PURPOSE: To eliminate influence due to a liquid temp. and a use cycle, by calculating the real residual amount of a battery by correction based on a capacity reduction ratio obtained by the liquid temp. signal of a liquid temp. sensor and a capacity reduction ratio obtained from a current integration value renewed at every definite time.

CONSTITUTION: The output of the liquid temp. sensor 12 of a battery 10 from ch1 of an A/D converter 7, a discharge current from ch2 through a current detecting resistor 4, an amplifier 5 and smoothing circuit 6 and battery voltage from ch3 are inputted to CPU8 and a discharge ratio is calculated from the battery voltage and the discharge current. On the other hand, a capacity reduction ratio is calculated from a liquid temp. through the table in a non-volatile memory 11 and a corrected discharge ratio is calculated from said reduction ratio and the discharge ratio while a capacity reduction ratio calculated from the total discharge amount rewritten at every definite time in the memory 11 is further multiplied by the corrected discharge ratio to calculate real residual capacity.