While it is nice to be able to refill tanks out in the field, getting left in the field due to a dead battery—isn't!
When H.A.M. first reviewed the Nomad (I and II), I brought up the issue about powering the unit without the engine running. The current draw is 26 amps, and depending on the vehicle in question, this amount of load is enough, that after about an hour of powering the Nomad, the vehicle in question might not have enough reserve to restart the engine, especially in cooler weather. My suggestion then, and now (!), is to start the engine and let it run while using the Nomad. And, the negative lead of the Nomad, should be connected at, or close to, the battery's chassis ground. This assures that the battery monitoring system is not bypassed.