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Old 04-12-2016, 09:27 AM   #1
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1979 31' Excella 500
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Oh no! Can these batteries be saved?

We're in the final stages of renovating our '79 Airstream Excella and this past week was all about polishing. I found a young crew to do the work and rented an indoor space for a week to get the job done. I went away for a week while they did their work and while the polish job is AMAZING it looks like someone flipped the breaker that was charging the AS batteries thinking it was for the overhead lights turning them off after the job was done. A few lights and appliances were left on in the AS and I came back to a bank of 4 x 6v batteries wired in series-parallel only putting out about 7v (in other words, DEAD).

I'm charging the bank back up now but concerned they were drained too much to make a full recovery. Prior to this they've never been drained below about 90% capacity (according to my battery monitor), and I bought them about a year ago. It's a 460ah bank, and I'm charging with a 15a (automatic) charger overnight to see if they can be brought back to life.

Any guesses? Do you think they're too far gone to make a recovery?

Interesting side note: some things in the AS still worked with only 7v... Battery monitor, SeeLevel Monitor, and the battery bank exhaust fan.
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Old 04-12-2016, 09:40 AM   #2
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When a battery gets that low its best to charge with 2 amp charge for longer time.
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Old 04-12-2016, 10:24 AM   #3
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Agreed a slow charge will be a much better approach. It is my belief that you should not keep the battery bank at full or near full charge, that they should cycle.
You may try discharging them and then slow charge a few times. This may help get them back.
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Old 04-12-2016, 10:27 AM   #4
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Once they are charged up, I would apply an equalization cycle to them.
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Old 04-12-2016, 10:53 AM   #5
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A week of being discharged, or a part of a week, is not terrible, especially on 6 volt golf cart batteries. They can stand more abuse than thin plate 12 volt batteries. Now, I am not saying that there was no damage done, but I think they will come back for now, and the net result would be a somewhat shortened overall life.

I do volunteer solar work for the Forest Service. Yesterday I discovered that one of the systems I built and maintain had an inverter left on all winter, a 0.3 amp load. The solar panel charging capacity here in the winter is not enough to sustain that load, so the batteries ran down to the low voltage cut off point and remained that way for months. The batteries are L 16 six volt units, already several years old. I don't expect that those batteries will come back, as the time of discharge was months, not days.

Battery life vs. discharge % is somewhat of a "black art" to determine. Some factors are the type and quality of the battery, the depth of discharge and for how many cycles, the time of leaving discharged prior to re charge. Chronically undercharged batteries are damaged, even though they are never fully discharged. A robust relatively newer battery, in good condition to begin with, fully discharged for a relatively short time, probably will come back and function well for a long additional time.

My best guess for your outcome.
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Old 04-12-2016, 11:36 AM   #6
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Slow trickle charge over the course of several days. Use lowest amperage possible. Monitor closely. Battery water will readily evaporate under constant charge conditions. Make sure water levels cover electrodes at all times.

Make absolutely sure you have plenty of ventilation. During periods of continuous charge, atoms of hydrogen and oxygen will break apart from water molecules and the result could be an explosion. I've seen this happen. You don't need that.

Tom
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Old 04-12-2016, 11:45 AM   #7
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Thanks for the quick advice everyone! I'm going to change the charger from 15a mode to 2a and let it sit for a while (a long while since I have 460 ah bank!).

Thinking I should install a low-voltage disconnect to prevent this from happening again. I have some questions on how to best implement that in my current setup... I'll start a separate thread for that.
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Old 04-12-2016, 04:19 PM   #8
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Treker...I've had a battery shop for many moons & run into your situtation all the time....

Here's the basic formula I use:
Amp Hrs ÷ chgr amps × 1.5 (fudge factor) = Hrs req'd to recharge.
460÷15×1.5=46 hrs
If @ 2amp rate...
460÷2×1.5=345 hrs

IMHO...you won't be able to recharge your batts at 2amp rate if they are at only 7 volts...too much internal resistance @ 2 amps won't cut it...

With your batts we would charge at high rate 40+ amps till 'warm' then reduce finishing chg @ 15-20 amp rate till hydrometer indicates full chg...

DO NOT DISCHARGE TILL DEAD & recharge as others have mentioned - lead-acid batts will not tolerate it - thats only for rechargable camera, phone batteries, etc...

Keep in mind that the longer RV batteries are allowed to be stored in a discharged condition - the shorter their ultimate life - just like your car battery, your RV batts need to be kept fully charged (& recharged in timely manner) for longest life...?
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Old 04-13-2016, 09:26 AM   #9
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Much better to have a slower charge and give these a chance to recover....PS he stated bank of 4 x 6v batteries wired in series-parallel & I am taking that as Golf Cart Batteries NOT the typical Deep Cycle RV

Nobody said "Fully Discharge" considering that this owner chose Golf Cart Batteries it is much more likely that he knows to only go about 50% to 60%.

Another consideration is that when they are at a serious state of discharge, 40% or more, they can Freeze! and Explode! if any charge or discharge is applied.
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Old 04-13-2016, 10:06 AM   #10
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I forgot to mention: Ambient temperature is also a factor. 65-75 degrees F is the ideal range for optimum battery performance.

Tom
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Old 04-14-2016, 10:18 AM   #11
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My two Interstates grp 27 are only 3 years old. I had an issue with the Parallax converter and my batteries got discharged too low when the batteries were less than a year old. I replaced the Parallax with a PD4655. Last year I decided to check the batteries with a hydrometer after they were fully charged. I found two cells down in specific gravity. I have an old 6 amp charger and I placed it on the affected battery. I let it charge at 15.5v for a couple of hours then checked the specific gravity. The cells came back. I performed the same charge on the other battery. The batteries seem to hold up better now since doing this. This winter I took the batteries out and the specific gravity was fine. I put them on a smart charger connected in parallel from November to just yesterday when I pulled our trailer home to get ready for a trip. Resting voltage 12.8. So I'm thinking I can get another season out of them.

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Old 04-14-2016, 01:44 PM   #12
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Just went out to see how the batteries are doing... Disconnected for all loads for an hour and they are hovering right about 13v. I'm guessing if I let 'em sit overnight they'd be around the nominal 12.8 range. Looks like these batteries could be saved after all!
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