By Hossein Maleki and John Johnson, Motorola Energy Systems Group, Lawrenceville, GA

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For the last few years, the notebook computer business has grown at an annual rate of about 25%, and the same growth rate is predicted to continue for the next few years. Such growth is attributed to the development of small, yet sophisticated notebook computers that have met both the users’ ordinary and complex application requirements. Today’s notebook computers may include 1.2 - 2.0 GHz CPUs (e.g. Pentium-IV), CD/DVD players, high resolution LCDs, ample disk space (100s of MB), wireless connectivity, and remote information sensing capabilities. Unfortunately, these components are less than 100% efficient, so considerable heat is generated during operation. In addition, rechargeable batteries self-heat while charging and discharging because of their inherent electrical, thermodynamic, and electrochemical impedances. Furthermore, Li-ion cells should be operated and/or maintained at temperatures lower than their chemical stability limit. Extended exposure of Li-ion cells to temperatures above 60°C will degrade their performance.

A combination of notebook computer features – compact size, high functionality, fast operating speeds, long run time, and high-resolution visuals – has created new challenges in notebook battery thermal management. Cell performance differs from manufacturer to manufacturer, heat generation varies with operating conditions, and products must be cost effective. These requirements have added a great deal of complexity to the thermal management issue. For practical reasons, thermal simulation plays a critical role in the design of highly reliable notebook computer batteries. Prediction of the thermal responses of a battery pack based on its real-time application requirements can save engineering resources and product design costs while ensuring quality and reliability. Motorola Energy Systems Group uses Icepak software to complete a full, problem-to-solution analysis for the development of batteries with optimum cooling capability.

Thermal profile of the electronics and cells inside of the notebook battery discharged at 55W constant power at 42°C ambient

As an example, a simulation of a notebook computer battery was conducted to determine the real-time thermal performance of the battery while charging at 2.2A, and discharging at a constant power of 55W, both at 42°C ambient. Results showed that the battery had a reasonable level of heat dissipation capability since the temperatures of its cells rose above 60°C for only a short time. However, the temperature rise of the charge control integrated circuit (IC) increased to near its critical limit of 73°C. Autopsy results showed that this particular IC had a small die-to-pad size ratio and a large volume of molding compound, both of which caused increased heat accumulation during the charge control process. Overall results indicated that the battery temperature rise during charge is dominated by the power dissipation from the control electronics, and that the temperature rise during discharge is dominated by heat dissipation from the cells. The results from this modeling exercise provided an understanding of the thermal response of the battery that helped to reduce the product design cycle-time.

Heat generation profiles of 2.2-A cells from four different manufacturers at 40°C ambient; the variation in heat released increases as the cells are discharged at higher power

Temperatures of control electronics and cells of the battery at the end of charging and discharging

Note that, during discharge, the Icepak results show cell temperatures that are higher than the experimental values. This is due to the fact that CFD captured the temperature over the entire body of the cells, whereas the measured value is the cell skin temperature, which is generally lower than the temperature at the center of cell, especially during discharge. CFD can therefore provide important information that could have been very difficult to obtain experimentally.

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