When I got my last set of batteries (150aH AGM, used), they had been tested on a high-amp car tester. But my friend who knows batteries pretty well said that was not a proper test, that what we should do was put a 25 amp load on, and see how long the battery took to get down to 11v or so. This method caused us to reject several batteries that had tested fine on the high amp tester. And the batteries I got have been good.This same friend, though, told me it was important to cycle batteries in long term storage (not just float them). Doing that, with the 25 amp load, one got away from me, suddenly dropped to 10.5v or so, and that killed it.I have also killed numerous 33aH batteries by putting a 100amp brief load on them, before I figured out that was not cool.So. How should I test these used AGMs I am looking at now? Should I rely on the sellers 150aH 15-second test? Should I do a smaller load for longer? What do you all do?

You need a known resistive load like ganged resistors, i.e. one that has been measured. For a 150AH battery, a 20 A load would be good. The load is connected and with timed duration. The battery is then measured for voltage, which indicates SOC. I don't know how you'll be able to do this at a battery reseller's shop. They are probably not going to allow it. Your choice would then be to buy one, test it and then purchase more. I think most purchasers take the word of the seller but there would be nothing wrong with checking voltage before purchase. A higher DC voltage would indicate that the batteries have been treated well.FWIW, I am using three small AGM telecom back-up batteries that were pulled for scheduled replacement. They work just as advertized.

According to our Interstate Battery rep, regular battery testers will not test AGM batteries. Interstate, at their warehouse has a $1500 tester they use. We opted to not spend $1500 for a tester we would rarely use. We have $500 Snap On resistance type tester with print outs and it will not work on AGM batteries. Doug

Sounds like good advice from your friend. The car battery tester is checking for cranking amps, which is all a car cares about. As RV'ers we care about the so-called "20 hour rate", providing a smaller current over a longer period.Not to discount surge capacity to much though, especially if you plan to use an inverter. It might need to pull 1500 watts or more from the batteries briefly, and that's closer to cranking amps than the 20 hour rate.

In all honesty, I don't think too many of us buy used batteries. At least I've never seen anyone mention "new to me" batteries.

GordonThree wrote:Sounds like good advice from your friend. WHAT ?? Most of his advice is dead wrong.....like the part about having to discharge them down to 11 volts for a test. The OP has already seen what that can do. And the part about cycling being better than a proper float during storage is just total bunk.A "standard" load test will tell you about the basic health of an AGM battery......as measured by the same spec's as a conventional wet cell. Odds are extremely high that if it passes that test there is little or nothing wrong with it that would interfere with normal use.Being obsessive about things like this often leads you to buy into obscure theories that often have no practical use.....even if they are technically "right".P.S. Some conventional wet cells can pass a high current load test when they are on the verge of total failure too. If the symptoms make you think they are about at the end of their useful life, then the odds are extremely high that they really ARE.

Testing AGM batteries? | Good Sam Community

37 Replies

MEXICOWANDERER
Explorer
Jun 03, 2016
Naio, download and read this PDF manual. You can trust the information.

https://www.tayna.co.uk/docs/datasheets/lifeline/lifeline-manual.pdf
GordonThree
Explorer
Jun 03, 2016
The cut-off voltage is going to be different depending on battery construction... Deka recommends 11.5 as a minimum for the bats I'm using, I set at 11.8 to be safe. I don't agree about the standard load test tells you anything other than the battery could start a car engine.
Sam_Spade
Explorer
Jun 03, 2016
GordonThree wrote:
Sounds like good advice from your friend.

WHAT ?? Most of his advice is dead wrong.....like the part about having to discharge them down to 11 volts for a test. The OP has already seen what that can do. And the part about cycling being better than a proper float during storage is just total bunk.

A "standard" load test will tell you about the basic health of an AGM battery......as measured by the same spec's as a conventional wet cell. Odds are extremely high that if it passes that test there is little or nothing wrong with it that would interfere with normal use.

Being obsessive about things like this often leads you to buy into obscure theories that often have no practical use.....even if they are technically "right".

P.S. Some conventional wet cells can pass a high current load test when they are on the verge of total failure too.
If the symptoms make you think they are about at the end of their useful life, then the odds are extremely high that they really ARE.
2oldman
Explorer II
Jun 03, 2016
In all honesty, I don't think too many of us buy used batteries. At least I've never seen anyone mention "new to me" batteries.
GordonThree
Explorer
Jun 03, 2016
Sounds like good advice from your friend. The car battery tester is checking for cranking amps, which is all a car cares about. As RV'ers we care about the so-called "20 hour rate", providing a smaller current over a longer period.

Not to discount surge capacity to much though, especially if you plan to use an inverter. It might need to pull 1500 watts or more from the batteries briefly, and that's closer to cranking amps than the 20 hour rate.
dougrainer
Nomad
Jun 03, 2016
According to our Interstate Battery rep, regular battery testers will not test AGM batteries. Interstate, at their warehouse has a $1500 tester they use. We opted to not spend $1500 for a tester we would rarely use. We have $500 Snap On resistance type tester with print outs and it will not work on AGM batteries. Doug
westend
Explorer
Jun 03, 2016
You need a known resistive load like ganged resistors, i.e. one that has been measured. For a 150AH battery, a 20 A load would be good. The load is connected and with timed duration. The battery is then measured for voltage, which indicates SOC.

I don't know how you'll be able to do this at a battery reseller's shop. They are probably not going to allow it. Your choice would then be to buy one, test it and then purchase more. I think most purchasers take the word of the seller but there would be nothing wrong with checking voltage before purchase. A higher DC voltage would indicate that the batteries have been treated well.

FWIW, I am using three small AGM telecom back-up batteries that were pulled for scheduled replacement. They work just as advertized.

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Testing AGM batteries?

37 Replies

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