Chargers for cordless power tools work by using electricity from the mains and charging a discharged battery.
The science of rechargeable batteries and how chargers can charge a battery is discussed on the page How does a cordless power tool battery work? Here we look at how chargers provide full and efficient charging and prevent battery damage.
The best chargers use the so-called three-stage charge or multi-stage charge. Nickel-based and lithium-based battery chargers use a three-stage system, although they work slightly differently.
3-stage charging
The three stages are called "bulk", "absorption" and "floating". Some chargers use a two-stage system with only bulk and floating stages; these chargers are faster but don't take as much care of the battery.
During the filling phase, the battery is charged to approximately 80% capacity. The electrical current remains at the same level, but the voltage (electrical pressure) supplied by the charger increases.
The absorption stage is when the voltage is held at the same level and the current slowly decreases until the battery is fully charged. It is also known as a "top-up charge" because it recharges the last battery charge. This takes a lot longer than the bulk stage because it has to be slower to prevent battery damage.
The floating stage of NiCd and NiMH battery chargers, also known as "drip charge", is when the voltage and current are reduced to a very low level. This keeps the battery fully charged until needed.
NiMH batteries require a much lower continuous charge than NiCd batteries, which means they cannot be recharged in a NiCd-specific charger. However, nickel-cadmium batteries can be charged in a nickel-metal hydride battery charger, although this is not ideal.
The floating stage of lithium-ion battery chargers is not continuous charging. Instead, charge pulses keep the battery charged to counteract self-discharge. Recharging may overcharge the lithium battery and damage it.
Full battery detection
Cheap chargers determine when a nickel-cadmium battery is charged by monitoring the temperature of the battery. This is not accurate enough and can damage the battery over time.
More advanced NiCd chargers use Negative Delta V (NDV) technology, which detects the voltage drop that occurs when the battery is fully charged. It's much more reliable.
NiMH battery chargers must use a combination of sensors to determine when a battery is fully charged because the voltage drop is not large enough to accurately detect.
Lithium-ion battery chargers have a more sophisticated computer chip that keeps track of cell changes. Lithium-ion batteries are more fragile and need more accurate detection methods to protect against damage.
