I have a battery meter readout that plugs into two Anton Bauer P-tap connectors that I'd be happy to give to someone for free. I had Steve Byro (typing that name just made me throw up a little in my mouth) build it for me to use with my Pro-Lite base back in the day. I haven't used it in years so it's yours if you can use it.
It switches back and forth between two batteries, reading the voltage. The frequency of the switching can be controlled by a small thumbscrew. It also has an on/off switch.
It isn't the most elegent product ever made, but it does work, and it is free. Let me know if you'd like it.