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Old 09-29-2013, 07:29 PM   #11
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Originally Posted by 1A Camper View Post
I have 2 group 27. THe group 24 battery case had to be changed out as it was too small. Also could not use the mounting frame for the group 27 either, had to make new frame. I use my battery one at a time. I add a switch on each. Only been out 2 day at a time. and 1 battery is doing fine for that.
You do know that you get less total capacity with the batteries separate than linked together???
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Old 09-29-2013, 08:05 PM   #12
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I upgraded my crappy Group 27 for a nice AGM Group 31 Deep Cycle. Last night I ran the furnace most of the night (probably kicked on a dozen times for about 10 minutes each) and my voltage was still 12.5 this morning, even after I recharged my kindle and phone overnight as well (I installed 12v USB plugs). Coupled with LED bulb replacements, a cut-off switch for the radio (so it doesn't phantom drain) and my power use is pretty minimal minimal unless I intend it (really the only phantom draw left is the CO/Propane detector in the power station).

I definitely recommend the replacement/upgrade to a Group 31. If you can fit two of them (I can't due to space and tongue-weight limits) you can do it either with two Group 31s in parallel, or a pair of 6volts in serial to equal 12v. Both work and there are pluses and minuses to both.
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Old 09-30-2013, 07:24 AM   #13
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You do know that you get less total capacity with the batteries separate than linked together???
The advantage of separate batteries is that one can be removed and taken someplace for charging with minimal effort. It is great for dry campers staying in one place for a while.
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Old 09-30-2013, 10:19 AM   #14
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The advantage of separate batteries is that one can be removed and taken someplace for charging with minimal effort. It is great for dry campers staying in one place for a while.
Yes, I am sure there are many reasons to run batteries separately, and that is a good one and it would be a good reason to have 2-12volts in parallel instead of 2-6 volt in series.

It is also true that you get somewhat reduced capacity because of Peukart's law and so in a situation where you really need all the amphours you can get, you may want to hook the batteries together.

Just some ideas to consider.
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Old 09-30-2013, 10:53 AM   #15
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Originally Posted by garbonz View Post
Yes, I am sure there are many reasons to run batteries separately, and that is a good one and it would be a good reason to have 2-12volts in parallel instead of 2-6 volt in series.

It is also true that you get somewhat reduced capacity because of Peukart's law and so in a situation where you really need all the amphours you can get, you may want to hook the batteries together.

Just some ideas to consider.

Here is a great article on the Peukart Effect.

Since amp draw determines total available capacity, (more amps; less capacity), sharing the amp draw across more batteries will result in a SIGNIFICANT improvement in battery life under load.

Remember using 2 6 volt batteries double the voltage and not the capacity, so the amps out of both are the same and the effect is felt equally on both batteries.

With 2 12 volt batteries you double the capacity and halve the amp draw on each battery.

So using the graph below with a 100 AH "stack" of "notional" batteries (2 100 AH 6 volt batteries or 2 12 volt 50 AH batteries), examine the capacity loss across the stack for a 30 amp continuous draw.

In the 6 volt scenario, your 100 AH stack will have its total available capacity reduced to 54 AH.

In the 12 volt scenario, the 100 AH stack (since the 30 amp draw is shared between the 2 12 volt batteries with each matched battery contributing 15 amps) will only be reduced to 68 AH.

Obviously, if the 12 volt batteries are not matched internally, they will have dissimilar amperage flows and the capacity will be reduced from the optimum 50% share. Matched means similar manufacturer, size, capacity, charge discharge cycles, and age.

If one (the lower internal resistance) is contributing 75% (22.5 amps) and one (the higher internal resistance) is "loafing" at 25% (7.5 amps), the effect will have a serious degradation on overall battery "stack" life, as the higher voltage (loafing) battery will start trying to charge (equalize with) the lower voltage (harder working) battery.

This "self charging"/reduced capacity effect with non-matched batteries does not happen with 2 six volt batteries. The older 6 volt one will just die even though the other is still "good" resulting in the entire stack dropping offline.

This is why changing out the entire stack (rather than just one) is critical in multi-battery installations in order to keep things in balance whether 6 or 12 volt.
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Old 09-30-2013, 01:25 PM   #16
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I've got to wonder, in a boondocking situation, how often the current draw is much over 5 amps. If you want your batteries to last, I wouldn't think it would not be high current for very long and not very often.
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Old 10-01-2013, 12:01 AM   #17
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I've got to wonder, in a boondocking situation, how often the current draw is much over 5 amps. If you want your batteries to last, I wouldn't think it would not be high current for very long and not very often.
Yep. What kills batteries are inverters, furnace, and incandescent lights.
Pretty much in that order.

a 2 921 bulb fixture draws 2.3 amps

a 400 watt inverter powering a 200 watt appliance draws 17 amps

Cut from another post:

LPG detector draws .2 amps
Single dual bulb light (198 bulbs) 2.3 amps
Single dual bulb light (LED panels) .1 amps
Light fixture over dining table (4 198 bulbs) 4.5 amps
Single reading light over bed (1 198 bulb) .8 amps
Recessed Halogen lighting, living room (6 lights) 8.4 amps Full on
2.1 amps Fully dimmed
Recessed Halogen lighting, over sink (1 light) 1.4 amps Full on
.4 amps Fully dimmed
Recessed Halogen lighting, over couch (3 lights) 4 amps Full on
1.2 amps Fully Dimmed
Refrigerator 1.2 amps
Furnace
(Suburban SF35) 7 amps
Water Pump (Shurflo Smart Sensor 5.7) 11.9 amps running, 12.6 max
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