A portable electronic device, comprising: a rechargeable battery; a controller (20, 50 + Q2, U909) configured to receive USB power from an external source and supply power to the rechargeable battery and one or more portable device components, the portable electronic device comprising the one or more portable device components; and a switch (Q1, Q3, Q908) configured to isolate the rechargeable battery from the one or more portable device components and control a supply of power to the rechargeable battery; a sensing circuit configured to compare a reflected system voltage level with a reference voltage level, the reflected system voltage level reflecting a system voltage level of the power supplied to the one or more portable device components, the sensing circuit configured to control the switch to limit battery charging from the external source such that: power is configured to be supplied from the battery to the one or more portable device components via the switch when the supply power is not available, wherein the reflected system voltage level also reflects the voltage level of the rechargeable battery; power is configured to be supplied from the supply power to the battery via the switch and to the one or more portable device components when the supply power is available, wherein the sensing circuit maintains the reflected system voltage level to be at least a minimum system voltage level required for operation of the one or more portable device components by modulating the switch. a rechargeable battery; a controller (20, 50 + Q2, U909) configured to receive USB power from an external source and supply power to the rechargeable battery and one or more portable device components, the portable electronic device comprising the one or more portable device components; and a switch (Q1, Q3, Q908) configured to isolate the rechargeable battery from the one or more portable device components and control a supply of power to the rechargeable battery; a sensing circuit configured to compare a reflected system voltage level with a reference voltage level, the reflected system voltage level reflecting a system voltage level of the power supplied to the one or more portable device components, the sensing circuit configured to control the switch to limit battery charging from the external source such that: power is configured to be supplied from the battery to the one or more portable device components via the switch when the supply power is not available, wherein the reflected system voltage level also reflects the voltage level of the rechargeable battery; power is configured to be supplied from the supply power to the battery via the switch and to the one or more portable device components when the supply power is available, wherein the sensing circuit maintains the reflected system voltage level to be at least a minimum system voltage level required for operation of the one or more portable device components by modulating the switch. power is configured to be supplied from the battery to the one or more portable device components via the switch when the supply power is not available, wherein the reflected system voltage level also reflects the voltage level of the rechargeable battery; power is configured to be supplied from the supply power to the battery via the switch and to the one or more portable device components when the supply power is available, wherein the sensing circuit maintains the reflected system voltage level to be at least a minimum system voltage level required for operation of the one or more portable device components by modulating the switch.