Which is better - (3) 12v batteries vs (2) 6v batteries

Hi BFL13,

Thermosiphon would work--but I'd want a 12 volt control valve and a thermostat.

Hi,

That is unwise advice. AGM's do fail and can create poisonous fumes when they do. It is prudent to have them in a sealed container which is vented to the great outdoors.

tplife wrote:
BFL13, AGM batteries don't have to be outside and don't have to be vented, so having them "warm" inside is a non-issue as well.

Quote by Pianotuna:

"Generally speaking, Optima are not as high a capacity in amp-hours "regular" AGM units such as Lifeline."
Yes, if you're comparing apples to oranges. Optima sells a group 24 Blue-top, the D-31M, in 75 Ah, and besides being less expensive, it has more reserve capacity than the Lifeline GPL-24T. I'm not that familiar with Lifeline, but I know that Optima makes more models than just the one I have.

BFL13, AGM batteries don't have to be outside and don't have to be vented, so having them "warm" inside is a non-issue as well.

pianotuna wrote:
Hi BFL13,

Under heavy loads use all the batteries. It is possible to bank switch, as I isolate my two banks. Charge them separately, or even charge one bank while using the other. This is especially nice when there is some solar energy to be had.

The only potentially safe way to use the heater exhaust that I can think of, is to have double walled tubing acting as an additional heat exchanger/extension exhaust pipe. Possible but pricy.

how about this "out of the box" idea to keep batteries warm. run a pipe from the hot water heater through the battery compartment to warm the batteries. cover the battery boxes with foam insulation to retain heat. if done right it should thermosiphon so you don't need the water pump to circulate the water. Use more propane, but for us that wouldn't be an issue.

Hi BFL13,

Under heavy loads use all the batteries. It is possible to bank switch, as I isolate my two banks. Charge them separately, or even charge one bank while using the other. This is especially nice when there is some solar energy to be had.

The only potentially safe way to use the heater exhaust that I can think of, is to have double walled tubing acting as an additional heat exchanger/extension exhaust pipe. Possible but pricy.

ktmrfs wrote:

BFL13 wrote:
I am not thinking so much of long battery life but more of getting through a long weekend in cold weather without needing a recharge before going home.

With the four 6s doing both furnace and inverter stuff and needing to stay above 50% to run the inverter, no can do. But if the 6s were free of inverter and now could run down to 30%, this could be done I think. I'll find out this winter.

You do lose on Peukert from not having all your batts in one bank, but you gain by being able to go to 30% on the 6s. I have no measurements of how that works out. Another situational thing I suppose.

The inverter use is controllable (eg choice of electric kettle or pot on the propane stove), but furnace use is not (for us!) Staying warm is a requirement. :)

Good explanation of the 24v idea. Especially since I don't want to get a 24v inverter now while the 12v ones are working just fine.

If you did have to recharge the pair of 12s before going home and the 6s were still above 30%, then your gen time won't be so much as doing four 6s, so that could be nice. Just have to see how it all works out this winter.

EDIT

PT, I didn't forget Peukert (see above) but I got forced into split 12s and 6s by Mex who warned me that my beat-up 12s would kill my nice 6s if banked. (I am not sure if that is true for temporary banking just when actually camping) Once I started the split bank thing, these other factors started to come into play. I am liking have the two banks more an more. Also don't forget I have four 6s plus the two extra 12s I got by chance, so I am not exactly hurting for AH, Peukert or not.

I think the same would apply if you had the more common set of two 6s and one 12 in a three-batt tray that could hold three 24s by footprint. If you only had a smaller inverter to do the TV/dvd, you could do some calculating to see if there would be any advantage in splitting with the inverter on the 12 and the rig on the 6s.

Remember, battery capacity drops significantly as temps drop. So wintertime camping hits you twice. First, the furnace needs to run more often, Second, the batteries, unless they are stored inside a heated compartment are losing AH capacity at low temps.

But I like your strategy, curious to see how it works out.

I wouldn't call it a strategy yet, but more of an experiment.

Camping in February here makes you aware of the temp/capacity issue. I have tried to account for that in my ugly graphs I post from time to time :)

The batteries inside where it is warm has always been a goal, but the complications of being in where it is warm and also ventilated are difficult. If you are brave (or optimistic!) with your AGMs, that can have them warm and able to do the inverter thing well in comparison with Wets.

I have always wanted to do something with the furnace exhaust heat, that goes to waste outside, to keep the batteries warm. Not got around to solving that either. (External battery bank where the exhaust passes through their container?)

Too many RV schemes, not enough time!

It is very interesting to me that the OP hasn't responded to any of the discussion points in this post... :?

As has been previously stated, in order to make an informed recommendation between using three Group 24 batteries or a pair of 6V GC batteries, we need to know a lot more information about the the usage scenario; What is the typical current draw? What is the expected max current draw? Have you done a daily power budget? How much solar are you considering adding? Do you plan to run the generator daily in addition to the solar? Why do you want to use AGM batteries? How will they be charged?

