BMS (and controllers) cut out at a certain voltage. It would be good to know the current capacity of your batteries (C rate), and the current draw of your controller (it really should say – and the vendor should be able to tell you). A 1C 8ah battery isn’t going to have much headroom to pull much current.

48v x 10amps = 500 watts. You probably are not pulling too much current (certainly not at 250 watts).

Remember watts = Amps x volts (referring to your meter question)

Your controller or your BMS could be flakey…