An energy source monitoring system evaluates the state of an energy source and responsively controls a power tool or other device. The energy source may be a power tool battery pack that includes multiple individual battery packs connected in series. The energy source monitoring system may be incorporated into a power tool, screwdriver, flashlight, audio and/or video player, or any other device. The energy source monitoring system intelligently exercises control over the power applied to the device. In one implementation, the energy source monitoring system detects when voltages in the energy source have fallen below a cutoff level, yet allows the device to continue running until the operator finishes the current device operation.