Given the limited information provided, and considering the fact that the OP said "the electric loads are modest" and wants a solution that provides "the most amp-hours at the best price", the leading candidate is a pair of reasonably priced 6V FLA GC2 batt's... such as the ones that are available from Costco or Sam's Club. With the tiniest bit of effort given to installation and maintenance, these will provide good capacity (200+ A/h) and offer the lowest dollar cost per Amp-hour delivered over their service life.

If the OP frequently boondocks, or has a high power inverter and/or a high current charger, or a mounting location with limited access/ventilation, or an unlimited $ budget, etc, etc...I might change my recommendation.
But for now I'm gonna say; (2) 6V batteries is "better" than (3) 12v batteries.

-Mark

BFL13 wrote:
I am not thinking so much of long battery life but more of getting through a long weekend in cold weather without needing a recharge before going home.

With the four 6s doing both furnace and inverter stuff and needing to stay above 50% to run the inverter, no can do. But if the 6s were free of inverter and now could run down to 30%, this could be done I think. I'll find out this winter.

You do lose on Peukert from not having all your batts in one bank, but you gain by being able to go to 30% on the 6s. I have no measurements of how that works out. Another situational thing I suppose.

The inverter use is controllable (eg choice of electric kettle or pot on the propane stove), but furnace use is not (for us!) Staying warm is a requirement. :)

Good explanation of the 24v idea. Especially since I don't want to get a 24v inverter now while the 12v ones are working just fine.

If you did have to recharge the pair of 12s before going home and the 6s were still above 30%, then your gen time won't be so much as doing four 6s, so that could be nice. Just have to see how it all works out this winter.

EDIT

PT, I didn't forget Peukert (see above) but I got forced into split 12s and 6s by Mex who warned me that my beat-up 12s would kill my nice 6s if banked. (I am not sure if that is true for temporary banking just when actually camping) Once I started the split bank thing, these other factors started to come into play. I am liking have the two banks more an more. Also don't forget I have four 6s plus the two extra 12s I got by chance, so I am not exactly hurting for AH, Peukert or not.

I think the same would apply if you had the more common set of two 6s and one 12 in a three-batt tray that could hold three 24s by footprint. If you only had a smaller inverter to do the TV/dvd, you could do some calculating to see if there would be any advantage in splitting with the inverter on the 12 and the rig on the 6s.

Remember, battery capacity drops significantly as temps drop. So wintertime camping hits you twice. First, the furnace needs to run more often, Second, the batteries, unless they are stored inside a heated compartment are losing AH capacity at low temps.

But I like your strategy, curious to see how it works out.

I am not thinking so much of long battery life but more of getting through a long weekend in cold weather without needing a recharge before going home.

With the four 6s doing both furnace and inverter stuff and needing to stay above 50% to run the inverter, no can do. But if the 6s were free of inverter and now could run down to 30%, this could be done I think. I'll find out this winter.

You do lose on Peukert from not having all your batts in one bank, but you gain by being able to go to 30% on the 6s. I have no measurements of how that works out. Another situational thing I suppose.

The inverter use is controllable (eg choice of electric kettle or pot on the propane stove), but furnace use is not (for us!) Staying warm is a requirement. :)

Good explanation of the 24v idea. Especially since I don't want to get a 24v inverter now while the 12v ones are working just fine.

If you did have to recharge the pair of 12s before going home and the 6s were still above 30%, then your gen time won't be so much as doing four 6s, so that could be nice. Just have to see how it all works out this winter.

EDIT

PT, I didn't forget Peukert (see above) but I got forced into split 12s and 6s by Mex who warned me that my beat-up 12s would kill my nice 6s if banked. (I am not sure if that is true for temporary banking just when actually camping) Once I started the split bank thing, these other factors started to come into play. I am liking have the two banks more an more. Also don't forget I have four 6s plus the two extra 12s I got by chance, so I am not exactly hurting for AH, Peukert or not.

I think the same would apply if you had the more common set of two 6s and one 12 in a three-batt tray that could hold three 24s by footprint. If you only had a smaller inverter to do the TV/dvd, you could do some calculating to see if there would be any advantage in splitting with the inverter on the 12 and the rig on the 6s.

Hi BFL13,

Your eyes may be open--but they are blinded by not seeing Puekert. If you use the twin twelves with the twin sixes as one large bank feeding the inverter you will have more power longer with a lower over all depth of discharge.

Many inverters alarm at 11.5 volts.

I would not take six volt batteries to 30% state of charge let alone 20%.

The twin twelve volts in my former RV did last for 9.5 years on usage of 50 days per year, until the new owner thought that the old magnatronic converter should be plugged in for months at a time. Even after that, the solar system "reserected" the bank enough to allow for lighting and running the fridge.