A battery pack includes a lead-acid battery and a charger port and an electronic switch connected between the two, the switch being controlled by a microcontroller which is connected to each of the battery and the charger port for separately monitoring the voltages thereat. The microcontroller operates under the control of a program which includes routines for comparing the battery voltage to the charger port voltage, for preventing closure of the switch unless the charger port voltage is at least a predetermined level and at least the battery voltage, for adjusting the duty cycle of the switch in accordance with the magnitude of the difference between the charger port voltage and the battery voltage, for detecting a bad battery by measuring the length of time required to increase the battery voltage from first to second predetermined levels, for reducing the switch duty cycle to a trickle charge level in the event of bad battery detection, and for disabling the bad battery detection routine in the event that a cigarette lighter adapter is coupled to the charger port. Audible and visible indicators may be provided to signal charging condition.