AN1333: Designing Multi-Cell Li-ion Battery Packs Using the ISL9208 Analog Front End

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: • 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. • The high voltage of the cells in the pack preclude the microcontroller 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 analog to digital (A/D) converter. • Because the microcontroller 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 microcontroller to protect the cells and the electronics in the pack. • In order to balance the cells in the pack, the microcontroller needs circuitry that will activate the balancing circuit of each cell. Most of these circuits are at a voltage too high for direct microcontroller control. The ISL9208 meets all of these needs and supports battery pack configurations consisting of 4to 7-cells in series and 1-cell or more in parallel. The ISL9208 is a very flexible device that can be used in a variety of ways to implement the battery pack. It provides integral overcurrent protection circuitry, short circuit protection, an internal 3.3V voltage regulator, internal cell balancing switches, cell voltage monitor level shifters, and drive circuitry for external FET devices that control pack charge and discharge. Each of these features have some flexibility in how they are used. 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 4to 7-cells1. Connection guidelines for each cell combination are shown in Figure 1.