So, if you hooked up one of these to a load you'd want a 100/20 = 5 amp load. Five amps @ 12 volts = 60 watts, so a 60 watt load would be appropriate to do a load test on one of your group 27 batteries.

As noted above, a halogen car headlamp might do the trick. These tend to be about 55 watts or 4.6 amps....a little less than the target 5 amp load. And, guess what? If your AGM battery were only about 92AH instead of 100AH -- as many are -- then the 4.6 amp load would be right on target :-)

Alternatively, you can probably find four 15-watt 12V bulbs at West Marine. Hook all 4 up in parallel and attach to the battery under test. Then, take periodic voltage readings over the next hours until you get to 10.5VDC.

If you only get 10 hours, that means the battery only has 1/2 capacity left.

If you only get 5 hours, that means the battery only has 1/4 capacity left.

When you're done testing each battery, be sure to fully charge it....don't let it sit discharged.

Good luck. Let us know what you find.


Last edited by btrayfors; 03-20-2013 at 05:22 PM.
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