--Electric Bike-- How to measure left percent of battery?
Assuming it is a lead-acid VRLA, then you basically only care about the voltage (ie. the voltage closely corresponds to the state of charge). Very often the voltages are printed on the battery or in the manual/datasheet.

For a "12V" battery the typical values are:


don't discharge below 10.5V
cycle use: charge up to 14.4V
standby use: charge and keep at 13.8V


To get values for 48V just multiply them by 4.