Testing AGM Batteries

I have 2 banks of AGM batteries in my power catamaran. Each bank is separately located in each pontoon and has 2 AGM batteries. Port side is having a hard time starting the port diesel engine. If I use the combiner switch, the port engine seems to start easier.

Question: How do you know when an AGM battery is nearing its end? How do you test the cranking power?

I also have a bank of AGM batteries that I want to test. Mine are around 6 years old. How long should they last?

UK Lele Girl said:

I have 2 banks of AGM batteries in my power catamaran. Each bank is separately located in each pontoon and has 2 AGM batteries. Port side is having a hard time starting the port diesel engine. If I use the combiner switch, the port engine seems to start easier.

Question: How do you know when an AGM battery is nearing its end? How do you test the cranking power?

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First step would be to check if the port side batteries are in fact being charged. Also, could be loose wires somewhere. Check the easy things first.

There are two different questions asked here: how to test starting batteries and how to test house batteries.

Starting is easy. Put a voltmeter across the battery terminals and crank the engine. If the voltage drops below 11 volts while cranking then they are weak.

Testing house batteries is a little more involved. Take your house battery capacity in amp hours and divide by 20. This is the amperage load that is used in the standard test for amp hours. Turn on enough inside lights, spot light, inverter with a 120V load connected, etc to draw that many amps. Start the test late in the day and then check it 12 hours later. If the lights are still bright then keep going. The batteries should peter out to below 10.5 volts after 20 hours if they have the rated amp hours in them. Any less time to reach 10.5 V like 12 hours means they have less capacity. If it is 12 then the actual capacity is 12/20 of rated and similarly for other times.

I typically get 3/4 capacity after a year and it stays that way until something happens like a shorted cell which kills the battery.

David

I would get a Midtronics MDX-640 battery tester.

cardude01 said:

I also have a bank of AGM batteries that I want to test. Mine are around 6 years old. How long should they last?

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Depends on how they've been treated. Maybe depends on which brand you've got too; dunno.

FWIW, our oldest starting/house bank of 3x Group 31 AGMs (Odyssey) has been through 9 seasons. Not saying that's average, though.

And even though that bank still seems fine... of course I'll be paying close attention to how well they still hold charge come this Spring.

-Chris

When my 4D Lifelines were 8 years old I had them load tested with an old fashioned load tester...the one that looks like a cheese grater with a couple of jumper cables attached. They were still good to go. Sold the boat two years later and they were still good. About two years after that the purchaser indicated that he needed new batteries.

Do a resting voltage check. Charge the batteries, let sit overnight with no load on them.Use a digital meter at the terminals and read the voltage. The "Gel" scale on this meter is close to the AGM numbers. The reading you get will indicate the % of charge they are retaining. 50% or lower, they're due to be replace..

Great info everyone. Thanks