you tell me........ http://www.emxinc.com/blueguard.html

It would need to control one hell of a relay to isolate the battery. Could be done, maybe a old ford starter relay. Probably be better to use it to isolate the fuel pump.

I would think fuel pump or ignition would be easy enough. Battery or starter may take a little much.

This is just a universal wireless controller, a set of relay contact to control other heavy-duty relays on the machine itself.

The output relay is only rated at 2 Amps. It could drive a high power relay for your battery cut off. There are some Ford style relays that are rated for continuous duty.

I purchased some Bosch style relays that are remote controlled from Ebay. I run it to the fuel pump.

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In electrical/electronics, almost anything is possible. That's why we have the term KISS.( Keep it simple, stupid.) The more complicated you make it, the easier it is to fail at an inopportune moment and then it can be difficult to troubleshoot.
Just curious, why would you want to have the battery cut-off on a relay? A heavy duty switch designed for that is as simple as it gets. If for anti-theft, I would go with a small relay to isolate the fuel pump, or a simple inline on/off switch for the fuel pump.
Tom

That would keep me from leaving my phone at the shop

I didn't plan on using one at all really. My relay is a barking dog. I was just thinking that people could use on of these "instead of" a battery disconnect. e.g. on the starter, ign switch or fp as already mentioned here.