I'm fitting a split charge system and would like to know the easiest place to connect the switching wire on the split charge relay (to flip it open when running) it needs to be on the dash light circuit, so the relay opens when the light goes out

A couple of us don't know the common phase 'split charge'. Care to elaborate on that point?

I'm adding another battery alongside the main one for accessories. There is a relay between the two batteries which isolates them when the car isn't running.

Ineed the relay switching wire feed x

The warning light basically compares the voltage put out by the alternator to the battery reference voltage. There's a blue wire that runs from the relay board to the warning light, and the other side of the light is hooked to switched power.

You can hook into that blue wire pretty much anywhere. At the connector for the light is probably the easiest (if you can find a "piggyback" connector, or if you want to crimp a new spade lug on with two wires instead of one) but it's not that hard to tap in elsewhere, You may be able to solder the new wire to the old one at the relay board connection, depending on how thick the new wire needs to be.

Not that I'm totally guessing about what kind of signal you want for the relay. You mentioned the alternator warning light, so that's what I went off of.

--DD

No your right Dave and thankyou.

As I have the interior out atm it may be just as easy to wire up behind the dash panel although is it easy to find/ get to the blue one in the engine bay on the relay board??

The blue wire is D+ on the alternator circuit. Here are the three connection points you could access in the engine bay.

I dont know what kind of current drive that Alt signal has.
It might not be enough to drive the battery isolation relay closed when running.

If not, you can use a DC SSR (Solid State Relay), they pull little to no current when actuated.

What is the point of having a second battery?

Stu

Er....

There are kits made for this and they are readily available at any auto parts or RV parts store. Ask for a "battery isolator kit". The isolator will allow you to run your engine off the primary battery and the accessories off the other (usually a deep cycle). When the engine is off, the 2nd battery is completely separate from the main engine (starting) battery. Once you start the engine, it allows the alternator to charge BOTH batteries.

These are really popular with VW Westfalia campers.

So I suppose everyone is supposed to know what that is?
Is it your flux capacitor, or your electronic ignition? I know.... The new megasquirt!
I would have guessed that it was an electric car, but you wouldnt be asking questions about batteries if that were the case...

Stu

+1
An RV store should have everything you need.

Andys