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Old 05-04-2013, 10:17 PM   #1
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Getting equal voltages on two 6-volt batteries?

I recently bought a Rockwood A-Frame with two 232 amp-hour 6-volt batteries that started out with unequal voltages. The voltage for one battery is 6.22 volts and the other is 6.08 volts. I only have a 12 volt charger. What's a good way of bringing both batteries up to 6.30 volts? I figure I'll only have to equalize the batteries once since they should charge at the same rate with a 12-volt charger once they have the same voltage.

It seems like the best way would be to discharge the higher voltage battery down to the lower voltage one, but I can't think of a good way to do this since I don't have anything that draws a lot of 6-volt current.

I'm considering getting a 6-volt charger but can't find one on Amazon.com that puts out around 20 amps when bulk charging. Any recommendations?

Any advice would be greatly appreciated. This is a great forum. Thanks!
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Old 05-05-2013, 07:33 AM   #2
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Always charge them in series with a 12 volt charger.

Use a battery desulphator across the pair to equalize them.
PowerPulse PP12L 12V Desulfator

Use a 3 stage dedicated charger and they should top right off.

You can permanently install the desulphator in the camper top keep those babies humming.
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Old 05-05-2013, 07:41 AM   #3
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First thing, if you haven't done so, is check the fluid levels. if any plates are exposed, add just enough distilled water to cover them. don't fill the cells when discharged as the fluid levels rise as the charge increases.

If your charger has an equalize mode, use that. In the equalize mode, the charger will put about 15 volts across both of the batteries. this has the effect of bringing the lower voltage battery up to saturation matching the higher voltage battery which will have already been at that point. You must monitor the fluid levels especially the higher voltage battery as it will start to off gas while waiting for the other battery to catch up.

If your charger doesn't have an equalize mode, well you do have options. You could hook a 100 watt light bulb to a set of alligator clips and use that to bring the voltage down. The bulb won't light, but it will drain the battery slowly. More bulbs will be more load. please be careful though as a battery is powerful even at 6 volts so don't short out the termials.
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Old 05-05-2013, 08:10 AM   #4
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elmonsanto,
Were these batteries brand new or did they come on a used trailer? Also, is the voltage taken after sitting for a bit, after charging, or when? It's seems odd that one would be at 80% SOC and the other is at 50% SOC.

Anyway, I agree with the above. Hook them up in series, check water levels (fill with distilled water to cover plates as needed), and charge using a decent three-stage charger. Measure the specific gravity and equalize as needed (only equalize batteries that are fully charged).

Test again and if the batteries demonstrate different SOCs then there could be a problem.
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Old 05-05-2013, 09:07 AM   #5
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thanks Triguy, I forgot to say to charge first, then equalize.
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Old 05-05-2013, 09:52 AM   #6
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No problem. I knew what you meant
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Old 05-05-2013, 12:21 PM   #7
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Quote:
Originally Posted by Triguy View Post
Were these batteries brand new or did they come on a used trailer? Also, is the voltage taken after sitting for a bit, after charging, or when? It's seems odd that one would be at 80% SOC and the other is at 50% SOC.
The batteries were brand new when we bought the trailer in Oregon. We camped for three days connected to shore power, drove down to the S.F. Bay Area and parked the trailer in a storage place about an hour from home. I checked the batteries a day after parking the trailer. One battery was 6.41 volts and the other was 6.28 volts.

I didn't know about parasitic battery drain at the time, so the batteries were connected for 9 days before I rushed over and disconnected them. I checked the batteries then and one battery was 6.22 volts and the other was 6.08 volts.

In 10 days, we're traveling north along the California and Oregon coasts where we'll be dry camping for the first 4 days. I'll be receiving a 1,000 Watt Yamaha EF1000iSC generator this week and I'll use it with a Schumacher SC-8020A battery charger to recharge the batteries. The charger puts out 20 amps when bulk charging. This will be the first opportunity I've had to recharge the batteries. Hopefully, not too much lead sulfate has built up on the plates. How effective are desulphators?
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Old 05-05-2013, 01:37 PM   #8
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Your batteries should not have any long-lasting effects from going down to 50% SOC. Its good that you have disconnected them now that you know about the parasitic drains. Did you fully charge the batteries before disconnecting? I assume you did but worth asking - batteries really do best stored with a full charge.

That second battery, which was at 80% SOC the day after you got home, is a little concerning. There is something going on that is causing that battery to lose its charge faster than the first battery. I suggest that you trouble-shoot that battery by testing it. You can take it to a battery place for testing but can also do it yourself. Your battery manufacturer (i.e., Trojan, etc) will have specific guidelines for maintenance and testing. Here are Trojan's for your reference.

About that desulfator, I don't use one like that so can't say. My Stanley smart charger does something similar, though, although they call it battery recondition. Battery recondition sends a series of electrical pulses to break up the crystalline form of lead sulfate and turn these chemicals into useful battery electrolytes. The process seems to be quite effective and my batteries are healthy.
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Old 05-05-2013, 02:09 PM   #9
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it could be that the batteries were not equally charged from new. Of course, each will be charged the same about, as you understand, and the low one has been "behind" since. I'll bet that once you get then equal, you won't have any other problems.
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