Ford starter relays are for intermittent duty use. Solenoids used for a battery disconnect, that also carry the load of the car while running, need to be for constant duty use. They may look the same but they are not the same.

We use constant duty solenoids for control of "ignition on" circuits, typically 80 -100A rated, but they aren't designed to carry the in rush of a starter motor.

There are battery disconnectors/solenoids/relays made for this and designed to carry higher amperage and be in constant duty use.. But you can't buy them at an auto parts for $15.






250A continuous, intermittent well over 1000A