Very Deep Discharge - How Bad?

[Intentional Drifter]

OK, here's the situation. I had to go back to the NW for two weeks to take care of some stuff and left the boat at a yard with a very good reputation and have them do some things (bottom paint, saildrives, etc.). They plugged in the boat while on the hard to keep the freezer and frig OK. Well, a storm came up over the weekend and knocked out the shore power. With the very hot weather, the batteries were deeply discharged before they discovered it. They say that they were discharged down to 8 volts. The batteries are Sonnenschein A200 gelcels in a 600 amphr bank. The batteries are 5 years old, but have been well kept and performing just fine, at least up to now.

They say they've been charging the batteries and they "seem OK", whatever that means.

So, please educate me. How bad is this likely to be? How can I tell if the batteries have been seriously harmed? Should I just bite the bullet and budget in for new batteries?

Thank you.

ID

[camaraderie]

I'll take a shot at this one. First...if they are indeed gels and not AGM's...they are the MOST tolerant of complete discharge of any battery type (flooded/AGM/Gel) and since this was a single event...the harm done may be minimal. On the downside...most yards will simply apply standard battery chargers of the automotive type to recover batteries. I am hoping they simply plugged in your boat and let your properly configured 3 stage charger deliver the right charge to your gels.

Your yard should have a dummy load battery tester that can stress your fully charged bank to see what kind of shape they are in. The will need a LARGE amp load model since your bank is large...say around 200 amps.

Like this one which can be had at grainger.com
Absent this type of load testing it is very difficlult to tell the actual damage done to your batteries. Fully charging them and then leaving them OVERNIGHT with no load on them will tell a bit...if the voltmeter reads less than 12.6V the next day...they have been damaged. If you run a big house load like a windlass until the V gets down around 11.8...and the batteries don't bounce back in short order to close to the start voltage, they are damaged. I'm sure you have lost some capacity over 5 years use but perhaps there is still more life in them. Good luck!

[Intentional Drifter]

Thanks, Cam!

That's very helpful. Yes, the batteries really are gels. I'm assuming that the yard simply let my on-board charger do the job, but I'll verify. It is a Victron interver/charger and is specifically set up for the Sonnenschiens.

ID

[Alan Wheeler]

Don't panic about it. If you had them discharged that deeply on a regular cycle, they wouldn't last that long. But oncein their life is not going to destroy them. Plus if they were 8V under a load, they would have actually been much higher that what the meter was displaying. If they sat at 8V unloaded, that is certainly very low and not good, but should survive this once or maybe twice.

[Pelagic]

As you mentioned your Fridge and Freezer, if possible take a look at the circuitry on the electronic controller for their compressors. On low voltage they will often cycle on/off and get pretty hot.

Best to have a smell and find backups if needed before they fail when you are out cruising…Good luck!

[Intentional Drifter]

Thanks, guys! Yes, they had a load on them. We'll have to see how they do with a load test, when I get back.

Thanks for the tip on the controllers. I didn't know that, but will have a look (and sniff).

ID