running dual batteries

demon1018 · Posted: Tue Oct 16, 2012 9:44 am Post subject: running dual batteries

I am working on a customers bus. He had 2 batteries installed and a small relay with small wires going to it . The wiring was all messed up so i pulled it out. My question is cant i just run batt cables from positive to positive and ground the two ground lugs on both batteries? All he is running is the radio and amp and its not big at all

WendyArmbuster · Posted: Tue Oct 16, 2012 9:52 am Post subject:

You could, but the relay keeps the main battery from draining when the bus is not running.
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1978 Westfalia

demon1018 · Posted: Tue Oct 16, 2012 10:02 am Post subject:

Ok but shouldnt the wires going to the batteries be at least the same size as the generator power wire?

demon1018 · Posted: Tue Oct 16, 2012 10:03 am Post subject:

Ok but shouldnt the wires going to the batteries be at least the same size as the generator power wire?

Tcash · Posted: Tue Oct 16, 2012 10:28 am Post subject:

Hi check out this link. Yes the wires going to the batteries should be at least 10ga. Good Luck
http://www.thesamba.com/vw/forum/viewtopic.php?t=297050&highlight=auxiliary+battery+relay]

demon1018 · Posted: Tue Oct 16, 2012 10:45 am Post subject:

The relay i have is the one from bus depot. It looks like a foglight relay with a 15 amp fuse in it. It doesnt make a lot of sense to me to run 10 gauge wires to a little relay. I asked bus drpot and they said that "you would have think the wires would be bigger". Should i use another type of relay?

busdaddy · Posted: Tue Oct 16, 2012 12:19 pm Post subject:

It's up to you and how fast you want to recharge or how drained the aux battery is. You could always have the small relay control a big one like this and use full size cables to link the batteries.

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Greezy Joe · Posted: Tue Oct 16, 2012 4:50 pm Post subject:

This was a factory set up in some buses, the wiring diagram is in the Bentley. I have done dozons of these thru a 30 amp general purpose relay. You DO NOT need heavy wires because the only thing running on that battery is the Interior lights. The relay just connects the 2 Batteries together only when the engine is running if the whole system is wired to the factory set up.
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Tcash · Posted: Tue Oct 16, 2012 5:49 pm Post subject:

Hi Demon, Here is some information on the Bus Depot Dual Battery Relay Kit.
http://www.busdepot.com/dual-battery-relay-kit
Since Bus Depot does not list the Gauge of wire. I did a little searching and came up with this. Not slamming Bus Depot, the poster may have had other issues. But it does list the gauge of the wire in the kit.
http://www.vgonman.com/auxbat.htm
It is not that a smaller gauge wire would not work. It is that it will not work as well. Let me give you an example. You hook a 1" hose and a 1/2" hose to a water faucet and put each end into two separate buckets. Which one will fill up first. Same concept with charging the batteries. You see post all the time, why is my Aux. battery not charging all the way. This is one of the main causes.
Take a look at the stock wire from the voltage regulator to the starter battery. Hopefully it is not speaker wire?
On mine it is a 6,0 mm2, this is slightly larger than a 10 Awg gauge wire but the yellow wire connectors for the 10 gauge wires are more available
Hope this helps you and your customer. Good Luck

demon1018 · Posted: Tue Oct 16, 2012 5:54 pm Post subject:

the second batt is not just for the dome lights. i dont know why you would even hook up a second batt just for that. hes basically using it for the radio but if he drains the battery down completly ,wouldnt you need a wire at least as big as the generator wire to deal with that. if not why is the generator wire that big going to the stock batt. from what i read most people use the second batt for heaters, refridg, etc .

busdaddy · Posted: Tue Oct 16, 2012 7:01 pm Post subject:

The wire from the alternator (or generator) is big because it has to charge the battery and run headlights, wipers, etc at the same time, plus a little more just for redundancy. Yes you can run all sorts of things on an aux battery and if it's run right down to nothing it'll suck up 15+ amps for the initial charge, better to go big if possible especially if you are still running that fridge and stereo after you fire up and leave for home from the beach with a dead aux battery.
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Rust NEVER sleeps and stock never goes out of style.

Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!

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gatorjos · Posted: Wed Oct 17, 2012 12:03 am Post subject:

That busdepot kit is useless in my opinion. It's got some 16 or 18 gauge wire and a 15 A relay. Not exactly $30 worth of stuff.

I bought a 40 A relay at autozone and connected the two batteries together with a 10awg wire. Works dandy.
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airkooledchris · Samba Member Joined: January 25, 2005 Posts: 2720

Make it really simple and install a marine battery switch in the engine bay:
http://www.amazon.com/Perko-8501DP-Marine-Battery-Selector/dp/B00144B6AE

Pull up to campsite, toggle to only battery 2. Toggle back to 'ALL' or just 1 before you leave.

Easy as pie. Ive done this in my last 3 busses now, works like a charm.

frankclimbs · Posted: Wed Oct 17, 2012 9:37 pm Post subject:


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