Pond Boss
Posted By: Bing Parallel wiring of batteries--size of wire - 04/17/12 11:51 PM
I plan to use two 12 vt. batteries in parallel in my bass boat this year. The object is to create more battery life per charge with the same 12 vt. output. The batteries will power the trolling motor, depth finder, live well and bilge pump.

The batteries will be side by side, so the wires will be just a couple of feet long. What size wire should I use between the battery posts?
Bing -- Over the years I've helped a number of friends do what you want to do, but mostly in campers and motorhomes. What you want to do is very common. If you go to an RV/motorhome center, like Camping World, they should have complete cable kits for connecting the batteries in parallel. I'm guessing they have similar kits, for boats, at the big box sporting goods stores, like Cabellas, BassPro, and Gander Mountain. I just couldn't find any by searching web sites.

Regards,
Ken
Posted By: ewest Re: Parallel wiring of batteries--size of wire - 04/18/12 12:50 AM
I did that on feeders. I could not tell that it worked any better.
If you're close to an Interstate Batteries center, they'll have what you need.
Go with the same size as is going to your starter and your ground lug.Go to your auto parts store, they should have what you need for battery cables. You'll need cables rated at carrying the full load. I think it's #4 gauge, but dont quote me on that.
Posted By: Bing Re: Parallel wiring of batteries--size of wire - 04/18/12 02:21 AM
By the way, my boat does not have an engine. Use it on my 3 acre pond and bought it without engine or wiring for an engine.
I guess you cant match up with your starter then. sorry, laughing. You will want to find out the total amp draw from all appliances, as if they were to be running at the same time.A general rule of thumb, is 10 ga. = 30 amps,8 ga. = 40 amps. I'd find that out, then go the next size bigger, to ensure equal discharge.
Any auto parts store should have wire with lugs swaged on the ends. Shorter the better. I wouldn't go any smaller than 6 ga wire. Chevy diesel trucks use 2 batteries, and those cables would work, but they are longer than what you need.

If you can get wire with swaged lugs that are sealed, even better. (to prevent corrosion.)
Posted By: JKB Re: Parallel wiring of batteries--size of wire - 04/18/12 09:46 AM
Minimum 6 ga if only a foot or two. Not quite sure what my camper cables are, but pretty sure they are quite a bit larger than 6 ga. My medium duty jumper cables are 4 ga. Didn't want to spend the money on heavy duty ones.

You can't size wires for DC as you do for AC. Voltage drop is terrible for DC compared to AC, so the wires have to be larger.
If you have a load of a motor, and lets say for example it draws 40A at 12V continuous, you then wire another battery in parallel with the first, now each battery will only need to source 20A to the load. So you will still need the heavy gauge wire to the motor from the battery pack, but you now only need 1/2 of the wire diameter between batteries. With more batteries, you can get fancy with the wiring to minimize copper, but I don't need to go there.

With that said, you are likely better-off using the same gauge as to the load for terminal sizes and posts to match up.

What you will find at the end of the day is longer run time, longer charge time, and probably better starting performance as two batteries can better-supply surge current to a starting electric motor.
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