Hey guys, one for the sparkies

This is probably a real dumb question but,

I have a 06 4.2 ute, I made my own duel battery circuit which has a wire tapped into the charge signal wire to switch a 12v relay (85 terminal)

Which brings on the charge light dimly, barely visible during the day but easy to see at night !

I also had another ute exactly the same when I did field service up north in wa with the same relay set up and same problem (diesel mechanic not sparkie)

I've never tested the voltage on the wire (as sometimes these can be half voltage) but used a 12v test light on it which lit up good when I installed it

I suspect to much draw for the wire but am a bit scared of frying something if I put more power in the wire to help hold the relay once switched ???

Cheers