A battery capacity and usage system, applied to an electric powered boat. A battery monitor is controlled by a micro-controller which includes a non-volatile memory for storing a lookup table. The lookup table comprises a database of characteristics of various batteries, including a relationship between the capacity and the charged voltage, the driving current and remaining hours of usage. The load of the battery is measured by a hall effect device. By monitoring the current micro-controller calculates the remaining time of the battery is obtained from the lookup table and displays the results on a digital readout installed on dashboard of the electric powered boat.