The internal sealed lead-acid (SLA) battery inside the MK-312BT is designed to last about twice as long as the one inside the original ErosTek ET-312B. The larger capacity requires an overnight charge or longer to reach a full charge, especially after many hours of portable use.
The chart below shows the battery voltage for the various percent charge (%) values displayed during power ON.
The displayed charge % level is more accurate when the MK is powered ON with the AC adapter cable disconnected.
It’s a good idea to connect the AC adapter to the unit in between uses. The internal charging circuit maintains the charge level and prevents over-charging of the battery.
Download [PDF]: MK-312BT Battery Volts vs Percent