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Old 04-13-2015, 11:08 PM   #1
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Adding 12 volt to 6 volts

my coach originally had 4..6 volt batteries.coupled with a 150 watt solar panel...they were great for about 6 years..we weren't dry camping that much, so when i changed them out...i went with only 2 ...6 volts...i'm finding that the lasting power is not that great..has anyone adding a 12 volt to the existing 6's and was happy with the results...
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Old 04-13-2015, 11:27 PM   #2
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It won't work as well as you would like it to. When connecting batteries in parallel, the best results are obtained if the batteries are all of the same type, same manufacturer, same amp-houir rating, and of similar age.

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Old 04-14-2015, 01:16 AM   #3
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We have used extra 12VDC RV/Marine batterys as extenders for boondocking in the past. It works just fine to add some capacity to your set-up.

Just be sure to connect the new 12VDC cell properly in parallel to the 2x6VDC bank or it will not work properly.

While it is true that battery banks used in a system work best when the cells are the same capacity and age, it is not a requirement.

Best luck
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Old 04-14-2015, 07:14 AM   #4
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I am fond of saying THERE ARE NO SIX VOLT BATTERIES IN RVs (and yes, I understand you (And I) have two six volt batteries... When they are out of the RV sitting on something else.. But when they are IN THE RV they are two halves of a single 12 volt battery.

This is a 12 volt battery -{Battery}+12V
This is a six volt -}Bat}+ 6V *or* +{ery}+ 6

This is the above two six volt batteries installed in an RV

-{Bat}+T-{ery}+ 12 v

So you are adding a small 12 volt battery to avery large one (Size 4-D electrically)

Suggestion..Go back to four GC-2 Not that much different in price than a good size 12 volt and way more capacity.
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Old 04-14-2015, 07:29 AM   #5
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Mixing 12v batteries with a pair of 6 volt batteries will increase your amp/hour (lasting time). However, dissimilar batteries have different recharge rates. Therefore over the long term. This solution will not provide the best solution. In some cases, the weaker batteries will harm the new batteries. That is why it is always recommended to have matched sets (age and size).
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Old 04-14-2015, 11:24 AM   #6
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Quote:
Originally Posted by shiggs68 View Post
Mixing 12v batteries with a pair of 6 volt batteries will increase your amp/hour (lasting time). However, dissimilar batteries have different recharge rates. Therefore over the long term. This solution will not provide the best solution. In some cases, the weaker batteries will harm the new batteries. That is why it is always recommended to have matched sets (age and size).
This is the best answer.

Can you add a 12V in parallel to a pair of 6V? Yes.

Will it give you more capacity? Yes.

Will it give you maximum battery life? No.

It works in the short term, but is not the ideal solution in the long run. Just because something appears to work, does not mean that it will consistently work well.
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Old 04-14-2015, 06:57 PM   #7
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Shiggs.. NO.. they have different CURRENTS

They true part is this: When discharging the bigger battery will provide more of the current and when charging take more of the current.

The MYTH part is that one will reach full or empty before the other.

What will happen is once a single charge/discharge cycle has completed they will be in FULL lockstep.. If, for example (And to make the math easier) The big battery is 200AmpHours and the smallr one 100 is this.

The big battery will provide 2/3 the power to the load (The smaller one supplying the remaining third) and likewise when charging the BIG battery will suck 2/3 the charge current, and the smaller one 1/3.

The perfect socialist setup, Each according to it's ability, Each according to it's needs. And Electrical Commune.

It can not be otherwise.. here is why: State of charge determines voltage and if they are DIFFERENT,,then the LOWER VOLTAGE battery sucks more charge or delivers less current till they are matched..then they lock together till the jumper cables go bad or come off or a cell shorts.
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Old 04-14-2015, 07:30 PM   #8
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I'm of the opinion that while this may not be "best practice", it makes little practical difference as long as the first pair of batteries is in really good shape. However, coupling up a not-so-good battery with a new one, whether same or different type, can give rather poor results. And too many people aren't knowledgeable enough about batteries to make an accurate call on that.

That said, I have done exactly what brelph asks about and got good service from it. I coupled a new AGM 12v to a 5 month old pair of GC2 6v's and it was fine. Still going strong when I traded the coach 5 years later.
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Old 04-14-2015, 07:32 PM   #9
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Same results here. Can't discount real world experience.
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Old 04-14-2015, 07:40 PM   #10
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If it's the cost for not having four 6V batteries, go to Costco and buy the Interstate GC-2 batteries they sell $83.99 at my local store.
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Old 04-14-2015, 09:44 PM   #11
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Our local Costco doesn't carry them, but Portland OR ones may. Just don't get there very often!
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Old 04-15-2015, 06:13 AM   #12
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Wa8yxm, I have to disagree to a point with your description. Yes, assuming a full charge cycle all batteries will equalize. However, in practice, each battery exhibits an internal resistance that at the terminals makes your description appear to be correct. Internally, the state of charge will be different if not charged long enough the come to the equal state.

Once the charge rate is reduced, the lower state batteries will attempt to absorb their needs from the fully charged batteries. This cycle over time will shorten the life of the better batteries.

Yes, I agree that practice vs ideal engineering is often two different cases. However, the most optimum situation is when all batteries are of the same age and size. Just look at any commercial installation like the banks used by the railroad or phone companies.

As an electronics engineer, I attempted to simplify my original response as most people will not understand the internal theory. You seem to have a good practice understanding from your own experiences. The bottom line is yes it will work but not the best choice.
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Old 04-15-2015, 08:16 AM   #13
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Quote:
Originally Posted by shiggs68 View Post
Yes, I agree that practice vs ideal engineering is often two different cases.
Reminds me of a good quote, I wish I knew the attribution...
"In theory, there is no difference between theory and practice. In practice, there is."
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Old 04-15-2015, 11:50 PM   #14
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thanks for all the feedback..i think in the end, i'll go back to the original set-up...add 2 more 6's...i like to run the coffee maker in the a.m and the other appl's without a lot of worry..
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