A Tale of Two Batteries - Page 2 - Fiberglass RV


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Old 12-15-2015, 01:15 PM   #15
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Originally Posted by Wayne Collins View Post
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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Originally Posted by Wayne Collins View Post
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.

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Old 12-15-2015, 08:10 PM   #21
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... To make a simple observation the amount of power you can get out of batteries is roughly equivalent to the weight of the things. ...
That's not guaranteed but I'd bet on it (for known trustworthy manufacturers).

Quote:
Originally Posted by Jon Vermilye View Post
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. ...
If anyone is interested Kirchhoff's circuit laws explains this. I went to the Wikipedia entry ( https://en.wikipedia.org/wiki/Kirchh...s_circuit_laws ) but it isn't for the faint or heart. I'm no engineer and I had to do a search to remind me of the name Kirchhoff.
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Old 12-16-2015, 07:52 AM   #22
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That's not guaranteed but I'd bet on it (for known trustworthy manufacturers).
"The race does not always go to the swift, or the contest to the strong, but that is the way to bet".

I just replaced the battery in my old daily driver, a Toyota Corolla Wagon. The "recommended" battery was a short little thing that came with a thick plastic spacer underneath to shim it up to fit the battery hold-down. I'm old school and was not having any of that. I asked for the best ($$$$) battery, and was rewarded with a full size (and MUCH heavier) battery that fills the whole space with actual dadgum BATTERY.

Walter, I wonder if the charger in your BF was correctly charging the AGMs? Isn't the optimum charging profile different for AGMs than flooded? You might want to sort that out as part of your battery purchase considerations.
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Old 12-16-2015, 09:08 AM   #23
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I am an old electrical guy and in my old fashioned way of thinking power and weight go together pretty closely. The new switching power supplies are a noticeable exception, but batteries of similar technologies are not.
If you are trying to charge batteries in parallel then they must be matched pretty closely or one will take most of the charge and the other one may not be completely charged.
When I was younger I worked on "foreign" cars (MG Bs especially). These cars had two six volt batteries and the first cold snap we stayed busy replacing batteries since in these things if one battery is bad then neither will charge or start.
So you pays your money and takes your choice. Basically either way you need pretty much matched batteries in the same condition.
Still for a small RV I think that two six volt golf cart batteries would be best if you want the capacity of two batteries. If you think of it your 12 volt battery is made up of six two volt cells and the two six volt batteries are made of of (wait for it) six two volt cells. They are just located in tow separate boxes.
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Old 12-16-2015, 01:03 PM   #24
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I re read some of the above posts particularly on the parallel 12 volt batteries providing more current capability the issue is the internal resistance of the batteries. while the internal resistance of the 12 volt batteries would be halved the true test is the internal resistance of the 6 vs the 12.
As usual Ohm's law will give some insight.
What is the result of the low internal resistance? the answer is the short circuit current and the terminal voltage under those conditions. The voltage drop under short circuit conditions is due to the internal resistance. If you check the short circuit current of the 6 volt batteries would be better than the 12 volt and the current available under discharge conditions would be better as well.
Still the available power will be roughly equal to the total weight for the chemical reactions given the same technology for each.
If you have room for two golf cart batteries this would probably be the way to go.
If you looked at the area of the plates per cell the cells of the six volt and the two 12 volt batteries is probably about equal.
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Old 12-24-2015, 06:08 AM   #25
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Never at a loss for learning something new here

I just love reading posts on this forum. Always reading something new. I'm hoping my battery questions fit on this thread, but if not, please kick me off moderator.

I have the Northern deep cycle battery that my used 2012 Scamp came with. PO just kept trailer plugged in to shore power all winter to keep battery charged, while I keep it in the garage on a battery tender (good one, with a charge and float mode) all winter, switching it off to charge my generator and lawn tractor battery every 3-4 weeks or so.

Might I expect after 3 years that my battery is on the way out? During the summer months, in between camping, I put the battery on the tender, as previously outlined. Also, I know battery charges while connected to the tug, and I believe it charges while connected to shore power, but I think I just learned what those PD wizard chargers are....Is the problem with a regular converter/power panel (one Scamp puts in as standard) that it puts a continuous charge on the battery, rather than having a "smart charger") which will recognize when batt needs a good charge, and then sort of cycle on and off if you will to maintain it (similar to my Battery Tender)?

Wondering how all these things might affect the life of my original, now 3+ year old battery.

Cheers,
Wendy
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Old 12-24-2015, 07:14 AM   #26
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The old constant charge converter/chargers will shorten the life especially if you don't keep an eye on the electrolyte level.
If you find the "water" had evaporated/boiled down then as long as the plated are covered do nor add water until the battery is charged (if in a discharged state) If you service a battery with low water in a discharged state the electrolyte will "boil over". The electrolyte will expand and push out through the vents and make a mess and also lose acid.
Checking often is important in this case.
I changed the old converter for a PD unit for the three level charge maintaining with the periodic "gassing" to keep the battery tuned up.
Personally I think that batteries last as long as they last.
I have had pretty good luck through the years, but I think luck comes with paying attention.
My next project (after finishing my rebuild) might well be installing solar panels for the just in case for hurricane season. That just in case goes with my Harbor Freight inverter generator (that is really pretty good).
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