Battery Charger Adapts to Multiple Chemistries

Jul 1, 2008 12:00 PM
By Terry Cleveland, Manager, Design Architecture & Applications Engineering, Microchip Technology, Chandler, Ariz.

Many handheld-device designers struggle with the choice of battery chemistry for new product definitions. In some cases, design engineers are transitioning from lower-density nickel-based chemistries to more dense lithium-ion (Li-ion) solutions. In other applications, the exact opposite is happening — some applications are switching from Li-ion to nickel-metal hydride (NiMH) chemistries.

This selection significantly impacts both the user and the designer with regard to cost, portability, safety and product life. Obviously, the battery lives of all rechargeable handheld devices are not created equal. Improper charge profiles can take the life out of a device. The following is a method for developing battery chargers that are programmable and adaptable to all rechargeable battery chemistries. This approach can be adapted easily to new chemistries and charge methods as they emerge.

Different Charge Profiles

Fig. 1 shows a typical charge profile for a Li-ion battery, and Fig. 2 is a typical NiMH charge profile. For many applications, there is a need to modify or adapt the typical charge profile. In these cases, a microcontroller-based mixed-signal design can be used to develop programmable charge profiles.

Li-ion batteries are recharged using a constant-current, constant-voltage profile. Prior to charging the Li-ion battery, a charge-qualification process measures the battery's voltage to determine whether it is deeply discharged (typically, below 2.4 V to 3.2 V per cell). If the battery is deeply discharged, the charge cycle begins with a precondition charge current, typically 5% to 25% of the fast-charge, constant-current value.

Once the battery voltage is above the precondition threshold, the constant-current charge phase of the charge profile can begin (Fig. 1). During the constant-current phase of the profile, the battery's voltage rises. Once it reaches the desired constant voltage, the charger must transition from constant-current to constant-voltage mode.

Charge termination occurs when the charge current during the constant-voltage phase is reduced to a percentage of the fast-charge constant-current value. In this example, 20% is being used as the charge-termination current. Manufacturers recommend anywhere from 7% to 30% for optimal battery cycle and capacity performance. This completes a typical charge profile for Li-ion batteries.

Besides the development of the constant-current and constant-voltage phases of the charge cycle, battery charger designs also require safety features. One example is a safety timer limiting the amount of time a charger will spend in a particular portion of the charge cycle.

For instance, timers limit the amount of time a charger will attempt to condition a faulty battery in the precondition phase, or the amount of time the charger will spend in the high constant-current phase or constant-voltage phase. Limiting voltage during the constant-current phase and current during the constant-voltage phase are important safety features for all battery chargers.

As shown in Fig. 2, the charge profile for NiMH/nickel-cadmium (NiCd) batteries is significantly different from that of Li-ion batteries, even though they both begin with a small conditioning current for deeply discharged batteries.

NiMH and Li-ion batteries are different in how the end of charge is detected. For NiMH, the end of charge is detected by measuring a reduction of battery-pack voltage or an increase in battery-pack temperature. A decreasing pack voltage or an accelerated increase in temperature are indications that the fast-charge current phase of the charge cycle is over and the charger should transition to the top-off phase of the cycle.

The top-off portion of the charge cycle is a reduced constant-current phase for a defined length of time. Typically, the constant current can range from 5% to 20% of the fast-charge current value. For NiMH battery-safety timers, charge-current limit and output overvoltage protection are important features, as they are in Li-ion battery chargers.

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