Recharging depleted AGM battery

Electrical and electronic topics for small boats
Foulweather Jack
Posts: 17
Joined: Tue Apr 19, 2016 3:56 pm

Recharging depleted AGM battery

Postby Foulweather Jack » Tue Jun 19, 2018 7:26 pm

Thanks to Jim and this forum I learned a year or so ago that the lead acid batteries that my Whaler dealer used to replace the batteries on my 2007 235 Conquest with a 250 Verado were incorrect. I was astonished to learn that they didn't use AGM batteries when the Verado owner's manual clearly states that only AGM batteries should be used. I pointed this out to their service manager and his response was along the lines of, "whatever--we wanted to save you money."

So this spring, at great expense, I had the boat serviced at the local Whaler dealer and the batteries replaced with the proper AGM batteries. Since I'd had difficulties with the old lead acid batteries maintaining a charge, I asked the service manager to check the alternator output on the Verado and I was assured that it was ok.

I discovered last week that the anchor light had been left on and the starboard (#1) battery was fully depleted. No problem, thought I, I can start the boat off the #2 battery, switch it to the #1 battery, and run it up the ICW for half an hour to recharge #1. No joy. Then I got my Schumacher battery charger, which has a specific setting for charging large AGM batteries, and, after several days of trying to charge the battery, there's still no joy. I get a "bad battery" warning on the charger after several hours of attempting to charge, and this after multiple attempts.

So a brief interweb search informed me that AGM batteries can be difficult to resuscitate if fully depleted. I'm wondering: do I have a bad battery? It was installed only a month ago. Or am I not recharging the battery properly? If it's truly a bad battery, I could have the dealer replace it (again) but I'd like to fix this myself if possible since getting to the Whaler dealer/service department would involve significant amounts of free time which I do not have.

Any advice would be welcome. It should be fairly obvious to all that I know only enough about electronics to be dangerous.

jimh
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Location: Michigan, Lower Peninsula
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Re: Recharging depleted AGM battery

Postby jimh » Wed Jun 20, 2018 3:23 pm

If you have completely depleted all charge from a battery, a charger connected to the battery might decide there has been a failure in the battery. A typical failure might be a shorted cell. In a nominal 12-Volt battery there are six cells, and at full charge they can be 2.1-Volts per cell. If there were a shorted cell, then with five cells the maximum voltage would be 2.1 x 5 = 10.5-Volts. If a charger were to see that voltage, it might decide it should not charge the battery at all.

If a battery still has six cells working, but they are all discharged to the lowest possible potential, the battery voltage would be about 10.5-Volts--the same as a fully-charged battery with one shorted cell. If the charger is confused by the very low battery voltage, around 10.5-Volts, it might decide it shouldn't try to charge the battery.

If a battery has been completely discharged, I prefer to begin the re-charging process with a very modest trickle charge, about 1-Ampere. I let the battery sit with the trickle charge for a while--hours or maybe a day--and then check the terminal voltage. Once the terminal voltage shows signs of rising, it means the battery is absorbing the charge. Then I switch to a faster charging current.

In your case, I suggest putting the AGM battery from the boat that is completely flat on a small and rather dumb charger that delivers only 1-Ampere. See what happens. You might find this works better than either the outboard engine charger or the boat charger.

Foulweather Jack
Posts: 17
Joined: Tue Apr 19, 2016 3:56 pm

Re: Recharging depleted AGM battery

Postby Foulweather Jack » Wed Jun 20, 2018 4:37 pm

Thanks, Jim. I've been trying to charge the battery a little bit at a time over successive days on low voltage and there are signs that it may be reviving.