The Complete Handbook of Power Tool Chargers

Chapter 1: Understanding Power Tool Chargers

A power tool charger is not a simple power adapter. It is a small, specialized computer and power supply that manages the flow of electricity into a rechargeable battery pack. Its primary function is to take the AC power from a wall outlet, convert it to the correct DC voltage and current, and then carefully deliver that power to a lithium-ion, nickel-cadmium, or nickel-metal hydride battery pack. The way it does this is critical. A poor or mismatched charging process can reduce a battery's capacity, shorten its usable lifespan, or, in the worst case, cause overheating that damages the tool or the user's property. Understanding how this process works is the first step toward making smart choices about equipment, extending the life of expensive batteries, and ensuring a safe and efficient work environment.

Let's start with the basic flow of power. When you plug a charger into a standard 120V (or 230V in many other countries) AC wall outlet, the charger's internal circuitry first steps the voltage down to something more appropriate for a battery pack. A typical 18V or 20V max lithium-ion pack actually has a nominal voltage of around 18V, but it charges to a higher "full" voltage of around 20V to 21V. The charger doesn't just apply a flat 20V DC and hope for the best. Instead, it uses a process called a constant current / constant voltage (CC/CV) charging algorithm, which is common to most modern chargers for lithium-ion batteries.