A battery device, comprising: a storage battery unit; a current sensing unit, coupled to the storage battery unit and a load for detecting a load current; a temperature sensing unit, configured to measure a battery temperature of the storage battery unit; a storage unit, configured to store a cycle number of the storage battery unit, a plurality of threshold intervals, and a plurality of charging voltage values, wherein each of the plurality of threshold intervals corresponds to one of the plurality of charging voltage values, respectively; and a processing unit, coupled to the storage battery unit and the storage unit, and configured to receive the load current and the battery temperature, wherein the processor is configured to perform a discharging record procedure, comprising: obtaining a depth of discharge, DOD, for the storage battery unit based on the load current, the battery temperature and the cycle number; and whenever the processing unit obtains the load current, the battery temperature, and the DOD, the processing unit stores the load current, the battery temperature, and the DOD in the storage unit; wherein the processing unit is also configured to perform a charging procedure, comprising: retrieving a maximum value of the plurality of obtained DODs which have been stored in the storage unit; identifying one of the plurality of the threshold intervals within which the maximum value falls to be a working threshold interval; selecting the charging voltage value corresponding to the working threshold interval to be a main charging value; and using a DC voltage set to the main charging value to perform a constant voltage charging for the storage battery unit. a storage battery unit; a current sensing unit, coupled to the storage battery unit and a load for detecting a load current; a temperature sensing unit, configured to measure a battery temperature of the storage battery unit; a storage unit, configured to store a cycle number of the storage battery unit, a plurality of threshold intervals, and a plurality of charging voltage values, wherein each of the plurality of threshold intervals corresponds to one of the plurality of charging voltage values, respectively; and a processing unit, coupled to the storage battery unit and the storage unit, and configured to receive the load current and the battery temperature, wherein the processor is configured to perform a discharging record procedure, comprising: obtaining a depth of discharge, DOD, for the storage battery unit based on the load current, the battery temperature and the cycle number; and whenever the processing unit obtains the load current, the battery temperature, and the DOD, the processing unit stores the load current, the battery temperature, and the DOD in the storage unit; wherein the processing unit is also configured to perform a charging procedure, comprising: retrieving a maximum value of the plurality of obtained DODs which have been stored in the storage unit; identifying one of the plurality of the threshold intervals within which the maximum value falls to be a working threshold interval; selecting the charging voltage value corresponding to the working threshold interval to be a main charging value; and using a DC voltage set to the main charging value to perform a constant voltage charging for the storage battery unit. obtaining a depth of discharge, DOD, for the storage battery unit based on the load current, the battery temperature and the cycle number; and whenever the processing unit obtains the load current, the battery temperature, and the DOD, the processing unit stores the load current, the battery temperature, and the DOD in the storage unit; wherein the processing unit is also configured to perform a charging procedure, comprising: retrieving a maximum value of the plurality of obtained DODs which have been stored in the storage unit; identifying one of the plurality of the threshold intervals within which the maximum value falls to be a working threshold interval; selecting the charging voltage value corresponding to the working threshold interval to be a main charging value; and using a DC voltage set to the main charging value to perform a constant voltage charging for the storage battery unit. retrieving a maximum value of the plurality of obtained DODs which have been stored in the storage unit; identifying one of the plurality of the threshold intervals within which the maximum value falls to be a working threshold interval; selecting the charging voltage value corresponding to the working threshold interval to be a main charging value; and using a DC voltage set to the main charging value to perform a constant voltage charging for the storage battery unit.