My boat battery has suffered a deep discharge and I want to charge it with the best strategy to give the highest odds of saving it. Maybe there's no hope, but it's a good battery and worth the effort. 12V AGM DieHard Marine PM-2.

Current open circuit voltage is 6.36V. Ugh. That's after jumping it yesterday for maybe 30 minutes to try to get on the water. Got the engines started with the jumped battery (car disconnected) in the parking lot, so we launched the boat, but by then the engines wouldn't start again. Had to call it a day. Didn't want to risk getting stranded on the river.

So, high current? Trickle?

 

I don't know if you have the means, but we used to do this to my submarines main battery on a regular interval. If we had a cell that we not holding its charge as well as the rest this could usually restore it. Its called an equalizing charge. A few precautions: Make sure that none of the plates are touching or shorted. Check the specific gravity of the electrolyte and restore/refill as necessary. I've included a link to the theory behind this type of charge. It kept our several hundred cell battery healthy and at full capacity throughout years of deep cycling conditions.

http://www.mydrv.net/equalization.htm

 

My boat battery has suffered a deep discharge and I want to charge it with the best strategy to give the highest odds of saving it. Maybe there's no hope, but it's a good battery and worth the effort. 12V AGM DieHard Marine PM-2.

Current open circuit voltage is 6.36V. Ugh. That's after jumping it yesterday for maybe 30 minutes to try to get on the water. Got the engines started with the jumped battery (car disconnected) in the parking lot, so we launched the boat, but by then the engines wouldn't start again. Had to call it a day. Didn't want to risk getting stranded on the river.

So, high current? Trickle?

Take it out and give a good charge for a day (24 hours). It is not enough to just charge it for say 30 minutes. Starting an engine is very hard for a battery. And quite often. If you only drive your car or boat for short trips. The battery will never have time to be recharged by the dynamo. I always charge my cars battery overnight about once a month. And perhaps you should do the same with your boat battery

 

OK, I've hooked a 10A charger to it for several hours now, with the charger set to "deep cycle". The battery appears to be taking a charge, and in fact the charger is cycling on and off due to overheat. (How the Hades can they rate it 10A but then it cuts out so much that the duty cycle is maybe half. Doesn't that mean it's a 5A charger?)

The open circuit voltage measured soon after disconnecting the charger is 11.6V. Better than this morning, but I'm getting that sinking feeling that maybe a cell is shorted. We'll see how it looks in another few hours.

 

It's a AGM battery so you can't run a true equalization cycle on it as this will cause outgassing and non-replaceable loss of the electrolyte in a no maintenance battery. If one cell is shorted the battery will seem to be charged but the voltage will drop to 10 volts as soon it's lightly loaded. If that's true it's scrap. If all cells are good but weak sometimes several full charge/discharge (to 10.5 volts under load) cycles might recover some capacity.

 

If that's true it's scrap

I've got one like that already, and I don't want another. It's got loads of capacity - at 10V. That's actually worse than scrap, because what happens with my boat starter is that it sits there drawing lots of current but not enough to quickly start the engine. So the starter overheats and that's an incredible pain to replace!

My DieHard in question now has about 8 hrs of charging on it, and shows ~12.3V when the charger clicks off. So my hope level is rising. The fact that it'll draw enough current at that voltage to overheat my charger is somewhat encouraging.

 

If its an AGM battery here is a link to how to properly revive it; http://www.optimabatteries.com/product_support/resuscitating_agmbattery.php

Another thing is you have to watch with the newer chargers. Most won't give a charge if the existing voltage is below a certain level. In that case you need to parallel a charged battery with it to trick the charger. Ask me how I found this out, I know have two new chargers after getting an old pick-up running so I could sell it.

