NOVELTY - A battery charging circuit for a portable device comprises: a switching regulator configured to receive a substantially direct current (DC) power source from one or more photovoltaic cells; and a controller configured to monitor the substantially DC power source.    USE - The battery charging circuit, battery charger, kit and method are useful for a portable device; and for charging a battery from a variable DC power source (all claimed).    ADVANTAGE - The battery charging circuit charges the battery to an adjustable regulated voltage level in a second mode when the voltage level of the variable power source is below the predefined threshold.    DETAILED DESCRIPTION - A battery charging circuit for a portable device comprises: a switching regulator configured to receive a substantially direct current (DC) power source from one or more photovoltaic cells at an input terminal and to charge a battery coupled to an output terminal; and a controller configured to monitor the substantially DC power source and to provide one or more control signals to the switching regulator to selectively operate the switching regulator in a first mode when a voltage level of the substantially DC power source is above a first predefined voltage threshold and in a second mode when the voltage level of the substantially DC power source is below the first predefined voltage threshold, where the switching regulator operates with a predetermined regulated voltage level in the first mode and operates with a variable regulated voltage level that tracks the voltage level of the substantially DC power source in the second mode. INDEPENDENT CLAIMS are:    (1) a method to charge a battery from a variable DC power source, comprising: sensing a voltage level of the variable DC power source, where the variable DC power source is provided by a photovoltaic array; selectively operating in a first mode when the voltage level of the variable DC power source is above a predefined voltage threshold, where the variable DC power source charges the battery to a predetermined regulated voltage level in the first mode and the predetermined regulated voltage level is approximately 4.2 volts; and selectively operating in a second mode when the voltage level of the variable DC power source is below the predefined voltage threshold, where the variable DC power source charges the battery to an adjustable regulated voltage level in the second mode and the adjustable regulated voltage level is approximately equal to the voltage level of the variable DC power source less a predetermined amount;    (2) an integrated solar charging battery package comprising: a battery layer placed on a bottom surface of an encapsulated package; a thermal barrier layer placed above the battery layer; a photovoltaic array layer placed above the thermal barrier layer; a protective layer placed above the photovoltaic array layer, where the protective layer forms a top surface of the encapsulated package; and a charging circuit configured to electrically interface the battery layer and the photovoltaic array layer, where the charging circuit occupies a portion of the battery layer in the encapsulated package;    (3) a battery charger comprising: a power regulator configured to receive a variable DC power source and to charge a battery; and a controller configured to sense the variable DC power source, to operate the power regulator in a first mode when a voltage of the variable DC power source is above a predefined voltage level, and to operate the power regulator in a second mode when the voltage of the variable DC power source is below the predefined voltage level, where the power regulator charges the battery to a substantially fixed regulated voltage level in the first mode and to an adjustable regulated voltage level in the second mode;    (4) a solar chargeable replacement battery package for a handheld portable electronic device comprising: a package having a substantially similar form factor as the standard battery specified by a manufacturer of the device; a battery placed on a lower portion of the package; a photovoltaic array placed on top of the battery and isolated from the battery by a thermal barrier, where the photovoltaic array supplies power to charge the battery; and a clear protective layer placed on top of the photovoltaic array, where the clear protective layer forms a top surface of the package; and    (5) a replacement battery kit for a portable electronic device, comprising: a replacement cover with a central opening; and a solar chargeable battery having a substantially similar form factor as the standard battery of the device, where the solar chargeable battery comprises: one or more photovoltaic cells configured for optical exposure through the central opening of the replacement cover after installation of the replacement battery kit; and a battery; and a charging circuit configured to charge the battery from the photovoltaic cells.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the solar chargeable battery system.    Photovoltaic arrays (100)    Full spectrum arrays (102, 104)    Power regulator (110)    Micro controller (120)    LDO regulator (130)    Lithium battery (140)    Mobile device (150)    Solar chargeable battery system (160)