This is the battery M18 battery adapter I use.

Amazon.com

While I like Nimrod's system I have Milwaukee tools to use the batteries on. I bought and 8.0 amp hr battery but a 5.0 amp hr would work fine for a 5 hour trip. I have both the black box and my 93SV hooked to it. It doubles as an power cutoff (just remove the battery).

I will tell you I get 20.4 Volts when the battery is fully charged. According to Garmin tech support, the black box is good with 32 Volts and the display would give an error message if it was getting too many volts. YMMV.

Red