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How do I test AGM batteries for damage from overcharge?

Jun 14, 2010
TBD Looking for my next boat CT
A solar charge controller failed on my boat allowing high output from one 50w solar panel to reach the batteries directly. The issue lasted about 4 days - started on July 22nd and lasted until about shortly after 12PM yesterday, when I was able to return to the boat and diagnose/disconnect it. (I copy-pasted a 3-hour interval sampling log from my Boat Command monitor). The overall bank is rated 220 AH
The batteries seem to be working OK, but if I have lost capacity I'd like to know; is this something I can measure?

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Oct 26, 2010
Hunter 40.5 Beaufort, SC
I'm sure @Maine Sail will respond if he sees this message but I recently did a battery capacity test using a constant current DC load tester I bought on ebay. I recognize its not a calibrated labaratory piece of equipment like Maine might use but it seemed to work well, hold current constant for the duration of the test and shut off automatically at 10.5 volts. I figure its a whole lot better than trying to control a constant current manually over 20 hours. I took the batteries off the boat and made sure they were fully charged and off the charger for at least 24 hours and in a controlled environment that was at 75 degrees. I was very pleased with how the test went and the results. I'm not sure of your bank capacity but the load tester is limited to 180W but I think you can add some constant load resistors to dissipate some of the load and get the load on the tester to within the 180W limit. Here is a picture of the unit I used. My bank is just 230Ah at the 20 hour rate so 11.5A. It tested out at 94+% capacity for a 3 year old wet cell lead acid battery so I am pleased plus after the recharge it seemed to stabilze OCV at a little higher than before the test meaning it may have regained a little capacity. As per Maine's website its a good idea to do a periodic capacity test so that should ease your mind.

Go to the Marine How To site for how to do a capacity test.

Feb 6, 1998
Canadian Sailcraft 36T Casco Bay, ME
First off:

What brand of AGM?

What brand and model of solar controller?

If they are Lifeline AGM's chances are they might be ok though potentially a little dried out. if they are any other brand they are likely damaged.

A constant current 20 hour discharge test is the only way to know for sure.. You can use a device like above or use the guidelines below.

“How can I conduct an accurate 20 hour capacity test?”

20 Hour Capacity Test:

Fully charge battery, equalize if possible, then allow it to rest disconnected for 24 hours

#2 Make certain battery temperature is between 75F & 80F

#3 Apply a DC load that = Ah Capacity ÷ 20 (small light bulbs and/or resistors can work)

#4 Connect an accurate digital volt meter to the positive and negative battery terminals

#5 Start the DC load and a stop watch at the same time

#6 As battery voltage drops, during discharge, adjust the DC load to maintain as close to the C÷20 rate as is humanly possible

#7 Immediately stop the discharge test when battery terminal voltage hits 10.499V

#8 Note the hours and minutes of run time on the stop watch and figure your percentage of 20 hours that it ran. This is your batteries Ah capacity or state of health as a percentage. For example if a 100Ah battery ran for 16 hours it is at 80% of its original rated capacity. Flooded lead acid batteries are considered, by industry standards, “end of life” when they can no longer deliver 80% or more of their rated Ah capacity.

#9 Recharge the battery immediately at the 20 hour rate. Follow this up with equalization level voltage and measure specific gravity until all cells match. (EQ – not for non-Lifeline AGM or GEL). A long slow recharge can have a slight reforming effect on flooded batteries and can actually recover some lost capacity. It is not uncommon for a battery to have more capacity after a 20 hour test than it had going into it.
Jun 14, 2010
TBD Looking for my next boat CT
Thanks, Rodd,
I'll do this after the boat is put on the hard. It wouln't be practical now, the boat lives on a mooring and refrigeration is running when we're there (and when we're gone). I also don't have easy access to shore power. The batteries seem to be holding up to an adequate extent. (3-hour intervals since yesterday shown below.)
To answer your questions: The failed controller was a Xantrex C12 (PWM) and was dedicated to one 50w panel. (I also have two 160w panels, each on their own dedicated Morningstar SunSaver MPPT 15A controllers). The batteries are Power-Tec AGM batteries Power-Tec Deep Cycle Marine Battery.
The failed Xantrex will be replaced with a Genasun 4A MPPT controller model GV-4-Pb-12V (IMHO appropriate for a 50w panel), but for the time being, the 50w panel is disconnected until I put it in place. (As you can see, the system is holding up the batteries without it, and the refrigeration is on, and the temps have been "heat wave" so it doesn't load up more than this. I should have the extra 50w back online this weekend, for even better daytime recovery even in overcast weather.)

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