Battery charging methods and systems for devices that have rechargeable batteries provide an efficient way to know when to charge a device's battery, and when to switch between the device's battery and an external power source as the device's power source. The methods and systems receive signals associated with an external information service including instructions to conduct processing, access thresholds for a plurality of power rates, obtain information about when different power rates are in effect and, after determining a charge level associated with a battery, compare the threshold of the current power rate to the device's battery's charge level. Based on such a comparison, the methods and systems can determine whether the battery should be charged and whether the device's battery or an external power source should be used as the device's power source.