Recharging depleted AGM battery

Electrical and electronic topics for small boats
Foulweather Jack
Posts: 19
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 flooded-cell 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 flooded-cell 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 okay.

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 web 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.

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Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

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: 19
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.

Posts: 137
Joined: Sat Oct 10, 2015 8:04 pm
Location: Indiana and Eastern UP

Re: Recharging depleted AGM battery

Postby Hoosier » Sun Aug 05, 2018 8:13 pm

Interesting. I just had a similar problem with my conventional house/starboard starting battery on an week long trip. The battery maintainer gave a "Bad Battery" indication one morning which necessitated replacing the battery with a "loaner" from another boater in our group.


After the battery was replaced I put the "Bad Battery" back on the charger there at the dock and it started taking a trickle charge of about 1.5-Amperes. After about an hour with still no "Bad Battery" indication, we put it on a 4-Ampere “Smart Charger,”! and after a couple of hours it read Full.

After I got home I put the battery on a smarter charger, and gave it an overnight 4-Ampere rate charge; in the morning it read Full on the charge indicator. This battery is less than two years old so I guess my friend is getting his loaner battery back.
1978 Outrage V20 with 2004 Suzuki DF-115. 1992 23 Walkaround with two 2010 Yamaha F-150s.