Sizing parallel battery cables

Bluedog225

Texas
Joined
Nov 18, 2019
Messages
484
I have two 12 volt, 100 amp/hr batteries (battle born) that I intend to run parallel. I’ll be running various 12 volt, low amp loads (lights, tv, radio). Charging will be from 300 watts of solar via a small controller.

What is the right way to think of the load between the batteries? Is it based on my maximum anticipated load or charging? Or 1/2 the load.

Short, heavy cables, I get. But seems like there is some math here I don’t understand.

Thanks
 

DThames

Photon Sorcerer
Joined
Nov 22, 2019
Messages
1,440
If you get a large stud type terminal block and wire both batteries to that block, with equal type/length cables, you can assume half the load from each battery. That would be the easiest way to know you are correct.

If you are going to put the batteries side by side and have a longer single run to the point of connection, I would just use the same cable between the batteries as for the main run. I would do that rather than worry with getting a different size cable/lugs and messing that.

edited to add....the load between the batteries will be about half in the second example above. First example almost zero current between the batteries. Almost zero unless the batteries are "exactly" the same.

Many builders suggest connecting main runs on each end of parallel group (below). It provides a better chance that equal power will be pulled from all parallel batteries.

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Bluedog225

Texas
Joined
Nov 18, 2019
Messages
484
This is one of those times I wish I were smarter (chuckle).

Arranging it per your diagram makes sense.

And I’ll size the cables between the batteries to the maximum load expected out of the combined batteries. If I understand correctly, this will necessarily provide a safety factor.

Thanks!
 

740GLE

Solar Addict
Joined
Apr 30, 2021
Messages
639
It’s always good to size wire gauge for future needs, at a minimum I’d say 4awg or 2awg should future proof your set up if you end up adding another battery and few more panels.
 

Alphacarina

Solar Enthusiast
Joined
Feb 22, 2021
Messages
214
I agree. Paralleling cables are short, there's little money to be saved by going with a smaller gauge. I'd go with the 2 AWG myself, no matter that load or charge current you expect to have

Don
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
1,682
Location
USA
Interesting update. BB recommends 1/0 cables.
Ultimately the correct wire choice is based on the current that will be flowing through the wires. Work out what your max current will ever be in your system. Let's say you will be charging at 50A. And let's say you only have DC loads that could add up to a max of 400W any one time. That's 400W / 12V = 34A. Clearly 50A is the higher current so you can choose wire appropriate for 50A which would be 6AWG. Or let's say you want to hook up a 2000W inverter. Now you have 2000W / 12V / 0.85 = 196A. Now you need to wire for 200A which would be 2/0AWG wire.

As you work this out, plan for the future. Your needs might change. Even if you have no inverter now, you might want to add one later. So it might be easier install the large wire now and then not worry about again. Or add the smaller wire now and replace it later if needed. Of course if you put in 2/0AWG wire now and then later decide you really need a 3kW inverter instead of 2kW, then you need to replace the 2/0AWG with 4/0AWG.
 
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