Abstract: A method and apparatus for monitoring the voltage discharge of a battery power supply while under load, and to manage the power supply by accurately calculating the time the useful charge is nearly depleted, providing a series of warnings to the user-operator of that fact, and subsequently performing a system shutdown before a complete discharge of the battery. If the invention is applied to a battery power supply for a computer, the series of warnings enable the computer operator to transfer data from a temporary memory to permanent storage in the time interval between a warning signal and computer shutdown; and since computer shutdown is effected before the battery is fully discharged a battery-destructive phenomenon known as a polarity cell reversal is prevented.