Hossein Maleki, John Johnson and Kevin Kitts, Motorola Energy System Group (ESG), Lawrenceville, GA

Demands for small and high power sources to operate portable electronics and their associated accessories are continuing to increase. Among these demands are increased power and reduced size for lithiumion (Li-ion) battery packs and their associated charging units. Li-ion batteries have become the power source of choice for portable electronics because of their high energy density, rate capability, and long cyclelife. However, they tend to selfheat during charge and discharge cycles, and lose capacity if exposed to or operated at temperatures greater than 65°C.

The internal peak temperature rise of the charger when fuel gauging (calibrating) eight batteries simultaneously is shown. The temperature of the load resistors (location 2) rises to ~88°C. Modeling also showed that the heat that evolves mainly from the load resistors causes the temperature of the back of the aluminum (Al) base (location 4) to rise above the critical limit (55°C), set by UL for metallic parts that could be touched by the end users.

The table above compares Icepak predictions to experimental data obtained while the unit calibrated eight batteries simultaneously

To charge a Li-ion battery, a charger needs to apply a controlled current to increase the Li-ion cell voltage from about 3.0 V to no more than 4.2 V. Overcharging could lead to capacity fading and thermal stability issues. Multi-unit chargers are more economical to operate than single-unit chargers, but they can run at higher temperatures, causing potential damage to the batteries and control electronics.

Motorola Energy System Group (ESG), a leading provider of complete energy system solutions for portable electronics, such as cell phones and laptop computers, has used Icepak to address thermal management issues related to a multi-unit charger for Li-ion batteries. This effort has allowed engineers to simulate the product’s thermal response for a given set of customer specifications, and confirm or make changes to the design before a new product is built.

Using Icepak, an eight-unit charger with maximum natural convection cooling was simulated. Early design validations demonstrated that Icepak predictions of temperature at several sites on the charger were in good agreement with measured data (see table at right). Through subsequent modeling, it was determined early in the design phase that the customer’s timeframe requirement for charging or calibrating (discharging a fully charged cell for capacity check) all eight batteries simultaneously was not possible. The charge step caused the temperature of the power supply to rise above its optimum operating temperature. Calibrating affected heat dissipation from the Li-ion cells and their associated load resistors.

Fin cooling (top) and fan cooling (bottom) show the temperature distribution on the outside surface of the charger. In both cases, the simulation was conducted with four batteries being charged and four batteries being discharged. Both configurations caused the charger to exceed the allowed upper temperature limit (55°C).

This charger has fins placed on the backside of the printed circuit board (PCB) beneath the load resistors. Additional modifications in this model included increasing the height of the back-wall of the Al-base, and thermal isolation of the back end of the PCB from the Al-base. These changes led to better cooling of the Al-base, maintaining a temperature below 55°C.

Icepak was also used to evaluate the effects of fan cooling versus fin cooling on the operating temperature of the unit while simultaneously discharging four batteries and charging four batteries. Results showed that the addition of a fan, meeting cost and design limitations, provides 15-17% more cooling to some parts of the charger.

After a number of modifications were tested, a final design was chosen. The series of simulations showed that the eight-unit charger, meeting customer design requirements, is capable of calibrating only three batteries, while charging five at the same time. This optimized solution, which includes detailed operating temperature information for all charger components, could not have been obtained without the combined strengths of the ESG engineering staff and Icepak software. The simulations demonstrated not only the limitations of the existing design, but also alternative solutions to improve the thermal performance of a multiunit charger. At Motorola ESG, CFD modeling with Icepak has proved to be a cost-effective tool for predicting the thermal response of electronic power sources.

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