At my work we have tons of 9 volt batteries for use with our wireless mics. At the bottom of the mic there is a light that turns yellow when it's time to change the battery. You can get a decent amount of time out of them before they die, but we throw them out right when they turn yellow, so we aren't caught in the middle of a show with a dead or crackling mic. Anyway, after considering the price of 9 volts, I figured I would just take them home with me, since they are going to be thrown out anyway. The thing is, they are worn out a bit. Would I be wasting my time? When I go into work the next time I'll whip out the ol' multimeter and see what kind of voltage these batteries have left at "yellow". What's an acceptable charge for an active bass?