[font=trebuchet ms]A classic gearhead response! :rolleyes:

All you really need to do is measure the voltage difference while turning the effect on and off. A weak battery can't supply current while it's under load, so the voltage drops noticeably. If you can get the meter probes onto the battery terminals while it's still in the pedal, you don't need to fart around with a resistor. (As second guesses go, I much prefer my own).[/font]