TECHNOLOGY FOCUS - ELECTRONICS - Preferred Circuit: In the battery charging circuit, the substantially DC power source comprises at least two photovoltaic cells connected in series. The battery comprises a lithium-ion or a lithium polymer battery. The switching regulator charges the battery using a constant-cur rent/constant-voltage algorithm comprising interleaving current regulation phases and voltage regulation phases, the switching regulator provides a substantially constant battery charging current during the current regulation phases, the substantially constant battery charging current has a predetermined current level when a current level of the substantially DC power source is above a predefined current threshold, and the substantially constant battery charging current has an adjusted current level that is approximately equal to the current level of the substantially DC power source when the current level of the substantially DC power source is below the predefined current threshold. The substantially constant battery charging current has a stepped rising edge near a beginning of each current regulation phase and the stepped rising edge comprises a set of incremental current steps with programmable step sizes and intervals. The switching regulator provides a decreasing battery charging current during the voltage regulation phases and stops charging the battery in the voltage regulation phases when the decreasing battery charging current reaches a termination current level. The termination current level, the predetermined current level of the substantially constant battery charging current, and the predetermined regulated voltage level are programmable parameters that are stored in the controller using a standard interface. The battery charging circuit further comprises a direction resistor coupled between the portable device and the battery, where the controller monitors the direction resistor to disable the switching regulator when current flow is detected from the portable device to the battery; a direction resistor coupled between the portable device and the battery, where the controller monitors the direction resistor to disable the switching regulator when current flow is detected from the battery to the portable device; and a status diode coupled between the substantially DC power source and a status pin of the switching regulator, where the status diode lights up while the switching regulator is charging the battery. The controller monitors the battery's temperature and disables the switching regulator when the battery's temperature is outside a predetermined temperature range, and the predetermined temperature range is a programmable parameter that is stored in the controller using a standard interface. The switching regulator and the controller enter a sleep mode when the voltage level of the substantially DC power source is less than the battery's voltage. Preferred Method: In charging the battery from the variable DC power source, the photovoltaic array is integrated with the battery and isolated from the battery by a thermal layer. The method further comprises sensing the battery's temperature, where the battery stops charging when the battery's temperature exceeds a predefined temperature threshold. The battery is installed in a device connectable to a substantially fixed DC power source and the variable DC power source stops charging the battery when the substantially fixed DC power source is connected. Preferred Package: In the integrated solar charging battery package, the battery layer has a thickness of approximately 3.5 mm. The thermal barrier layer comprises polyimide film with a thickness of approximately 25-50 mu m. The photovoltaic layer has a thickness of approximately 140 mu m. The protective layer is substantially transparent and has a thickness of approximately 70-90 mu m.