method and apparatus for accurately measuring charge storage system operating conditions in a vehicle . a measurement circuit produces an electrical response at an output which varies linearly with battery voltage levels . the circuit is calibrated to provide an output only for a voltage range above a specified battery voltage level in order to improve the resolution in detecting these levels using a standard electrical level detector such an ammeter or voltmeter . the measurement circuit is connected to a display circuit for indicating at least one charge storage system operating condition based on the terminal voltage of the battery . the measurement circuit also includes an output damper to limit the large , rapid output variations experienced during vehicle start and circuitry for compensating for variations in circuit performance due to temperature variations .