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AGM super cycle sudden voltage drop under small load

Hello I’ve got 3x170 super cycle batteries in a bank in a boat. They are less than 2 years old, been lightly used never using more than 150 amps and normally kept on float by a dc-dc charger and a lithium bank powered by solar.

It’s been very cloudy so I turned the dc-dc charger off and everything was fine until the around 40 amps had been discharged via the fridge then suddenly under 7 amp fridge load the voltage would drop from 12.4 to 11.6 and the fridge shuts down, the voltage recovers to 12.4v then the fridge starts again and cycle goes in. These batteries have not had a hard life in fact very little use. Is this the problem and is there something I can do? Thanks in advance.

AGM Battery
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Bob K avatar image
Bob K answered ·

When I've experienced that with lead acid batteries, it usually indicates one or more bad internal cells. There's enough voltage there to read "good" when the battery is resting, but as soon as I put it under a load, it can't meet the demand. Yes, it can still happen with batteries even though they're newer or had an easy life.

I recommend you check all connections to make sure they're good, not corroded, etc. Then, load test your batteries. Either buy a load tester or have it done at an auto supply store.

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john345 avatar image john345 commented ·
Yes will do thanks.
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JohnC avatar image
JohnC answered ·

Hi @John345

How cold are they? Pb batteries can get very sluggish and suffer from this when cold. They also need a higher V to charge. Maybe your Float V is a little low anyway, and they may not be fully charged..

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john345 avatar image john345 commented ·

Thanks what float voltage would you suggest? It’s currently at 13.8v

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JohnC avatar image JohnC ♦ john345 commented ·
@John345

Depends how long you float them. 13.8V is highish, and suitable for a few hours a day, but not 24hrs. Some might say 13.2V for continual use, but might be too low. These for 25degC temp, and need compensation.

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