Brock,
Not to be a know-it-all (I hate those guys
), but I would be hesitant about using a solid state relay for a circuit that included a starter. The starter will spike a few hundred amps initially. It is my expectation that a mechanical relay has the mass to absorb the brief but sizable heat load generated from this momentary current flow, but a solid state digital relay would not.
However, a starter already has a relay (the solenoid). If you do not mind diverging from the traditional aircraft style of a-master-relay-for-everything, you could simply wire the battery straight to the starter, just like every other non-aircraft setup. You would then provide a solid state relay for everything else (the panel, the starter switch, etc).
I would suggest this only for a battery location immediately next to the starter. The point of the master relay is to minimize the length of the always-hot main wire coming off of the battery. If your battery is next to the starter, then there is no difference between a foot of always-hot wire going to the starter vs. going to the relay. In other words, if your battery is in the tail... then yes, use a mechanical master relay in the tail immediately next to the battery.
With all of that said, there are already high-load automotive grade solid state relays available. No need to use a PCB type. I have an automotive solid state relay installed in my Thatcher CX4 and it serves the essential buss (the ECU, the fuel injectors, the coil packs, but NOT the starter). There is a picture of the installation here:
https://worktablecnc.us/projects/thatcher-cx4.html#ecu-and-electricalThe relay is a NTE-R51-1D70-12F. 70 amps, 12 V, with a flange. It was the mounting flange that made this one appealing to me: I then could then just wire directly using female blade connectors without having to source a socket.
I have stolen an image from the internet and attached it.
Anyway, I hope this helps.
Robert (BobbyBreadwinner)
Oregon, USA
