BU-414: How do Charger Chips Work?

When first introduced in the 1980s, charger chips simplified the design of NiCd and NiMH chargers as batteries with these chemistries were difficult to charge. Li-ion is simpler and most modern charging chips also include the protection circuits that are needed to safe charge Li-ion. These include current and voltage regulation, FET switches and may also contain charge status indicators and cell balancing. Added to most chips is a time-out-timer that halts charge if predictable symptoms do not occur as expected when charging a flawed battery.

Advanced chips also feature pre-charge conditioning (boost) to wake up an inactive battery, as well as a sleep mode that lowers the housekeeping current of the circuit while the battery is in storage. Some chips also initiate a charge if a parasitic load lowers the battery voltage below a preset threshold while residing in a charger.

Although charger chips are easy and economical to use, they have limitations. Most offer a fixed charge algorithm that does not permit fine-tuning for specialty uses. Chips are made for a given battery and may not accommodate different chemistries as requested by the user or read a battery code that may be embedded in a battery holder. Nor do most chips adjust to an optimal charge current when charging an aging battery with reduced charge acceptance.

Microcontrollers offer an alternative to charger chips. Although the design cost is higher because of the extra programming time needed, manufacturing costs are compatible to charger chips. It should be noted that the charge chip or the microcontroller only form a small part of the charger circuit; the bulk of the cost lies in the peripheral components, which include solid-state switches, signal lights and the power supply. The parts cost is directly related to wattage.

Factory-configured charger modules are available that are set to the correct voltage, current and algorithm. Some have seamless DC-DC conversion to allow charging a battery with a higher voltage than the input provides. Options include SMBus, solar charging, discharge for calibration and display. Using soft-programmable charger modules resembles the ready-made AC power supplies that became popular in the 1990s as a lower-cost alternative to building one’s own charger for each application.