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How does a cordless power tool battery charger work?

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Chargers fill up batteries with power to recharge them for cordless power tools. Cordless power tool battery chargers work by using electricity from the mains and filling up a discharged battery with power.
The chemistry and physics behind a refilling a rechargeable power tool battery. The science behind rechargeable batteries and how the chargers can refill the battery with power is discussed on the page How does a cordless power tool battery work? Here we look at how chargers ensure a full, efficient charge and avoid damage to the batteries.
Chargers often use three stage charging to charge lithium ion and nickel batteries. The best chargers use what is called a 3-stage charge or multi-stage charge. Both nickel and lithium-based battery chargers use a 3-stage system although they work slightly differently.

3-stage charging

NiCd battery charger for a rechargeable battery for a cordless power tool. The three stages are called ‘bulk’, ‘absorption’ and ‘float’. Some chargers use a 2-stage system which involves just the bulk and float stages; these chargers are faster but don’t take as much care of the battery.
Bulk stage charge up to 70-80% battery capacity. The bulk stage is when the battery is being charged to about 80% capacity. The electrical current stays at the same level and the voltage (electrical pressure) being applied by the charger increases.
Absorption charge or stage is when the battery is filled up the last 20%. The absorption stage is when the voltage is kept at the same level and the current is slowly decreased until the battery is fully charged. It is also known as the ‘topping charge’ because it is topping up the last of the battery. It takes a lot longer than the bulk stage because it needs to be slower to prevent damage the battery.
A trickle charge is like a trickle of water except the water is electricity. The float stage of NiCd and NiMH battery chargers, also known as the ‘trickle charge’, is when the voltage and current are both reduced to a very low level. This maintains the battery at full charge until it is needed.
NiMH batteries cannot be charged in a NiCd battery charger because it will damage the battery. NiMH rechargeable batteries require a much lower trickle charge than NiCd batteries which means they cannot be recharged in a NiCd-specific charger. However, NiCd batteries can be recharged in a NiMH charger, although it is not ideal.
Float stage of a lithium ion rechargeable battery The float stage of Li-ion battery chargers is not a trickle charge. Instead, pulses of charge keep the battery topped up to counteract the self-discharge. A trickle charge could overcharge a lithium battery, damaging it.

Detecting a full battery

Temperature is not a very good indicator of when a nickel-based battery is full. Cheap chargers detect when a NiCd battery is full by monitoring the temperature of the battery. This is not really accurate enough and can damage the battery over time.
Voltage drops when the battery is full, this is a better indicator for NiCd battery chargers. More advanced NiCd battery chargers use Negative Delta V (NDV) technology which detects the drop in voltage that happens when the battery is full. This is much more reliable.
A mixture of sensors monitoring temperature and voltage is est for finding out when a NiMH battery is full. NiMH battery chargers have to use a combination of sensors to detect when the battery is fully charged because the voltage drop is not large enough to be detected accurately.
the best chargers have microprocessors to make sure they work efficiently. Li-ion battery chargers have a more complicated computer chip that monitors the changes in the cells. Li-ion batteries are more fragile and need more accurate detection methods to protect against damage.