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Designing multi-cell Li-ion battery packs using the ISL9208 analog front end

Posted: 07 Aug 2007 ?? ?Print Version ?Bookmark and Share

Keywords:microcontroller? battery pack? multi-cell Li-ion?

A microcontroller provides the primary control of the operation of the battery pack. However, several factors in the multi-cell series Li-ion pack require the use of circuitry around the microcontroller.

? ?The voltages involved in a multi-cell series battery pack (up to 30V for 7-cells in series), are far higher than most MCUs are rated. So, the pack needs a voltage regulator to power the microcontroller. The MCU cannot just operate on the voltage from one of the string of

Li-ion cells (typically 3.0V to 4.2V) because higher current from only one cell will cause an imbalance in the battery pack. This will shorten the life of the pack. A later discussion highlights the effects of unbalanced cells and how to rebalance the pack.

? ?The high voltage of the cells in the pack preclude the MCU from reading the voltage on each cell as needed to properly manage the charge and discharge limits in each cell. So the pack needs circuits that level shift the voltages across each cell down to a ground referenced voltage that the microcontroller can read using its internal A/D converter.

? ?Because the MCU is relatively slow to respond to high speed overcurrent events, (such as a short circuit condition) the pack needs circuits that shut down the pack quickly and autonomously of the MCU to protect the cells and the electronics in the pack.

? ?In order to balance the cells in the pack, the MCU needs circuitry that will activate the balancing circuit of each cell. Most of these circuits are at a voltage too high for direct MCU control. The ISL9208 meets all of these needs and supports battery pack configurations consisting of 4- to 7-cells in series and 1-cell or more in parallel.

This application note discusses some of the hardware and software design decisions and shows how to select external components for a multi-cell Li-ion battery pack using a microcontroller and the ISL9208 analog front end.

Please view the PDF document for more information.

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