a testing device applies time - varying electrical excitation to a cell or battery and senses the resulting time - varying electrical response . computation circuitry within the device uses voltage and current signals derived from the excitation and response signals as inputs and computes values of elements of an equivalent circuit representation of the cell or battery . the internal temperature of the cell or battery is calculated from the value of the time constant of a particular parallel g - c subcircuit of the equivalent circuit . the battery &# 39 ; s internal temperature is then either displayed to the user , used to apply appropriate temperature corrections to other computed quantities , used to detect thermal runaway , and / or used to control an external process such as charging of the battery .