Connecting 12 Volt Batteries in Parallel

[charliem]

[font size="3"]Elsewhere on this forum there has been some discussion of mounting extra batteries on the rear bumper to shift weight off the tongue. While it can be done, connecting two 12V batteries in parallel, but physically located apart such as front and rear of the camper, is not a trivial exercise. In order to maintain maximum performance and life certain conditions must be met. First, the batteries should be of equal capacity, from the same vendor, and of the same age. Second, the path from charger/load to and through each battery must be electrically identical. This means equal lengths of equal sized wire or equal resistance paths. Doing this ensures both batteries will share equally in charge and discharge current. Failing this condition the larger capacity battery and/or the one connected with the lower resistance path will hog the charge/discharge current, thus limiting total available capacity and shortening its life. It's not as simple as just running two wires between the batteries and tying them in parallel.  This is the reason series connected battery banks are preferable to parallel connected banks. If a second battery is desired, but simply cannot be accommodated on the tongue, mounting one GC2 on the tongue and another on the rear might be an option. Wiring and connecting would be simpler.[/font]

[wvabeer]

I would recommend two 6 volt in series. That way one battery won't suck the life out of the other.

[charliem]

I would recommend two 6 volt in series. That way one battery won't suck the life out of the other.

[font size="3"]Exactly my point. [/font]

[fasteddieb]

I'm in the process of mounting a spare 12v battery to the rear bumper of my 21BHS. Pictures to follow when completed in new thread.

Not planning on wiring the location right now, just carrying a spare to swap out if the main battery gets low.

But if I did wire it...

I've had vehicles where the negative battery terminal is just connected to the frame, sometimes via a braided cable. Is there a reason one could not simply run a single large gauge wire forward from the positive terminal, and let the frame be the negative ground?

I'm thinking along the lines of just a knife switch or equivalent up by the front battery box to switch the trailer power from front to rear battery.

Thoughts?

[admin]

I've done that on a one of my jeeps before. I don't know if it was the best option but it certainly worked for my needs. I actually had the second battery hooked up to one of those small solar maintainers while not in use. That way I knew was always good to go.

[charliem]

I'm in the process of mounting a spare 12v battery to the rear bumper of my 21BHS. Pictures to follow when completed in new thread.

Not planning on wiring the location right now, just carrying a spare to swap out if the main battery gets low.

But if I did wire it...

I've had vehicles where the negative battery terminal is just connected to the frame, sometimes via a braided cable. Is there a reason one could not simply run a single large gauge wire forward from the positive terminal, and let the frame be the negative ground?

I'm thinking along the lines of just a knife switch or equivalent up by the front battery box to switch the trailer power from front to rear battery.

Thoughts?

[font size="3"]Eddie,

IMHO the welded aluminum frame is a very good common ground point. Some will disagree and note the auto industry has gone away from that practice, but I stick with my position. I have used it on all my 12V electrical mods. It does add extra points subject to loosening and corrosion, but just make good connections with SS hardware and a few extra won't matter. It simplifies the wiring SOME, but there are some added wires to be run. As for the knife switch, it will work but you lose the advantages of the two batteries sharing the load. Some decrease in total capacity due to Mr. Peukert and more hassle. Commercial marine A/B battery switches are available. All that said, why not just bite the bullet and go to a pair of 6V golfers?[/font]

[david]

Using the aluminum trailer frame as the negative wire to the battery is good. It is like a hundred 00 gauge wire- no voltage drop. Scrape the mounting point first, use SS fastners and coat it with grease  to keep the corrosion down.

BTW, voltage drop is dependent on the total round trip wiring distance: pos battery lug to converter and back to battery neg lug. Using the frame as one leg, essentially reduces it to a single pos wire leg, not a round trip.

But running a wire all of the way from the bumper forward to the existing battery uses 20+ feet of wire plus another 10' of existing wire back to the converter vs maybe 10 feet directly to the converter panel depending on where it is in the trailer. That extra distance creates a big voltage drop which won't let the battery charge very fast.

Here are some examples. While charging at 20 amps, a reasonable number for a discharged Group 24 battery, you will get the following voltage drops between the converter and the battery on the bumper using 10 gauge wire:

.21V 10' one way (pos wired to converter, neg to frame)
.41V 20' one way (pos wired to converter, same for neg)
.62V 30' one way (pos wired to front battery and then back to converter, neg to frame)
1.24 60' one way (pos wired to front battery and then back to converter, same for neg)

Anything over a few tenths of a volt drop is going to significantly reduce your charging rate. The converter thinks it is putting out 14 volts, but in the case of the last entry above, the battery is seeing 12.8 volts. A battery won't charge much at 12.8 volts. But of course the actual current in this example will be more because the lower current results in less voltage drop which increases the current. It will balance out somewhere between near zero and 20 amps.

David