A battery tester determines a remaining level of charge of a battery mounted within a separate electronic device having an audio jack. The battery tester includes a plug and a circuit having a high impedance input amplifier. At least one electrical contact of the plug is electrically coupled to an input of the high impedance input amplifier. The plug is removably insertable within the audio jack such that the battery of the separate electronic device is electrically connected to the input of the high impedance input amplifier. When electrically coupled to the battery, an output of the high impedance input amplifier provides a signal proportional to the remaining level of charge of the battery, whereby the remaining level of charge of the battery is obtainable by the battery tester without having to remove the battery from the electronic device.