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Old 09-18-2011, 09:54 PM   #1
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Dual 12 Volt battery question

Came across an extra 12volt dry cell the other day, only couple months old and fully charged (score)...

So,, considering a little boondocking this fall and was thinking about wiring them up in parallel (pos to pos, neg to neg, right!

Question I have is if it matters if the batteries are different types Iie, one is a "sealed" battery and the other is one that you need to maintain.

I assume not, but just wanted to make sure...

Oh- follow up question for the extra beverage ..... I've installed a disconnect switch on the negative side (between terminal and frame). If I add another battery, is there anything I should consider? I read something somewhere about "ratings" so need to check that out... I think the switch is fairly decent, 500 amp rating?????

Thanks in advance...

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Old 09-18-2011, 10:18 PM   #2
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If the batteries aren't identical, it won't matter at first. The strongest one will wind up charging the weaker one, blah, blah, blah....
As for a disconnect, 500 amps is WAY OVERKILL. The breaker feeding your trailer is most like 50 amps or less. Since you didn't post trailer make or model, that is only a guess. The new Cardinals with auto level systems have change to 100 amp breakers, but that is the highest I've seen in a while.

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Old 09-18-2011, 10:33 PM   #3
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yeah... I think it's a rating for the switch.... reccomended by another FRF member... The TT is a 30 amp unit, but I think the switch is rated for the battery???

Here's the link of the type of switch..

Longacre Economy Battery Disconnect Switch -
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Old 09-18-2011, 10:51 PM   #4
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The other thing you should consider is the charging characteristics of the dry cell. Your charger is designed to switch between stages based on the "feedback" of a wet-cell battery. Having the dry cell in parallel may fool the charger and possibly cause some issues. If it were me, I'd not put them in parallel and use them separately (switch over when one is depleted).

I'm assuming by "dry cell", you're talking about a different chemistry than lead-acid.
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Old 09-18-2011, 11:11 PM   #5
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Here are some thoughts you might consider, lots of fun reading at the least.
Yes, it matters a great deal. Batteries of different size and type are best used as separate entities. This means if you switch them rather than paralleling them, good for you. Marine supply stores sell switches for this. Of course, simply changing the leads from one battery to the other works equally well.
As previously explained, they will discharge from the strongest to the weakest, making both fail over a period of time. How long this is depends on the quality and strength of both batteries, and how much you use these if in parallel, and your charger.
Furthermore, charging batteries on the same system will overcharge one while undercharging the other, as a rule.
While you have a find, I’d certainly have you consider keeping one isolated, and then replace the low-charge cell when it’s depleted.
Alternatively, you could purchase charging systems which will charge dissimilar units without harming either. That’s relatively expensive, but depending on how much bd’n you do, it may be worth the trouble. Personally, I’d choose the swap method and when both batteries are toast, replace them with 6v golf cart Trojans, at the least, new deep cycle batteries.
BTW, 6v’s have bigger plates and greater distances from the battery floor to those plates which- discharges over longer periods, takes more charges over the life of the batteries, the sulfates don’t short out as easily because of the greater distance to the cell, and have generally more capacitance. Disadvantages include cost, size, weight, re-wiring, and new cases.
Generally, choices for batteries are wide depending on just what you are doing, but for camping, a deep discharge cell is a better choice than a start battery. Therefore, AGM, GEL, Lead Acid-- similar to a car battery, and 6v gc batteries fit your bill.
Finally, discharging batteries greater than 50% isn’t good, but really bad. Consider keeping your batteries above 70% for best life. Charging these with a good quality 3-stage charge system, bulk-float-trickle is more costly but does a better job keeping your cells around. 1 or 2-stage chargers can overcharge a battery, boiling the life out of them, and smart chargers will measure the temp of the batteries, temp of the ambient, be calibrated to the type- GEL, 6v, etc, and monitor the charge rate including voltages etc. Since voltage is more critical on GEL and AGM, not as much on 6v, you’ll find that charging GEL’s like a 6v will bulge the battery making its life very short.
With safety in mind, remember that most batteries store a huge current reserve and that shorts cause sparks, explosions from H gas and acid. Take proper precautions working around these by removing watches, wearing safety glasses, venting the area for a reasonable period of time, and using proper insulated tools and adequate wire.
In summary, it’s a science within which I’m still learning, practicing, spending money.
Happy camping.
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Old 09-18-2011, 11:22 PM   #6
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have them able to wire up separate. they will fight and kill each other over time. best to have one at a time working. use one, charge the other, swap and charge other etc. solar panel ftw!
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Old 09-19-2011, 08:40 AM   #7
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I think the OP said:

1) you have two lead acid batteries; one a maintenance free type (sealed top) and one (the OEM) maintenance type battery.
2) The on/off switch is on the ground leg of the OEM battery
3) The OP asked two questions: Can this be done and Is this a good idea?

So If I got that right here is my take:

Most likely the battery you found is a car battery. The plates are thin and designed to give up it's charge in a burst (like to start a car); not over time. The found battery will NOT add any significant capacity to your system. It's AH capacity might only be 10-30AH. Your OEM Dual Purpose Battery will spend most of its capacity trying to keep your car battery charged.

Answer to first question: Yes. Positive to Positive - Negative to Negative will still give you 12 volts. As long as you hook the 2nd battery up to the first and NO WHERE else. Even your battery on/off switch will work to isolate both batteries.

Answer to second question: Definately not. Mixing battery types will result in LESS capacity than your single OEM battery alone. Due to internal resistance losses of the battery bank trying to equalize itself constantly.

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Old 09-19-2011, 11:33 AM   #8
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OK- Thanks All

Taking all in... I think it's just best to keep the batts separate and use the extra one as spare....Doesn't seem to be as straightforward or effecient given the parameters and different batt types- just need to be more dilligent on power usage when off the grid

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Old 09-19-2011, 11:52 AM   #9
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Please don't get me wrong. I don't mean to squash investment in your RV, but rather, enjoy helping you in the right way.

Of interest, none of the RV's I've had were wired correctly, this one is no exception. The fusible links, one of which, wasn't connected at all, and the other had a lose lug nut. Never have I seen a ground worth anything. The bolt threads and star washer were essentially the only connection points to the chassis, and that's usually found on every RV. Imagine some poor sole in the middle of nowhere and little mechanical ability.

You can see that battery options are wide and varied, but worth the time when and if you boondock a lot. Consider the book: Living On 12 Volts With Ample Power, and another well-known author, Nigel Calder, who's written at least three quality references.

The note on safety is part of my thing, whereby no object is worth getting hurt.

Nevertheless, a bit of research could save hours of frustration and huge expenses and one's self. BTW, prior to spending 4.5 years aboard our sailboat touring the Pacific Ocean, my wife and I were happy we spent equal time learning about power.

After all, few current bushes and repair facilities exist while in the middle of the Pacific. Those 1,800 miles in any direction to anyone, sailing at 7 knots average, make for long voyages without DC.

Get the book(s), you'll be glad you did.


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12 volt, battery

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