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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 i

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.

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1. A battery tester for determining a remaining level of charge of a battery mounted within a separate electronic device having an audio jack, comprising a circuit including a high impedance input amplifier and a plug having a contact that is electrically coupled to an input of said high impedance i

1. A battery tester for determining a remaining level of charge of a battery mounted within a separate electronic device having an audio jack, comprising a circuit including a high impedance input amplifier and a plug having a contact that is electrically coupled to an input of said high impedance input amplifier, said plug being removably insertable within the audio jack such that said contact of said plug electrically connects the battery to said input of said high impedance input amplifier, said high impedance input amplifier having an output that produces a signal proportional to the remaining level of charge of the battery, whereby the remaining level of charge of the battery is obtainable by said battery tester without having to remove the battery from the electronic device. 2. A battery tester according to claim 1, further comprising an indicator electrically connected to said output of said high impedance input amplifier, said indicator producing an audible or visible indication of the remaining level of charge of the battery based on the output signal of said high impedance input amplifier. 3. A battery tester according to claim 2, wherein said indicator includes at least one of a microprocessor, A/D converter, digital display, and bar graph indicator. 4. A battery tester according to claim 2, further comprising a power source that powers said indicator and said high impedance input amplifier, a housing within which said high impedance input amplifier and power source are contained, and in which said indicator comprises a visual display on said housing for providing said visible indication of the remaining level of charge in the battery. 5. A battery tester according to claim 2, wherein said indicator includes a microprocessor that calculates the remaining level of charge of the battery based on at least one of steady state charge, initial peak, and decay of charge. 6. A battery tester according to claim 2, including a housing within which said high impedance input amplifier and power source are contained, wherein said plug extends rigidly from said housing and includes at least two electrically isolated contacts. 7. A battery tester according to claim 2, including a housing within which said high impedance input amplifier and power source are contained, wherein said plug is a stereo plug including a base cylindrical contact, an intermediate cylindrical contact, and a tip contact that are insulated from one another. 8. A method of determining the remaining level of charge of at least one battery contained within a battery compartment of an electric musical instrument, comprising the steps of: inserting a plug of a battery tester into an audio jack of the instrument to electrically couple the at least one battery to the battery tester without removing said at least one battery from within the instrument; and observing an indication of the remaining level of charge produced by the battery tester in response to said inserting step. 9. A method according to claim 8, wherein said inserting step includes electrically coupling the effective circuit impedance of an on-board tone control circuit of the instrument and the at least one battery in series to an input of an amplifier in the battery tester. 10. A method according to claim 8, wherein the observed indication is produced by an indicator circuit of the battery tester that is electrically coupled to an output of said input amplifier, the output being proportional to the remaining level of charge of the at least one battery.