I struggle with this as well, in my particular case I eventually figured out when I pulled the case of that the manufacturer had put the sticker backwards on the case and I had the B-,A+, V- & V+ wires in the reverse order. Once I wired it up to follow the markings on the PCB instead of the case I started getting answers.

My XY-MD02 from aliexpress has a red LED on the board that is constantly on when it has power on V- & V+. That was a clue.