A Tale of Two Batteries - Fiberglass RV


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Old 12-15-2015, 06:50 AM   #1
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A Tale of Two Batteries

A Tale of Two Batteries

When I bought my Bigfoot, one of the features that appealed to me was that it came with two batteries, and both outside the cabin, so no storage space was sacrificed.
On the Bigfoot they are 12 volt batteries and so wired in parallel.
But in many discussions on the forums, 6 volt batteries wired in series are strongly preferred.
At the time I questioned that preference, thinking 12 volt units made more sense. My reasoning was/is that when one of the batteries dies, you are in much better shape if the remaining one is 12 volt and still able to power all the dc circuits. On the other hand, a single 6 volt battery is basically useless.
The counter arguments were that with only half the number of cells in the same volume, the 6 volt batteries will have heavier plates, provide more amp hours, and be more durable (last longer).
Certainly the second point makes sense.
Another argument was that in parallel, if one battery died, it would kill the other one as well, from the excess draw.
Because that viewpoint seems almost universal I had about caved and was planning to switch sometime soon. I figured my current 12 volt units were due replacement anyway.
In the meantime, from my battery monitor, I was not getting the charge life I would expect, but being inherently lazy, I didn't investigate whether that was true, or what the cause might be, since I was planning on new batteries anyway.
Then all of a sudden, after disconnecting shore power in preparation for a move, I had no 12 volt power at all. Ouch, what could that be? But I needed to get down the road and figured to investigate when I got to my next destination; my daughter's home. On the way I stopped at an Advance Auto and had the batteries tested. Sure enough, one read fine, at about 13v, but the other was essentially dead at about 0.6v. When i rewired for the one good battery, I've been good since, about three weeks now. At the time I was still in 6 volt thinking mode and asked if they carried any.They did not and didn't know where I could find them, another point in favor of a 12 volt choice when on the road.
With some more thought, the upshot of all this is that I'm now firmly back in the 12 volt battery mode. If I'd had 6 volt units and one failed, I'd have no easy way to continue. And it's clear that one dead 12v battery will not necessarily kill the other, at least not right away..
I'll still buy two new batteries soon, when I have time to decide which are the best available.
So the answer is that probably both arguments have some truth, but I now know which is best for me.

Walt
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Old 12-15-2015, 07:38 AM   #2
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Walt
I always used battery switches and manually selected batteries for charging and use with my sailboats (dual 12V batt.). Blue Sea has a device to control charging that goes along with their switch. The switches alone are much cheaper.
Eddie
http://www.amazon.com/Blue-Sea-Syste...lue+sea+switch

http://www.amazon.com/Blue-Sea-Syste...battery+switch
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Old 12-15-2015, 07:56 AM   #3
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I guess I need some edumacation here,
What's the purpose/advantage?

Walt
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Old 12-15-2015, 07:59 AM   #4
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You didn't mention the age of that failed battery or the caliber of the charging system you have. Both play into battery life and on-the-road failures.

While not unheard of, battery failures are fairly rare, considering proper charging and reasonable life expectancy. That said, the 6 volt batteries are more robust and failure is less likely, but reasonable care, proper charging and a realistic replacement time is needed. If one simply waits until a battery fails before replacement it's sorta like driving on a 12 y.o. tire until it fails, it's gonna happen, usually at the most inconvenient time.


One needs to be realistic about battery life, they don't last forever, but a good 3-stage charger will help it get closer.



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Old 12-15-2015, 08:08 AM   #5
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Good points. I put new batteries in when I bought the trailer less than three years ago, and installed a new 3 stage converter less than two years ago, as a Christmas present from my kids.

Walt
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Old 12-15-2015, 08:36 AM   #6
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The switch lets you disconnect the batteries for storage or choose which battery you want to use or charge. If you have a battery monitoring system it lets you look at each battery seperately. Also unless both batteries are selected one bad battery will not discharge the other battery. In a boat you don't want your house load to and discharge your start battery. Also in a boat you can select both batteries for starting if needed.
The switch will let you better manage your batterys and prevent you from being stranded with two dead batteries.
Eddie
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Old 12-15-2015, 08:41 AM   #7
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Thanks. I can see the advantages for a boat. Not so much for a trailer.

Walt
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Old 12-15-2015, 11:07 AM   #8
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I believe two 6 volt batteries are better than one 12 volt, but I do not see an advantage of two 6 volts over two 12 volts.
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Old 12-15-2015, 11:10 AM   #9
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Advantage of two 6V over two 12V is about 80 pounds.
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Old 12-15-2015, 11:22 AM   #10
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Originally Posted by WaltP View Post
I guess I need some edumacation here,
What's the purpose/advantage?

Walt
To what terminals were your charge wires connected?
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Old 12-15-2015, 11:49 AM   #11
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To those who know simple electrical circuitry and think which terminals don't matter, think again. I did. The imbalance in current draw caused by resistance in small gauge cabling can have a major impact.

