Dual battery wire gauge? - Fiberglass RV


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Old 01-01-2015, 09:32 AM   #1
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Dual battery wire gauge?

I am thinking of adding a second battery to my casita. I will be mounting the second battery on the trailer tongue. The existing battery is is the back of the trailer so there will be about 15 feet of wire between them. I will probably go with two 12 volt batteries in parallel for now, but I may switch to two six volts in the future. Is a ten gauge wire sufficient and would it be sufficient in either case? Any thoughts?
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Old 01-01-2015, 08:24 PM   #2
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You don't have an inverter installed do you?

Both of my campers came set up for 30A DC service. 30 amp inline breaker and appropriate sized cables. If you figure you have the same 30 amps, at 30 feet (you add both positive and negative leads), you need 4awg.
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Old 01-01-2015, 09:02 PM   #3
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Originally Posted by padlin00 View Post
You don't have an inverter installed do you?

Both of my campers came set up for 30A DC service. 30 amp inline breaker and appropriate sized cables. If you figure you have the same 30 amps, at 30 feet (you add both positive and negative leads), you need 4awg.
Bob, don't you mean 30 Amps AC?
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Old 01-02-2015, 07:50 AM   #4
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I think he means the DC system. 4 gauge seems kinda big, but thats what the chart says? I'm not using an inverter, so I will not be pulling large loads off the dc system. There is a ten gauge wire going to the 7 pin plug for power that is rated for 30 amps so I thought 10 gauge might be adequate for the second battery. Probably the largest load on the wires would be while I am charging the batteries with my 200-300 watts of solar power or the casita's converter/charger. Does anyone know what the charge rate of casita charger is?
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Old 01-02-2015, 08:31 AM   #5
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Also to consider. The current draw from the second battery should be half of the total draw since loads will pull equally from both batteries assuming you are using two batteries of equal capacity. If thats the case, then bobs chart says 8 gauge is sufficient. If I double up conductors and run four ten gauges (easy wire to get and run), then that gives me about a 7 gauge equivalent.
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Old 01-02-2015, 09:09 AM   #6
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Dual battery wire gauge?

Cutting the required capacity in half is a bad assumption. All it takes is a bad cell or a broken wire that you won't know about, and you've just overloaded those wires.

What amperage is your charger? I have a pd4045, and for a 13' run of wire, 4 gauge was needed, and actually at the minimum side, but all that will fit in it, anyway.

That chart posted is for a .44v drop in the wiring. That's a big drop if you're somewhere just running on batteries.

~12.7v is fully charged for a battery, 12v is considered dead. With that wiring, it's already half dead (system voltage, not the battery) before you've started.

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Old 01-02-2015, 10:59 AM   #7
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The exiting wires going to my single battery are 10 gauge. I dont know what the capacity of the casita charger is. 4 gauge is jumper cable size (impractical really). Maybe getting the second battery nearer the other one is a more practical solution?
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Old 01-02-2015, 11:37 AM   #8
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I have bought a number of jumper cable sets at yard sales for less than $7-8 and use them as a source of heavy cable when it's needed. be sure to get the more flexible kind.
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Old 01-02-2015, 12:19 PM   #9
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Doesnt the battery connect the negative lead to the frame?
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Old 01-02-2015, 04:58 PM   #10
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Originally Posted by hotfishtacos View Post
Bob, don't you mean 30 Amps AC?
Hi Steve
I could certainly be wrong, but when I traced the system out, the breaker is between the battery/s and the converter. In the Escape with the 2 6 volt batteries it's between the batteries and the factory battery disconnect switch.

Too cold and dark to go double check.
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Old 01-02-2015, 05:27 PM   #11
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Dave
You can of course do anything you want, sounds like you already know what you want to do. I'm no engineer but the 2 batteries should be looked at as one. Don't think going with smaller cable because you are drawing half your power from each holds water. 4 awg is not all that big, a quick look shows jumper cables from 4 to 1/0 awg, lower being better at transferring power to the battery being jumped.

Here's another chart that shows both the 3% and 10% voltage loss, you can go with the 10awg if 10% is acceptable to you, your call.

How much loss do you think there is between the existing camper battery and the TV's alternator using 10awg?

I believe this one came from the Blue Sea site, cut just the wire selection portion.
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Old 01-02-2015, 05:40 PM   #12
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Thanks bob for the charts. I'll probably do some experimenting. If I moved the second battery to the back next to the first that should solve this problem right?
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Old 01-02-2015, 06:26 PM   #13
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Yup.
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Old 01-03-2015, 09:45 AM   #14
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Originally Posted by dslaggie View Post
The exiting wires going to my single battery are 10 gauge. I dont know what the capacity of the casita charger is. 4 gauge is jumper cable size (impractical really). Maybe getting the second battery nearer the other one is a more practical solution?

If you don't know what the capacity of the charger is, how can you properly size the wire? This is flying by the seat of the pants. It has to take the max amps of the charger or the system, whatever is higher.

4 gauge is quite the opposite of impractical (at least with my setup). It will take my 45 amps of charging, provides a very small voltage drop, and keeps my trailer from burning. 10 gauge will not. Ok, mine is protected by a 50 amp marine switchable circuit breaker so that won't happen, but still, it keeps the circuit from tripping under normal use.

10% voltage loss puts system voltage with a fully charged battery under what's considered dead for a battery, before you even use it. Why bother with dual batteries then? You've already lost the race before you've even started.




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