Lantern batteries are a standard part of my 6V troubleshooting kit. Great for light sockets and horns and having for long road trips when I expect issues. Rewiring my '35 was made easier with this temporary source.
Powering up a fuel gauge and sender would be easy, but last time I did it for a '36 years ago was with a wet-cell 6V. Gauge is in the box and sender in the vise.
