48V is 13 groups in series. Since a fully charged cell group is 4.2V, that means full charge is 54.6 volts.
The battery BMS will isolate the interior cells from the outside world if any cell group gets below its minimum, which is usually set around 2.8V. Also prevents recharge around the same value.
Are you going to open the battery up, and do a voltage check on all 13 groups? That's the best way to tell the battery status. Watch out for accidental shorts if you do it.