Method and apparatus for monitoring batttery capacity with charge control

Abstract

A battery detect circuit (32) is connected to the battery with the current input to the battery and extracted from the battery measured with a sense resistor (50) and then converted to charge and discharge pulses with a V/F converter (52). A microcontroller (64) is operable to increment a Nominal Available Charge (NAC) register (180) during a charge operation, and to increment a Discharge Count Register (DCR) (184) during a discharge operation. The NAC register (180) indicates the available charge, which value is output to a display (34). The maximum value to which the NAC value can rise is limited by a value stored in the last measured discharge register (182). This value represents the value stored in the DCR (184) whenever the battery is discharged from an apparent full state to a fully discharged state. This results in a qualified transfer to the LMD register (182) such that no knowledge of the actual battery charge is necessary. The charge rate is varied as both a function of temperature, voltage and also of the state of charge. A fast charge rate is initiated when the charge state is above a predetermined low voltage threshold and less than full. When the charge state reaches a level within approximately 80% of full charge, the charge rate is reduced. At full capacity, the charge rate is further reduced to a top off charge rate which continues for approximately two hours. If the temperature falls below a predetermined low temperature threshold, the charge rate is again reduced to a safe, slow, effective rate. Trickle charge is provided to replenish charge adaptively in accordance with a self-discharge counter. The trickle charge operates to momentarily pulse the battery (10) at a predetermined charge rate.