Here’s one that every guitar player, bass player, or other musician or engineer who uses pedals, battery-powered devices, active guitars or basses, or acoustic guitars with onboard preamps should have in their toolbox: the Keith McMillen Instruments Batt-O-Meter. The Batt-O-Meter is a battery tester with an integrated 1/4″ jack/probe, which means you can check the batteries in your pedals without taking them out! Just plug the B-O-M into your active guitar’s jack, into a battery-powered pedal’s input, or any device that uses battery power and a switching jack. The B-O-M will tell you exactly how many volts the battery is delivering and how many hours are left before the battery runs out of power. Never get caught in the studio or onstage with a dead battery again!