Why is my boat battery not holding a charge?

Duncan Kent

Chris Mardon is struggling to understand why his boat battery is not holding a charge. Duncan Kent has some suggestions

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My sailing yacht Spook has two AGM lead/carbon service batteries of 110Ah each in parallel on a 12V system.

It has many items connected including a NASA battery monitor.

A mysterious fault has arisen in that the batteries look 90% state of charge (SoC) when we go to bed but have collapsed to 10V by 8am the next morning.

We run the engine for two hours and the batteries charge up to 90% and 12.9V which is maintained by the solar panel during the day.

We have had no 240V power available on a visitor pontoon. We’ve switched every circuit off and on but none have drawn unusual current.

The fridge shuts down automatically when the voltage drops below a set point (around 12V I think), and we have switched off all other electrical circuits except lights overnight.

At 1am the monitor showed the battery voltage at 12.8V but at 7am it had collapsed to 10.9V.

The batteries are four years old and have always been kept at over 89% SoC on a 240V charger when on her marina berth.

Do you have an opinion as to what you think are the most likely causes for this mysterious voltage collapse?

I bought the lead/carbon AGMs because they were supposed to have a much longer life than lead-acid batteries. Could one have failed? How can I tell?

Chris Mardon

Duncan Kent replies: Four years is not an unreasonable lifespan for lead-acid marine house batteries, even the more advanced AGMs like yours.

AGMs usually last longer than regular flooded or VRLA batteries, but they are very fussy about how they are charged.

They need to be fully recharged frequently (preferably using a smart mains charger) to retain their capacity.

Two common problems often occur.

A battery bank on a boat

Battery not holding a charge: AGM batteries are fussy about how they are charged. Credit: Barney Green

Firstly, the percentage state of charge (SoC) reading on any battery monitor is almost always inaccurate unless you follow a very rigid charge and resynch protocol, so folk often think they’re fully charging the battery when they’re actually not.

Secondly, I’d always check the voltages at the batteries themselves using an accurate multimeter, after disconnecting all loads and charge sources and waiting an hour.

If you suspect a faulty cell then separate them, charge each battery separately, then test by putting a known load on each one for a specific period of time.

When running your engine, they should show a voltage of around 14.4V initially.

If not, then I suspect a faulty alternator.

You don’t say what mains charger you have but it’s also important to ensure it is set to the AGM charge regime.

I have no idea what loads you have been running on them, but if they’re dropping below 11.0V at any time other than when there’s a very heavy current draw such as an inverter, then I’m afraid they’re very likely to be scrap now.

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