A while back I was in a shop, and saw a Keith McMillen battery tester. You can plug into a pedal’s input jack, and it tells you whether or it’s time for a new one. Great idea for $30!
There’s an easy way to check to see if the battery in your stomp box is dying or dead for FREE, though. If you have a nine-volt AC adapter (and who doesn’t?), just plug it into the pedal’s AC jack while the pedal is already switched on with the battery. If the LED gets brighter, it means the battery has fallen below than nine-volts. DU-uh, right? I discovered this trick after leaving my pedals plugged in overnight many times…
This only works on a stomp box that has an LED, of course. In some Boss pedals, for example, you may have noticed the word “Check” above the LED. It may not be obvious but this was originally intended for checking the battery-charge. This works even better in the dark, obviously. It’s not always easy to tell how much is left in a battery from a dimmed LED, but if you plug in an AC adapter while the pedal is engaged, it’s very easy to see the difference between how much more power the pedal is getting from the adapter.
After installing a fresh battery, the voltage will quickly drop to between 8~9V. When it falls to below that, it’s probably time to get a new one, but it depends on the device. 3~5V is almost certainly useless, but if you like the sound of a fuzz with a weak battery, you can save these weakened batteries. Unfortunately, the Keith McMillan Battery Tester does not tell you how many volts you get from a battery, like a multi-tester would.
With a fresh battery, occasionally the LED shines brighter than with an AC adapter! This is because a new battery may actually have enough juice (or current) to put out closer to 9.6V. The acid/juice in battery cells is like water in a ballon, and the voltage is like the air-pressure. So less current means less voltage, just as less air volume means less pressure forcing its way through the opening.
A regulated Roland/Boss AC adapter puts out exactly 9V, but some other brands like Line6 will put out 9.6V, and you can also measure this with a $30 multi-tester. Every home should have a multi-tester, but BE CAREFUL! I’ve never shocked myself testing an AC adapter, but one I did trip all the fuses in the house. Not good for downloads.