Virtually all battery-based powermanagement designs depend on the associated battery, so design starts by picking the specific battery type. The battery may be the non-rechargeable primary type or the rechargeable secondary type. (For more, see “Batteries 101: From Nickel To Lithium And Beyond” at www.electronicdesign.com, ED Online 20737.) The most widely used rechargeable battery-based systems may employ:

• Nickel cadmium (NiCd), which finds use where long life, high discharge rate, and economical price are important
• Nickel-metal hydride (NiMH), which has a higher energy density than NiCd, at the expense of reduced cycle life
• Lithium-ion (Li-ion), which is used where high-energy density and light weight are of prime importance
• Li-ion polymer, with chemistry similar to the Li-ion in terms of energy density

Battery-Charger ICs
The charge and discharge capacity of a secondary battery is in terms of “C,” given as Ah. The actual battery capacity depends on the C-rate and temperature. Most portable batteries are rated at 1C. A discharge of 1C draws a current equal to the rated capacity. For example, a battery rated at 1000 mAh provides 1000 mA for one hour if discharged at a 1C rate. The same battery discharged at 0.5C provides 500 mA for two hours.

The performance and longevity of rechargeable batteries depend mainly on the quality of the chargers. One type of charger (used only for NiCd) applies a fixed charge rate of about 0.1C. A faster charger can take three to six 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 packs because high currents can affect the lithium. With most lithium packs, a charge above 1C isn’t possible because the protection circuit limits the amount of current the battery can accept.

Precise full-charge detection of nickelbased batteries requires special 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 into 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’s now a trend toward lower trickle-charge currents.

NiMH battery chargers use a combination of NDV, voltage plateau, rate-oftemperature- 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 cutoff control tend to deliver higher capacities than those charged by less aggressive methods.

Li-ion chargers use a voltage-limiting device. However, Li-ion batteries have a higher voltage per cell, tighter voltage tolerance, and the absence of trickle or float charge when reaching full charge. 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. Because Li-ion batteries can’t absorb overcharge, these batteries should not use a trickle charge. Overcharging can cause the cell to overheat.

Li-ion batteries have good cold and hot temperature charging performance. Some cells allow charging at 1C from 0°C to 45°C. Most Li-ion cells prefer a lower charge current when the temperature gets down to 5°C or colder. Avoid charging below freezing.

Several charger ICs for Li-ion batteries make it possible to charge the battery via a USB port or an ac adapter. For USB operation, the user can plug the USB cable into a desktop or laptop computer and use the 5-V output to charge the battery pack in a cell phone or PDA.

Texas Instruments’ bq24150 batterycharger IC is a compact, flexible, high efficiency, USB-friendly, switch-mode charge-management IC for single-cell Liion and Li-polymer batteries (Fig. 1). The I²C interface allows the programming of the charge parameters and reports charge status to the host. It integrates a synchronous pulse-width modulation (PWM) controller, power MOSFETs, input current sensing, high-accuracy current and voltage regulation, and charge termination, into a small wafer-level chip-scale package (WCSP).

Continued on page 2

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Reader Comments Can a set of 1.2V 2450 mAh be left in the charger after charge is complete. I noticed that the batts were warm when fully charged, but at room temperature when charging terminated and light on charger stopped flashing and stayed solid green. Also does leaving the batts in the charger after charging damage or reduce the battery's life. Best regards, BrianJ Brian -March 20, 2009
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