Justin replied:

I think it sould be fine to do as you described, but you will have to set up a system that ensures you remember to activate/deactivate the switches everytime you start and stop. Adding relays to the system may extend the life of the switches.

Potential problems: If one of the batteres are flat and you start the car with the switches open you run the risk of frying the wiring, unless it is very heavey (starter/jumper cable sized) wiring. Also if the switches are left open and one battery goes dead, it will flatten the other batteries.

I understand you can buy special relays from 4wd shops that automatically snese a voltage drop (flat battery) and will automatically isolate that battery. However I don't think these relays are suitable for starting the car. and they reatail for around $60 - $100)

On my car (84 SWB Pajero) I am in the process of setting up a dual battery system. I am considering using a heavy duty diode to recuce the chance of the aux battery draining the main battery and a relay to connect/isolate the batteries for charging. If I need to start the car with the aux I will use a jumper lead.

Please feel free to correct me I am telling furbies :o)
Justin

P.G. (Tas) replied:

G'day Axel + Karen, What you are trying to do is fraut with Danger. (1) I am be concerned with the gauge of the wire. Should you forget to turn the switches off EVERY time you stop the motor, a restart could result in instant FIRE! The reason being with all batteries being tied together they will all share the stating load. On some engines this can be up to 300 amps! (2) If your vehicle has an engine management computer, switching in batteries in this manner can cause electrical spikes, instantly frying the computer. Solution? There are many, but what I am about to do is buy a marine grade battery switch. These allow you to connect to Bat1, Bat2 or Bat1+2. They handle up to 325 amps which make them safe for starting and give you various options for use. Hella make such a switch (P/No. 2767 250amps continuouis/325amps peak for 30 seconds) or there are many clones available. Clone prices start at about $40 from Marine shops, but be sure to check peak/continuous amperage currents with them. This switch allows the use of appropriate size battery cable. The downside? You must make your selection of batteries before turning on the key. This eliminates electrical spikes. In some circumstances, it may be better to start the car on Bat1, run for an hour or so, then stop, turn off the key, then select BAT1+2. This will ease the load on the alternator. If you want to run a third battery, this can be paralelled off the second battery and Hella again make an isolating switch that you can use to disconnect the two if you wish (again using good size battery cable. These switches are used on race cars to quickly isolate the electricals in the event of an accident). Let me know how you get on. Hope this helps. Cheers!

Pat replied:

Hi Alex+Karen, This is how I've set up my second battery and I've had no trouble with it at all.
1. Find a wire that has a positive charge with the ignition on BUT cuts out when you hit the starter. (Try the + source to the radio or lights.)
2. Buy a very good quality heavy duty relay.
3. Wire the relay up so that power to activate the relay comes from the wire found in 1 above.
3. Run two wires (I use 6mm two core flex) from No1 battery to No2 battery with the positive wire through the relay.
4. Run all your accessories from the second battery.
This is what happens. A/ Charge is supplied to the 2nd battery when the engine is running. B/ The second battery is isolated while starting. C/ The second battery is isolated from the first when the ignition is turned off.
Hope this helps. Pat

John replied:

You can find a power wire in your wipers motor that will work your switch and turn off with the motor.

Mal Try replied:

A and K, I have a HJ60 series LC thus no computer and high starting amps. I have fitted a second and third batteries. The Hella battery management system failed, the diodes and relays system was unreliable so I went for a single switch( Hella, like a big red key) .The two auxiliary batteries are wired in parallel and isolated from the start battery, which is also in parallel, by the switch. The alternator is wired to the start battery only when the switch is off, and to all batteries when it is on. I also have a volt meter in the auxiliary battery circuit. When I am driving I have the switch on and all batteries are being charged. When I stop for any length of time and have the fridge on etc. I turn the switch off. Thus the start battery is isolated and only the auxilaries are being used. When I take off again I turn the switch on and all batteries are once again being charged. I have a great system of not forgetting to switch the switch - make it the responsibility of your wife or partner - they hate being isolated by flat batteries in the middle of the Simpson! The switch is best located near the ignition key and it becomes automatic - switch one ,switch the other. The volt meter lets you know how the auxiliary batteries are going while at rest. The cableing I use has a copper diameter of 10 mm and as nothing gets warm it must be OK. Hope this helps. It certainly didn't cost much. Mal. P.S. I also carry a Cristie charger.