B0B is a very simple way to fly your Mavic Mini without the DJI genuine battery but with the telemetry of voltage available on the RC. B0B is not a battery, it's rather a cheap platform to experiment with any battery you wish.
B0B is able to communicate with the ESC board of the drone giving it just enough information to be accepted at boot and to monitor the voltage during the flight.
BOB's web page here : http://aeropic.free.fr/B0B
more information can be found here: https://www.thingiverse.com/thing:4544954
detailled information in the readme file here: https://cdn.thingiverse.com/assets/49/92/b3/0d/90/B0B_read_me.pdf
