Connecting Two disimilar bateries with A-B-Both switch.

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Jim Hilburn

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Jun 13, 2017
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Hi, first post here.
I went to a bluegrass festival and was having trouble with a Walmart Group 29. I was in a line-up so couldn't go to Walmart so had Carquest deliver one, a group 27. Once I got back home, I took the Walmart back and they are replacing it.
This leaves me with 2 new but dissimilar batteries. I would like to set it up with a battery 1,2 or both switch so I can go between them as opposed to paralleling.
Now the tricky part. I have a 60 w. suitcase solar panel. What will happen if I disconnect the coach, put the switch on both and try to charge-maintain both batteries with the solar, given they are different kinds of batteries? Or should I just give them equal time on the charger, separate from each other? Thanks in advance.
 
Well first. the difference between a Grou 27 and 29 is not great.. But when it comes time to replace them PLEASE consider GC-2  YOu will gain about 10% more battery and just about double the usable power due to the GC-2 being a genuine DEEP CYCLE v/s the MARINE battery you have now.

Next.  Many will tell you a bunch of fiction about how the smaller battery will charge faster (not true) and signal the converter "I'm full" and the larger will never fully charge (Again not true).  NOTE: THIS I(S TRUE WITH BATTERIES IN SERIES, and that's where it comes from. but you are running in parallel.

There is no problem with runing a group 29 and a group 27 in parallel.

What will happen when charging/discharging?

Well the state of charge determins the battery voltage.. Thus the battery with the LOWER voltage (Lower State of charge)_ will suck most all the charge current till they are in sync. and there they will stay IN perfect lock step sync.

And on discharge the battery with the HIGHER voltage will provide all the current till they sync up again,

and once syced. be it charging or discharging,  they will form the perfect commune. Each providing power according to its ability, each eating charge current according to it's need. They basically become one battery.

The only difference is .. generally, the older battery will die first,  and when that happens it can take a bit of effort to figure out which one died.
 
John, thank you for that information. I'm going to be starting with 2 brand new batteries. Do you think it is OK just to parallel or worth installing the switch in case one battery was to fail?
By the way, I have installed a Powermax smart charge converter in place of the original.
 
I would just parallel them unless you have another reason for isolating them from each other. The two batteries are the same type (flooded lead acid) and age (new), and the difference in amp-hour capacity between a 27 & a 29 is only about 12%. Close enough to be considered "same batteries".
 
A further reason is that they will tend to balance when first connected in parallel. That could result in high current between the batteries if one has a much greater charge (higher Voltage) than the other.

Ernie
 
Thanks to everyone.
I just ordered some 5 guage jumpers. The RV is a 23 ft. BT Cruiser and we use very little power normally. However would like to be able to use the furnace for at least a couple of cycles per night without taxing the system too much. These should be OK for this application don't you think?
 
While on the subject, what's the better buy, GC-2 or AGM?
I'm sure there are plenty of discussions on this that I haven't looked up yet.
 
GC2  is a size, not a type, but the GC2 that RVers typically get is a 6v golf car deep cycle. They are produced in high volume for the golf business, so generally are hard to beat on a cost per amp-hour basis.  You can get size GC2 in the AGM type as well, but they are pricey.

The big AGM advantage is that they are truly maintenance free, whereas flooded cell deep cycles need some TLC at least annually. More often if the charging system is less than ideal. But 12v AGM deep cycles are a specialty battery type with a small market, so pricing is high. 

If you want to discuss RV battery type and criteria, please read my RV Battery article in the Forum Library first. See Choosing a battery
 
Jim,
I use a Boat 1,2 or Both switch with two golf cart batteries on one side and a Group 27 on the other.  I usually set it to both for charging and use unless I am concentrating on a specific battery situation.  If I use the furnace at night I may set it to one side or the other to assure I don't draw down all three batteries to a uncharged state but other than that my battery use is limited to a light load.  I do not use a 12v to 120V inverter.  That might alter my thinking.  All three batteries are holding up well after three years.
 
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