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As most of you, my car is 6V.
That said, I have an accessory that is only available in 12V.
My thought is to have a separate 12V battery hooked up to this accessory, as it is a low power draw, I will simply charge the battery at a time the amperage drops too low.
My question is this:
Can I run a hot lead to the accessory and simply ground the positive to the frame like the 6V does? (This is fine with the accessory, it is positive ground)
As they are two separate circuits I expect they could both use the same ground. I can run a ground wire as well to the accessory, but I am trying to see if I can drop the redundant ground wire run.
Thanks,
Bob J.

If it's low current, why not use a 6 to 12 volt inverter?

To answer your question, yes you most likely can share the frame for a ground. Just make sure the 12v wire does not connect to anything else. Not knowing what you are wanting to install, no idea if there could be any feedback paths to any 6v item if something else is connected to the accessory. That would be the only thing that might be an issue.

If there is no motor or solenoid coil involved with the accessory and it is low current, as Kev said you should be able to find a 6 to 12v inverter that will work. Amazon has a few. Just make sure to get one able to carry the current required and for good measure it might be a good idea to get one with a few more amps rating than needed so you have a safety margin and the inverter is not running at close to maximum capacity all the time.

If there is a motor or other inductive load in the accessory then some inverters are rated for electronic use only and could fail if something that can pull a surge current or produce a high back emf is connected. Make sure you get one rated for motor use if you have something of that nature.

Thanks!
Bob J.

Yes it's fine to have the positive of the 12v connected to ground and just run the negative power wire. Though you don't necessarily need a 12V. You can run a separate circuit from your car into the positive terminal of another 6V battery then the the negative side of that battery will have 12V to ground. Then if needed, you can disconnect the little 6V, connect the circuit from the car to the negative, and ground the positive; this will charge the 6V battery with the car's charging system. You could even wire in a crossover switch to switch the battery between powering your accessory, and charging. Only a DPDT would be required.

Great idea, thanks!
Bob J.