Thanks for the info RadioTwo.
The second battery will be a leisure battery for the camper conversion I'm doing to the Sprinter. Yes, I came across the self-switching combination relay option when researching the forums earlier. Decided against it due to it only being rated at 30amps and the alternator could generate more. But had not considered using it just to switch my 70amp relay. Seems a bit of overkill though since I already have a suitable relay. I just need a suitable (accessible) signal wire. But from where ....?
You will be surprised, 30amp will be sufficient most caravanner's use those self switching units for both battery charging and running the fridge's (as I do)
Most batteries will take less than 30 amps, unless you are drawing a lot off it at the time of charging!
Where are you thinking of putting the battery ?
Just to go back to the wiring of the relay, I know how it is wired but harder to explain !
these are the pin outs on the relay:
pin 85 goes to the sense wire on the alternator ( the wire that goes to the warning light on the dash, some times labled as WL)
pin 86 goes to earth.
pin 30 goes directly to the van battery
pin 87 goes to the leisure battery
The way it works is: when you turn on the IGN (turn the key) and the battery light comes on, there is a 12 volt feed from the switch, and it goes through the bulb to the alternator and earth's,but when you start the engine and the alternator starts to charge it puts more than 12 volts on the other side of the lamp so it goes out, but that means the terminal 85 becomes live when it is charging, so switches the relay and in turn charges the leisure battery.
Hope that helps
If you need any more help, please PM me