2nd and 3rd battery charge line size

[rockcrawler31]

Hey all, i'm going to piss off the dual batteries in the front of the cruiser and put a pair of gel batteries in the tray at the back. I'm not planning to use the second batteries to start the car, just to run fridges etc.

My question is, the cable that will run to the back to charge the batteries, can it be something like 5-6mm core diameter instead of the 13mm starter cable that i have currently with the batts next to eachother. The way i see it, the alternator only uses a 4mm cable to charge the main batt so surely that's all i need to charge the deep cycles. The smaller cable will be cheaper (lots!!) and easier to run. I'm either going to use a marine voltage sensing relay or just a manually activated solenoid.

Also, how would a pair of oddysey deep cycle batteries go with running a pair of bushranger max air compressors? would the current draw be too high and more suited to a cranking battery? The reason i ask is that the compressors are going to be mounted closer to the rear batteries meaning a shorter cable length.

[-Scott-]

The narrow cable will act a bit like a negative feedback loop - higher charge current will increase the voltage drop on the cable, and reduce the charging voltage at the batteries.

When the batteries are deeply discharged, the narrower cable restricts the charge rate - recharging will take longer than it otherwise might. Other than that, it will work.

[ausoops]

to simplify what scott said, your major problem over any length is not going to be the current carrying capacity of the cable but the voltage drop per metre. the longer the run the larger the Vd

[rockcrawler31]

so if 5mm core dia is possibly too small, what would be the optimum size to go from the front battery to the tray?

[drivesafe]

Hi Rock, because you will be trying to charge two batteries at the end of a long run of cable, you will need at least 6B&S ( 13.5mm2 ) for the very reason Scott and ausoops posted.

The cable size is paramount to you not only charging your two batteries but will also help the batteries to keep a decent operating life span. This in itself makes spending the money now on decent sized cable more sense as it will save you a lot more money down the road because your batteries lasted longer.

You will need both positive and negative cables.

Cheers.