 

Did not say this. But if you battery was 100% flat. The charge current can be quite big. And as a result the the safety fuse may kick in quite often, and then reset after a while. But the charging will take days. If your charger has a high low range. Start charging it in low current range. Then the charge current drop somewhat say after 6 to 8 hours switch to the high range

 

I've seen some lead acid batteries "resurrection" "or revitalising" circuits online like this one: http://www.barkeraircraft.com/files/Pulse3_web_layout_.pdf

They all seem to rely on a boost circuit charging the battery and apparently it needs to be attached for days to work properly. But I have not used it and have no idea if it really works.

 

I've got one like that already, and I don't want another. It's got loads of capacity - at 10V. That's actually worse than scrap, because what happens with my boat starter is that it sits there drawing lots of current but not enough to quickly start the engine. So the starter overheats and that's an incredible pain to replace!

My DieHard in question now has about 8 hrs of charging on it, and shows ~12.3V when the charger clicks off. So my hope level is rising. The fact that it'll draw enough current at that voltage to overheat my charger is somewhat encouraging.

Sorry, but I don't think that's a good sign of a healthy battery. What's normal is about 13.2 dropping to 12.6 open voltage. That could easily mean it's overcharging 5 cells with a risk out-gassing cooking the other cells. Look for bulging on the sides.

 

Well, giving it a decent charge for a day then letting it sit for a couple of days. Measure the open circuit voltage and record it. Then try to use it with a device that can draw several amps for even a few minutes and measure the voltage. If it measures 10V - 10.5V then I think you might be out of luck. I have a battery that does just this. It charges to normal voltage levels and appears good, but when you got to ask of it any meaningful current the voltage drops to 10.xV. It's scrap or at least not usable for it's intended purpose. For me I can operate a bedside LED lamp for eons with it at the current demand is next to nothing.

 

I think I'm in the clear, fingers crossed. I let it charge for ~12hrs yesterday. When I stopped, the 10A charger was still cutting out and in due to overheat. The battery was ~12.1V a few minutes after disconnecting. Not quite what I was hoping for but not bad.

To my surprise, it was at 12.3V this morning. Maybe it cooled overnight? I resumed charging and it immediately went into "sawtooth" mode, bouncing between the voltage setpoints. I let it go for a while longer but basically it behaves like a fully charged battery. I know it'll start the boat - it's a smaller load than a car engine - but only time will tell how it lasts.

My take-home lesson is that an AGM battery is a very different beast than a normal automotive battery. If a brief jump doesn't get your car going, the battery is usually toast in my experience. Not so for this AGM battery - you have to give it a nice long charge. Happy to have dodged a bullet this time.

 

I'm waiting to hear back with your results. Bring a spare battery with you...

 

Once the plates lose coating materials,the battery wrong hold
a deep charge. One bad plate can keep it from holding charge,
regular or marine.

 

Am pessimistic.
Nothing to lose by trying the methods exposed in these responses; but there is a rule for lead-acid :
"A battery that stays discharged over 24 hours will never recover its reliability/performance"
So take it as a lesson.
In the eventual case the battery allows starting after recovering attempts, you would have a risk situation of uncertainty after a few hours at rest in the middle of the river.

 

I'm not optimistic either.

A local museum has a number of retired military vehicles that all have 24v electrical systems. We had a number of 6TMF batteries (high capacity 12v truck batteries) that were not maintained properly. I'd tried charging several of them over several days' time. One of them was charged to 12.7v, and would start the engine a couple of times when in series with another good battery, but subsequently failed to start it after sitting for a few hours.

Your battery will likely perform in a somewhat similar manner; it'll start the engine until you really need for it to start the engine. Then you'll be looking for a paddle.

 

Bring a spare battery with you...

Not much point in carrying around two bad batteries or one bad battery and one good battery. Might as well return the bad battery for recycling and have one good battery with you.

 

All concerns noted. I'll be very cautious and carry cables on the boat until I regain confidence in this battery.

I usually test-start the engines before leaving home and also make a rule of never pushing away from the dock without starting the engines, except of course the one time I pushed off with this dead battery. I had to throw the anchor several times to pull myself back upwind to the dock.