Keither McMillen Instruments Batt-O-Meter Guitar Pedal and Preamp Battery Tester

The Batt-O-Meter  is the first battery tester geared for musicians. No need to remove the batteries from your guitar or pedal, you can test the remaining hours of battery life and voltage of 9 volt batteries while they are still inside gear through a 1/4-inch plug that is attached to the Batt-O-Meter. The Batt-O-Meter can test stand-alone batteries: 9V, 1.5V (AAA, AA), and 3V and it also works with Alkaline, Rechargable, and old school Carbon-Zinc batteries. The Batt-O-Meter requires its own 9 volt for operation, which it can self-test.