Dual battery setup

[mhoey]

Hi guys

I am at the stage of setting up a dual battery system (Ironman 275 amps) in my Mitsubishi L200 2011.

Due do the lack of space in the engine compartment for my Power Sonich deep cycle AGM battery (PS-121000) I have chosen to relocate it in the rear cabin behind the passenger seats.
The kit came with AWG 6 cables but not enough lenght now that I install it in the rear of the car and not under the bonnet.
The distance I will have to run the cables from the main battery is about 7 meters (23 feet).
Will AWG 6 be enough or should I go with a large mm2 cable?

I will charge the aux. battery with the alternator/ solar power and 220 volt plug in charger.

All the best

Mick Hoey
www.ride-chile.com
Danish living in Santiago, Chile

 

[machinisttx]

No, 6awg is not large enough. The actual circuit length is not 23 feet, but 46 feet since you have to add in the ground circuit path as well. Industry standard voltage drop is no more than 10% from what I can find, but IMO that's excessive. I don't know how much current you expect to pull from the second battery, so I'm just going to throw numbers in the calculator and see what it comes up with. I'm going to guess that you're running a winch and a battery isolator. Depending on exactly how you're wiring it up, the load may be shared by the batteries or it may come from one, so I'll use realistic numbers for both.

Numbers are from a typical winch drawing 450 amps at full load on 12VDC:
Power draw split between the batteries

With a 225 amp load and 6awg, the voltage drop will be 4.14 volts, so that's about a 35% loss.
With a 225 amp load and 4awg, the voltage drop will be 2.48 volts. About a 20% loss.
With a 225 amp load and 2awg, the voltage drop will be 1.66 volts. About a 15% loss.
With a 225 amp load and 1awg, the voltage drop will be 1.35 volts. About 11% loss.
with a 225 amp load and 1/0awg, the voltage drop will be .99 volts. About 8% loss.
with a 225 amp load and 2/0awg, the voltage drop will be .8 volts. About 6.7% loss.
With 3/0awg, the voltage drop will be .64 volts. About 5%.

If you wire it so that the winch only draws from the second battery,

With a 450 amp load and 6awg, voltage drop will be 8.28 volts. About 68% loss.
with 4awg, the voltage drop will be 4.97 volts. About a 40% loss.
With 2awg, the voltage drop will be 3.31 volts. About a 28% loss.
With 1awg, the voltage drop will be 2.69 volts. About 20% loss.
with 1/0awg, the voltage drop will be 1.99 volts. About 17% loss.
with 2/0awg, the voltage drop will be 1.59 volts. About 13% loss.
With 3/0awg, the voltage drop will be 1.28 volts. About 10% loss.
with 4/0awg, the voltage drop will be 1.01 volts. about 9% loss.

Realistically, the current draw will be slightly less with the engine running since you should be at around 13.8VDC, and the alternator will be supplying some power. That said, the alternator cannot keep up with the current draw of the winch and system voltage will quickly drop if you're winching a heavy load or for long periods of time. Once voltage drops, amp draw will increase and at that point it's time to shut down and let the batteries recharge or risk burning up the winch motor and damaging electrical devices in the vehicle.