Im not too sure what I'll need.
Ive added a second battery and alternator to my car. I want a relay to be turned on by the ignition, to switch on the charge/excite circuit of the alternator.
At the moment the excite circuit is coming straight off the ignition, with a charge light. But this is causing engine interference (a whistle) that disappears, if alternator number two is excited by battery number two directly. Thus the desire to have a switch/relay.
If you're not familiar with how alternators works. They need 12 volts to excite the electromagnet that produces power from the motion of the engine turning it.