Posted by John T on October 30, 2011 at 07:23:34 from (173.243.181.15):
Thanks yall for your input on my post of yesterday where I was looking for engineering and scientific reasons, if any, why its better (or worse) to use two 6 volts batteries in series versus two twelve volt batteries in paralell if I want to maximize energy storage capacity (Amp Hours) in my RV for extended periods of dry (no electrical hook ups) camping like we do in Utah or Colorado BLM lands. Right now I'm running three identical Group 29 (125 AH each if I recall) 12 volt RV/Marine batteries in paralell (375 total AH) plus have a rooftop solar panel and an Onan genset if n when needed, by starting the RV evey few days if needed, the 80 amp alternator (plus solar panel) keeps the batteries topped off and I can stay (with no hook ups) about as long as the first wife can stand it lol
Here are a few comments/reasons some other electrical engineers provided plus an excellent website I got explaining the proper method to wire multiple batteries.
Ive learend from this experience (why I love yall and this website) and am sharing n passing this along FWIW...
Thanks again, An ever curious Old John T
There is a LARGE difference in the CONSTRUCTION of a 6 volt deep cycle battery compared to a 12 volt combination battery. solid lead plates vs wafer plates, wider spacing, plastic spacers vs cardboard spacers. 12 volt wafered plates are very easy to warp when deep discharges occur and this can easily short out plates. Even if you find a 12 volt that has half the power of a 6 volt there is still the longevity issue. I use 6 volt batteries about 8 years per set and have never gotten more than about 4 from 12 volt batteries before they're toast.
I believe that many of the popular Trojan 6 volt deep cycle (golf cart) batteries - that are in physical sizes "close to" the popular 12V 105-120 AH size batteries - run in the 210-230 AH range. Hence .... two of these Trojan 6's in series provides around 210-230 AH of total current flow driven by 12 volts of potential, while two of the 12's in parallel provides around the same 210-230 AH of total current flow driven by 12 volts of potential.
Also, the "two 6's versus two 12's" argument usually revolves around a two 6's advantage due to them being "true deep cycle batteries with thick plates" (so they supposedly last longer), plus other minor considerations such as there being plenty of room below the plates for sludge to accumulate without covering any of the plate area over time.
I don't buy either of the points above:
First - you gotta compare true deep cycle 6's to two true deep cycle 12's before buying, which is hard to do since these kind of 12's are hard to find reasonably priced, so most folks follow the crowd by going down the easy path and buying the 6's. However, two 6's in a double bank have to supply twice the current per battery whenever being used - so of course their plates have to be thicker to get the same life as two 12's in a double bank supplying one-half the current per battery. Remembering just enough to be dangerous of my chemistry courses, I'm assuming here that it's lifetime current delivered per battery that ultimately determines erosion/oxidation rates of the plates - hence the 12 V battery's smaller mass plates last the same length of time due to their being destroyed only around about one-half as fast over the battery's life.
Second - since each 12 volt battery supplies only one-half the total current over it's life, the amount of sludge that accumulates in the bottom is less, so the area below the plates doesn't have to be as large, anyway.
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