First you need to determine what type of batteries are in the unit. From the voltage, most likely either Ni-cad or NiMh ( Nickle Metal Hydride) Any charger designed for your battery type will work as long as the output voltage does not exceed 4 volts. Some chargers are designed for either type and some are type specific. Even those small battery chargers cn com ein three different versions. 1. manual- Charges until you disconnect it. 2. Auto Charges for a certain amount of time and shuts off. Fully Auto - Charges untill the correct voltage is reached, then goes into a monitor mode and keeps the battery topped off. Might be your best bet is to contact to maker and get a new charger if the tool is worth it. CAUTION The wrong charger could cause big problems - from ruining the tool to starting a fire. Use Care