Walt
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Old 12-15-2015, 11:53 AM   #12
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Originally Posted by dvrcdlady View Post
I believe two 6 volt batteries are better than one 12 volt, but I do not see an advantage of two 6 volts over two 12 volts.
Two batteries in series may not charge equally. If one is at a lower voltage than the other, the charger can't tell the difference and may overcharge the higher one, while never bringing the lower one up.
You ARE better with two 12 volt batteries in parallel. that way it doubles the time it takes to run them down.
As another person mentioned, there are devices that will charge each battery separately, so they stay in balance.
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Old 12-15-2015, 11:57 AM   #13
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If you have two batteries in series then the same charging current goes through both, no matter their conditions.
This is one issue where the plumbing analogy works exactly.

Walt
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Old 12-15-2015, 12:08 PM   #14
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If you have two batteries in series then the same charging current goes through both, no matter their conditions.
This is one issue where the plumbing analogy works exactly.

Walt
Right. and your OK as long as both batteries carry an equal charge. As long as the load draws equally from both. but, IF one battery becomes more discharged than the other, not so. Even something simple like a dirty terminal that causes more resistance in the wiring can upset the balance. Stay with a parallel circuit.
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Old 12-15-2015, 01:15 PM   #15
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Two batteries in series may not charge equally. If one is at a lower voltage than the other, the charger can't tell the difference and may overcharge the higher one, while never bringing the lower one up.
You ARE better with two 12 volt batteries in parallel. that way it doubles the time it takes to run them down.
As another person mentioned, there are devices that will charge each battery separately, so they stay in balance.
Holding many things constant:
one 12 volt battery consists of 6 cells in series in one box
one 6 volt battery consists of 3 cells in series in one box
two 6 volt batteries consists of 3 cells in series each box with a total of 6 cells in series in two boxes
The difference is packaging.

Not holding things constant (in the real world) often 6 volt batteries are designed for industrial or golf cart use and are made more robustly. They have heavier less porous plates and can support higher amp/hour capacity.
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Old 12-15-2015, 03:53 PM   #16
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Right. and your OK as long as both batteries carry an equal charge. As long as the load draws equally from both. but, IF one battery becomes more discharged than the other, not so. Even something simple like a dirty terminal that causes more resistance in the wiring can upset the balance. Stay with a parallel circuit.
The current must be equal between the batteries. They are in series, current is "stuff" and you cannot have more "stuff" going through one battery than the other since there is only one path. Any additional resistance caused by dirty terminals, etc will effect the current in both batteries.

As to the choice, I agree with the packaging description. For the same weight, and deep cycle attributes, there is little difference between a pair of 6V or a single 12V. You will usually pay less for the same amp hours & true deep cycle batteries purchasing 6V batteries than a single or pair of 12V batteries.

I just did a comparison between 6 & 12 Volt batteries for the same amp hours, and for around 232 amp hours you save over $200 going with a pair of 6V batteries than any combination I could find of single or dual 12V batteries.

On the other hand, if you run a high powered inverter, there is an advantage to a pair of 12V batteries (again, with the same amp hour and deep cycle attributes). A pair of 12V batteries will have lower internal resistance (because of the paralleling) so they are less likely to cause an inverter to shut down due to low input voltage. Only a problem with draws of 50 - 60 amps or more, so for the small inverter user not a problem. A solution used by larger RVs is to install 4 6V batteries in series/parallel, but that would be a bit heavy for small fiberglass trailers (typical weight of a 6V, 232 amp hour battery is 65 - 70 lbs each)
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Old 12-15-2015, 06:39 PM   #17
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If you have to have the back up capability provided by two 12 volt batteries then the argument (discussion) is over as you have decided. However, as initially attractive as the 2-12 option appears I see other things far outweighing any supposed advantage. For one thing, worst case I can get my trailer 12V needs met for a short time from the Tug just by plugging in the 7 pin. That will at least get me to the battery store if one or both 6V dies.

I looked at all of the options before I decided on two 6V golf cart batteries. My decision was made because two 6V batteries provided the most capacity (AH), for the cheapest cost, that still fit the foot print of the space I had for batteries.

I have also installed a new, appropriately wired converter/charger (PD Wizard) along with a Trimetric monitor. I plan to take care of my battery investment and given that I doubt one of them would up and die without notice.

To the OP, where the batteries in your Bigfoot real deep cycles or something else?
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Old 12-15-2015, 06:46 PM   #18
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I replaced the batteries as soon as I bought the trailer, less than three years ago. They were not researched for quality at the time; just AGM with good dates from Advance Auto. I needed something quick and figured they'd do for the short term. Yup, pretty short I'd say, but no regrets. Now I have time to make a more informed, long term choice and spend some real money if needed.

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Old 12-15-2015, 06:53 PM   #19
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The two 6 volt batteries in series are electrically better for the above reasons. The charge currents are the same. As long as the 12 volt batteries are matched then they will charge equally. Different internal resistance and then the charge curents (as above) are different.
To make a simple observation the amount of power you can get out of batteries is roughly equivalent to the weight of the things.
Two 6 volt golf cart batteries would probably be best.
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Old 12-15-2015, 06:59 PM   #20
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Okay, I had typed out a long, detailed response and then scrapped it to avoid contention. But now Jon has stated the main point and I think it's important.
It doesn't matter what the condition of the batteries, the connectors, or the cables. The current is travelling in a single, closed loop and can only be the same at every point in the loop when the batteries are in series, so the charging current can only be identical in the two batteries. It's necessary to recognize that in order to understand anything in the system.

Walt
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