A heat sink for a handheld device is formed as a one piece chassis for supporting heat-producing circuits inside the device while having heat radiating surfaces outside the device. In another embodiment a battery pack control circuit under microprocessor control allows battery packs having different battery chemistries and characteristics to be interchanged in the same device without switches or adapters. Control of operating modes of the device despite changes in the battery pack including battery chemistry, state of charge, available voltage, or desired operating mode are automatic. The microprocessor measures the battery pack characteristics, using look up tables in a suitable program to adjust the circuit operating modes. The microprocessor also responds to sequential switch contact closures in setting operating modes.