Batteries in parallel: A dance with the devil?

[KMAG YOYO]

OK, sorry for the clickbait title. I've read all the info that says running batteries in parallel is asking for trouble. But, there are several advantages:

-Lead-Acid batteries discharged at a slower rate are able to use more of their capacity. The current draw on each of two batteries in parallel is half the draw on a single battery with the same load. See my other thread: The reality of batteries in which we learn that 1+1=2.7

-12v Deep-cycle batteries are cheap!! Costco sells group 24's for about $70 (last I checked). So I can get two 75 Ah batteries for $140 or a single 150 Ah battery for $250+. 6v golf cart batteries ain't cheap either.

-Proper connections allow even loading of parallel batteries, preventing one battery from charging/discharging more than the other. That's my theory anyway.

This all assumes that you start with batteries of identical model and age. The Kirkland batteries are relatively inexpensive so I think I might guinea pig this thing for y'all. Anybody have evidence to prove me wrong before I take the plunge?

 

[ccbreder]

It all depends on how much amp hour capacity you need or want. group 24's are not deep cycle batteries no matter what the label states. Two six volt golf cart batteries are true deep cycle. In series connection they give you 12 volts and 235 amp hour rating. Cost less than $110 each. Will last longer than any 12 volt group 24. All the things you say about proper connections is well proven and you are correct.

 

[BradKW]

I've been click-baited.

 

[SternWake]

Most issues with paralleled batteries are due to installer error or ignorance.  6v batteries in series removes the possibility of unbalanced wiring.


A pair of group 24s in parallel will not last half as many deep cycles as a pair of GC batteries in series, even if they were discharged to the same percentage and otherwise treated the same.

Excepting AGMS, there are only a few true deep cycle 12 volt batteries.  Any battery labelled marine/trolling/RV/Deepcycle is just a starting battery with slightly thicker plates.  If high inverter loads are not required,  and the battery compartment is 11.5" tall or higher, the person buying a pair of group24's over a pair of gc-2's is simply shooting themselves in the foot.

If one only needs 150AH total, then look at the Trojan-t-1275.  It is one of the few true flooded 12v deep cycle batteries, but will still not have the longevity of the T-105 and the absorption stage will take longer than on GC-2s.

As always the battery longevity is most determined by how well it is recharged.  Promptly and to a true 100% for maximum longevity.  Anything less will yield less lifespan, to some degree or another.  100% on wet battery is only able to be determined by a hydrometer.  How many amps it accepts at absorption voltage is a good clue too, but can be misleading on a sulfated battery.

6v gc-2 batteries are easier to recharge fully, requiring less time at absorption voltage than 12v marine/RV group 24/27/29/31 batteries for the Specific gravity to max out.  A battery 100% recharged cycle after cycle will last twice as long as the battery recharged to only 95% cycle after cycle.  Most solar controllers on the stock charge algorithm and automatic charging sources flash the green light/ indicate full charge in the 92 to 95% area, yet this is enough for most people to claim their batteries are full, completely erroneously.  Most smart chargers have to be tricked, by loading the 'fully charged' battery until voltage drops below ~12.7v and restarted, multiple times, before Specific gravity will max out.  Especially on a deeply cycled battery, and especially a 12v flooded marine battery, and especially a battery that has been cycled many times without reaching a true 100% state of charge.

More capacity, true deep cycle plate construction, easier to recharge fully, double the cycle life, at least. 
 The only way in which two group 24's would be superior is under huge inverter loads, they will hold a higher voltage for longer before the low voltage alarm on the inverter starts screaming.

Call yourself a Guinea pig, but this experiment was figured out long ago and the loser is the group24 battery, or any wet/flooded 12v battery compared to a GC-2 6v in deep cycle duty.

AGMs are another story with their own issues + and -.

 

[ZoNiE]

Just go buy two 6V GC2's and run them in series. Much better battery, much better life. Just replaced my old 5 year old Costco sourced Interstate GC2's with two new Trojan T-105's. On Solar, I should get 10-12 years out of them...

 

[highdesertranger]

I would like to know where the info came from that running batteries in parallel is asking for trouble? I have been doing it for 40 years. diesel trucks all come with 2 batteries wired in parallel from the factory. what others here have said about 6v batteries in a series is true, you will get much longer battery life. highdesertranger

 

[KMAG YOYO]

Thanks indeed for the education. Part of my motivation is availability, there aren't a ton of places locally you can get a decent price on a deep cycle battery. Because of the weight and the fact that they are made of hazardous materials means buying online = expensive shipping. If I can find GC-2's at Costco for a decent price then I'll go that way.

 

[SternWake]

The initial outlay might be more expensive, but when they last twice as long the savings is real. 12 volt group 24/27/29/31 wet flooded batteries at best, are marine batteries, and even the best of these( Trojan, Crown,Deka,USBattery), can only achieve half the cycle life of a 6v golf cart battery.

The flooded marine batteries sold in AP stores/wally world are usually only able to achieve 3/4 the cycles of the 4 quality brands listed above

http://tools.trojanbattery.com/dealers.aspx

 

[ccbreder]

Costco, Sam's Club, and my favorite, Batteries Plus Bulbs. They all have golf cart batteries. I bought Duracell labeled, 235 amp hour at $109 each at Battery Plus Bulbs. I have six batteries connected in series as three sets and then paralleled for 12 volt bank.

 

[SternWake]

Depending on the Location, i believe the Duracell GC-2 are EastPenn/Deka made, and a very good GC-2 battery.

The minor price increase for so much extra lead and capacity and longevity should be a no brainer. The 12v options in this range simply cannot handle deep cycling the way a GC-2 can.

 

[KMAG YOYO]

I went by my local Batteries Plus, you're right you can't beat 235 Ah for $220. GC-2s are the way to go for sure. 

Thank you for showing me The Way of Battery Virtue and Goodness.

 

[ccbreder]

You may have to pay a recycling charge or provide old batteries as exchange. My store worked with me on this.

 

[PapaMuns]

The more I read about golf cart batteries the more I like the idea of them.  Before I really started researching I assumed that deep cycle batteries in a parallel system couldn't be beat.  I've never been able to convince a battery company to waive a core charge.

 

[ccbreder]

Once the owner helped me as he had scored before my purchase. The next bank I helped a friend clean up his garage by taking his old batteries.

 

[MikeRuth]

The more I read about golf cart batteries the more I like the idea of them.  Before I really started researching I assumed that deep cycle batteries in a parallel system couldn't be beat.  I've never been able to convince a battery company to waive a core charge.

I'm not saying they will every time but At Battery Company, Inc. in Valencia did indeed wave a core charge on my purchase. 
I did bring in one regular deep cycle battery when purchasing two T-105's, they waved the core charge for me on the second one. 
the T-105's were dated less than 60 days old. 

If in So cal, http://www.atbatt.com/contact-us

Mike R

 

[AuricTech]

I would like to know where the info came from that running batteries in parallel is asking for trouble?  I have been doing it for 40 years.  diesel trucks all come with 2 batteries wired in parallel from the factory.  what others here have said about 6v batteries in a series is true,  you will get much longer battery life.  highdesertranger

I seem to recall reading on the NAWS forum (linked in my .sig file) that having more than two batteries in parallel is where issues of uneven loads/uneven charging become a problem.

 

[ccbreder]

It all depends on the cables. http://www.smartgauge.co.uk/batt_con.html