AGM batteries, charger on or off?

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Oct 5, 2007
Vessel Name
Anastasia III
Vessel Make
Krogen 42
My bow thruster is powered by AGM batteries. The only charging source is a three stage smart charger, no charging off the engines. Since I only use them occasionally, I normally don't leave the charger on. Just turn it on the day before I'm going to use them to top them off, then recharge them afterwards. There are no other loads on them.

The batteries are showing "weak" on a load tester (toaster as Arild calls it) after about 6 years. I've put my desulphator on them and expect it'll bring them back over time, but I'm wondering if it would be better to leave the charger on all the time?

The last time I used them, I charged them up then turned the charger off. After about a month they were reading about 12.3 volts. Overnight charging brought them up to 13.3 or so. They dropped to about 10v on the load tester.
Hi Keith,

check out the cruisers forum, search agm's and read Rick's(Bremerton) posts on breathing new life into dead batteries.

If you contact him directly, he will probably walk you through some of his black magic.

good luck
Keith, is that charger charging any other batteries?...or just the thruster AGM? My first inclination is to always leave the charger on. My second inclination is that the battery is 6 years old and I think you have got your money's worth
Nope, just those. However, my house and start banks are 7 years old and going strong, probably because I've had the desulphator on them all the time. Didn't do that with the forward battery bank, but it's on there now. After two weeks, still showing low, so I'll go another two and check them again.
What the sellers and manufacturers don't tell you about AGM batteries is that after ANY discharge the batteries MUST be recharged to 100%. If you can't or don't do that the battery life is reduced considerably. That may be why your house banks are fine but these batteries are not. It may be time for replacement and if the charger is operating properly it should be left connected. Chuck
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