Designing Multi-Cell Li-ion Battery Packs Using the ISL9208 Analog Front End Application Note July 17, 2007 AN1333.0 Description Battery Connection The ISL9208 supports multiple series connected Li-ion cells. The bottom three cells (CELL1, CELL2, and CELL3) and the top cell in the string are always required. CELL4, CELL5, and CELL6 are optional. This allows the ISL9208 to be used in battery packs of 4- to 7-cells1. Connection guidelines for each cell combination are shown in Figure 1. 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. 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. They are: 7-CELLS VCELL7 6-CELLS VCELL7 ? The voltages involved in a multi-cell series battery pack (up to 30V for 7-cells in series), are far higher than most microcontrollers are rated. So, the pack needs a voltage regulator to power the microcontroller. The microcontroller 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. CB7 VCELL6 CB7 VCELL6 CB6 VCELL5 CB6 VCELL5 CB5 VCELL4 CB5 VCELL4 CB4 VCELL3 CB4 VCELL3 CB3 VCELL2 CB3 VCELL2 ? The high voltage of the cells in the pack preclude the microcontroller f