Patent ID: 8841871

Claim:
A control circuit for a power tool, comprising: a detection circuit, electrically connected with a battery pack, which comprises an internal resistance R b and a temperature factor C t , and configured to measure a voltage between two measuring points (V′) and measure the temperature of the battery; a calculating control circuit, configured to calculate a calculation result of battery capacity according to the voltage between the two measuring points (V′) measured by the detection circuit, and configured to control the power supply for a motor in conjunction with at least the temperature of the battery pack, wherein the calculation result of battery capacity (V) comprises the voltage between two measuring points (V′), a voltage consumed by the battery pack internally (I·R b ), and a voltage consumed by the battery in a discharge loop (I·R c ), wherein there is a discharge loop internal resistance R c ; a battery capacity indicating circuit for indicating the calculation result of battery capacity; and, a current measure and calculating circuit for measuring a current I flowing through the motor, wherein the formula for calculating the open circuit voltage V of the battery pack is: V=V′+I·R c +I·R b ·C t .