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Old 07-24-2015, 04:00 PM   #1
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Dual Batteries - But should I use only one?

I have two batteries. When one starts to go bad, it drains the other, and always presents the complication of trying to figure out which one of the two batteries is bad and will not hold a charge. So now I am wondering if it might be worth my while to manually manage which battery I am using and/or charging.

For instance, I now have one battery that is virtually new and another battery that has about 6 months left on a full replacement warranty.

I could, for instance, use solely the older battery, thinking it will fail before the 6 months and I will get a replacement.

Or...use only the new battery since it is theoretically stronger and more robust. This may lenghten the life of the older battery since it won't be used regularly.

In either case, the other battery would always be available in reserve to use if the first should be discharged or fail.

What do you think?
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Old 07-24-2015, 04:03 PM   #2
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I think you should use both batteries wired in parallel, it will double battery life.
Also both batteries should be replaced at the same time. If one battery fails when they are wired parallel, it will take the good battery with it
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Old 07-24-2015, 06:07 PM   #3
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[QUOTE=handn;1657661
If one battery fails when they are wired parallel, it will take the good battery with it[/QUOTE]

This has not been my experience. In this instant case I saw both batteries go down to 8.5 volts three times. So I decided to diagnose by charging batteries completely, then separating them to diagnose. One battery dropped to 11 v after an hour, and continued down to 9 volts after about 2-1/2 days. The other battery is at 11.55 after being disconnected for 3 days from the bad battery. So, I replaced the one that cannot hold any charge and am also keeping the one that seems to at least stay up for a while.
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Old 07-24-2015, 06:37 PM   #4
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It is not so simple as any of those answers. In your test where one battery dropped to 11 volts in an hour, was that under load or open circuit? If open circuit, that battery is dead. 11.55 after three days isn't great either, it should settle around 12.2. Second, if the batteries are wired in parallel and one cell shorts, that can take the other battery down too. But that isn't the only failure mode. In general I would connect the batteries in parallel all the time. If one develops a shorted cell, pull it out and replace it. Battery losses are reduced whenever you can discharge more slowly -- two batteries in parallel will last longer than one battery

Are you sure these batteries have been serviced only with distilled water? Is there any contamination you can see in the electrolyte?
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Old 07-24-2015, 06:48 PM   #5
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Dual Batteries - But should I use only one?

I'd look at the charging system first. That three stage kind that some talk about.

Then I'd check the batteries monthly with a hydrometer (which is more a more accurate indicator of battery serviceability than volts) and to see if they're thirsty if so top them off correctly with distilled water.
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Old 07-24-2015, 06:56 PM   #6
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Sounds like the bad battery has a dead cell
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Old 07-25-2015, 10:55 AM   #7
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Always replace in pairs. If one fails the other isn't far behind anyway. If you use two six volt batteries in series you essentially have only one twelve volt battery anyway. But the sixers will last a lot longer too.
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Old 07-25-2015, 12:48 PM   #8
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Go gel! I love my gel batterys and ya might be happy with one!
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Old 07-25-2015, 12:55 PM   #9
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Always replace in pairs. If one fails the other isn't far behind anyway. If you use two six volt batteries in series you essentially have only one twelve volt battery anyway. But the sixers will last a lot longer too.
Agreed, agreed, agreed.

Whether it's a series pair of 6V's, or a parallel pair of 12V's, they should be considered *one* battery. Always replace both, even if just one seems bad.

And the resting voltage of a good 12V battery, after being charged, is 12.6V, not 12.2, which is actually about 50% discharged.

In the test where one battery went to 8V, and the other to 11V, both are toast and should be replaced.
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Old 07-25-2015, 01:15 PM   #10
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Originally Posted by TankerIP View Post
So now I am wondering if it might be worth my while to manually manage which battery I am using and/or charging.

In either case, the other battery would always be available in reserve to use if the first should be discharged or fail.

What do you think?
I think that installing an A/B switch and manually switching between batteries is more of a problem than it's worth. What that will do is ensure that the two batteries are constantly at different charge states by draining one at a time, which will cause a problem if you ever want to recharge both at onceó the one with the higher state of charge will be overcharged before the one at the lower state of charge is fully charged, exactly the problem that you're trying to avoid in the first place.

But if you're happy also charging one battery at a time, then I don't see a problem with what you're proposing. I don't think you need to use one battery at a time and manually switch between them just because of a possibility that one will go bad, but my job isn't to make your decision for you, only to provide information to let you make your own decision.
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Old 07-25-2015, 08:04 PM   #11
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11.5 volts is considered dead in a 12v battery. Dead after sitting by itself for 3 days is not good. That is a dead battery.
Google 12v battery voltage vs capacity. You should find a chart that will show you that 11.5v is basically 10% capacity left and you shouldn't go below 50 % regularly if you don't want to kill the batteries quickly.
Make sure you keep an eye on fluids in the new batteries.
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Old 07-25-2015, 08:46 PM   #12
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Agreed, agreed, agreed.

Whether it's a series pair of 6V's, or a parallel pair of 12V's, they should be considered *one* battery. Always replace both, even if just one seems bad.

And the resting voltage of a good 12V battery, after being charged, is 12.6V, not 12.2, which is actually about 50% discharged.

In the test where one battery went to 8V, and the other to 11V, both are toast and should be replaced.
That depends on the composition of the battery. 12.6 resting voltage for a Lifeline AGM is 25% discharged. A 'full' Lifeline is 12.8-13.2 VDC. Other batteries vary by manufacturer.

Best to contact the battery manufacturer for their actual specifications. Some batteries, like Lifelines are at 50% when 12.2VDC is reached. Others, usually liquid cells, are full at 12.6 and at 50% when 12.0 is reached. YMMV!!!

You are correct in your thinking, as any number of batteries in a battery bank should be thought of as 1 battery, and should be purchased and installed at the same time. Ideally, they should all be from the same manufacturing lot too!!
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Old 07-25-2015, 10:01 PM   #13
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What is wrong with this logic? Two batteries in parallel, charge will always equalize between the two. If one has high internal leakage, it will be the same as if both have excessive leakage. The weak battery, as it discharges, takes charge from the good battery and is like a discharge load to the good battery. Lead acid batteries do not like to be left with a constant load on them until they are fully discharged. As long as the batteries are identical, there won't be a problem, but they are likely not identical to start with and they will move to become more dissimilar over time.

When two six volt batteries are in series, the two are independent. If one has higher internal leakage, its voltage will decline, as will the total voltage of the two in series but this will have no effect on the better battery.

I plan to move to two 6V batteries in series when my existing single Group 27 fails.

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Old 07-27-2015, 08:27 AM   #14
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Nothing wrong with that logic, Al, and two T105s or better quality GC2 batteries provide more watt-hours per dollar than about anything else. The point people miss is that when you connect batteries in parallel, the charge will not "equalize" between the two of them. As you said, if one has high leakage it will drain them both, but that leakage does not charge the leaky battery. The OP asked about whether he should try to get a little more useful life out of one nearly-dead and one somewhat better battery by connecting them in parallel or using them one at a time. He hasn't answered about the other conditions that might apply, but the general answer would be to connect them in parallel if there is useful life left in them -- if not, don't bother trying to charge the dead one. I think he wants to be sure the nearly dead one will be truly dead when he takes it in for warranty replacement. There are sure-fire ways to kill a battery pronto, no need to delay the agony...
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