A 48v battery is fully charged at 54.6v. The low voltage cutoff is around 39v. It is best not to discharge more than 80% of the capacity for good cycle life. 80% DOD is around 43v depending on cell chemistry. Li-ion has a flat discharge curve. The voltage will drop from 54.6v down to 50v fairly quickly then level off. Below 42v the voltage drops off quickly and there is not much remaining capacity left. The cells heat up at this point as well.