Posted by John T on October 29, 2011 at 11:59:42 from (173.243.181.15):
This is more of an RV question but can apply to tractors or my RV when I go to a Tractor Show, I also posted it on RV Net so here goes:
Okay, I'm a retired Electrical Engineer so understand most of this topic, HOWEVER Im always puzzled when I hear someone say "its better two operate two six volt batteries in series then one twelve volt" (or two 12's in paralell if you run two batteries).
FIRST THE GIVENS: Now if maximizing your total "stored energy" is your main concern (thats mine when I dry camp), let me offer this.
1) Stored Energy is measured in Volts x Amps (instantaneous power) x Time, thats what the utility charges you (Watt Hours) for and thats one battery energy storage rating i.e. its Amp Hours of STORED ENERGY.
2) If you run two 6's in series, the voltage is additive, but the CCA nor the AmpHrs of stored energy is additive i.e. if you have two 115 AH 6 volt Trojan Golf Cart batteries wired in series, the total AH the two can store remains only 115 AH (its just that its now at 12 volts. The thing is the two in series cant pump out any more amps then either one can (CCA doesnt double), the amps dont add like if they were in paralell. Similar, if one can provide x amps at 6 volts for x minutes (stored energy), then two in series still can still only deliver that same x amps for the same x minutes. CCA NOR AH ADDS IF WIRED IN SERIES. In a series circuit the amperage is the sameeeee.
3) HOWEVER if you place two 12's in paralell BOTH the AH and CCA is additive i.e. if you have two 115 AH 12's in paralell, the total stored energy is now 230 AH AND MORE STORED ENERGY IS WHAT I WANT DRY CAMPING. Similar, if one can pump X amps of CCA, the two in paralell can pump out 2X of CCA.
4) Now, deep cycle Marine or RV or Golf Cart batteries are ALL designed more for a slower deeper discharge and then they get replenished versus an auto starting battery. You hear people bragging about using two Trojan 6 volt golf cart batteries in series, but they, like marine or RV deep cycles, are still designed for deep cycle use, so unless theres some hugeeeeeee difference in quality and design I just dont see why the two 6 volters in series is so much better????????
5) CAVEAT its true if two are in paralell and one is bad it can cannibalize the other. Thats why I ALWAYS run the same type size brand and age of batteries if wired in paralell. However, this isnt quite as bad with two in series (cannibals) although the two are no better then the weakest.
QUESTION So if I want to maximize my STORED ENERGY I say (and I do) use two twelve volt batteries in paralell instead of two six volt batteries in series. Now tell me why thats wrong if you can show it is????? I'm lookin for science and engineering here not just me n dad prefer two 6 volt Trojans and used em for years. If AH and CCA adds if wired in paralell but not if wired in series and more AH of stored energy is my goal (more dry camping) THEN WHY WOULDNT I USE TWO 12 VOLTERS IN PARALELL?????
As Jimmy Buffett sings in "He went to Paris lookin for answers" Im lookin for answers to questions that bother me so lol
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