Years ago in the 70s, I had a 6V VW and a 12V radio I wanted to install. I installed a 12V battery and ran a big screw through the top center of the battery to get the 6V to start and run the VW. Worked but I don't remember how well the battery charging worked out.
I use this meter from MJPA.com. Fast and easy, Volts, Amps, Watts.
https://www.mpja.com/Panel-Meter-Snap-in-Multi-Function-AC-Volts-100Amps-and-22KW/productinfo/32403+ME/