Battery Charger ICs

Performance and longevity of rechargeable batteries depends on the quality of the chargers. One type of charger (used only for NiCd) applies a fixed charge rate of about 0.1C (one tenth of the rated capacity). A faster charger takes 3 to 6 hours with a charge rate of about 0.3C.

A charger for NiMH batteries could also accommodate NiCds, but not vice versa because a NiCd charger could overcharge a NiMH battery. Lithium-based chargers require tighter charge algorithms and voltages. Avoid a charge rate over 1C for lithium battery packs because high currents can induce lithium plating. With most lithium packs, a charge above 1C is not possible because the protection circuit limits the amount of current the battery can accept.

Charging of NiCd and NiMH batteries starts with a charge-qualification mode based on battery voltage and temperature. If the battery voltage is less than the charger IC's internal minimum threshold, the IC goes into a charge-pending state. This condition indicates the possibility of a defective or shorted battery pack. To revive a fully depleted pack, the charger IC trickle-charges the battery.

Following qualification, charging continues while monitoring charge time, temperature, and voltage for adherence to the termination criteria. The charging cycle ends with a trickle maintenance-charge that continues until the voltage reaches its end-of charge value.

Precise full charge detection of nickel-based batteries requires ICs that monitor battery voltage and terminate the charge when a certain voltage signature occurs. A drop in voltage signifies that the battery has reached full charge, known as Negative Delta V (NDV).

After full charge, you can trickle charge a NiCd battery to compensate for its self-discharge characteristics. The trickle charge for a NiCd battery ranges between 0.05C and 0.1C. To reduce memory effects there is a trend towards lower trickle charge currents.

NiMH battery chargers now use a combination of NDV, voltage plateau, rate-of-temperature-increase (dT/dt), temperature threshold and timeout timers. The charger utilizes whatever comes first to terminate the fast-charge. NiMH batteries that use NDV or the thermal cut-off control tend to deliver higher capacities than those charged by less aggressive methods.

Li-ion battery charging usually involves a constant current/constant voltage process. The charger IC for a Li-ion battery initiates the charge cycle by ensuring that the battery is not in deep discharge. If the battery voltage is below 2.5V, it is recommended to trickle charge the battery with 5mA to 10mA until the output is above 2.5V. At this point you can charge the battery with constant current until it reaches its top off voltage (4.2V for a typical single Li-ion cell) or a time-out occurs.

For the constant-voltage portion of the charge cycle, an accurate termination voltage is recommended. The higher the termination voltage accuracy, the more energy the battery will store. Since Li-ion cells do not exhibit a memory effect, less accurate termination does not harm the cell but simply stores less usable energy in the battery. The charge cycle is completed by disabling the charging process when the termination current drops below a minimum recommended level, typically 50mA or less, depending on the manufacturer's recommendation, or if the circuit times out.

Charge time for Li-ion batteries charged at a 1C initial current, is about three hours. Full charge occurs after reaching the upper voltage threshold and the current drops and levels off at about 3% of the nominal charge current. Increasing Li-ion charge current has little effect on shortening the charge time. Although it reaches the voltage peak faster with higher current, the topping charge will take longer. Overcharging can cause the cell to overheat.

Li-ion batteries have good cold and hot temperature charging perf