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1 Cell Balancing
Application Report
SLUA463 – July 2008
Cell Balancing With the bq77PL900
Willy Massoth ...................................................................................................... Battery Management
ABSTRACT
The bq77PL900 five to ten series cell lithium-ion or lithium-polymer battery protection
and AFE IC includes a cell-balancing function. This document describes how to use the
cell-balancing feature of the part in a battery pack application. Boosting the current
capability of the IC using external FETs is described. The algorithm for balancing in
stand-alone mode is described as well as considerations for implementing a
host-controlled balancing algorithm which avoids damage to the IC.
Contents
1 Cell Balancing ....................................................................................... 1
2 Description of Internal Cell-Balancing Circuit ................................................... 2
2.1 Internal Cell-Balancing Circuit Design .................................................. 2
2.2 External Cell-Balancing Circuit Design ................................................. 2
3 Stand-alone Balancing Algorithm ................................................................. 4
4 Considerations for a Host-Balancing Algorithm................................................. 8
5 Additional Timing Information ..................................................................... 9
List of Figures
1 Internal Cell-Balancing Circuit .................................................................... 2
2 External Cell-Balancing Circuit .................................................................... 3
3 Internal FET Characteristic ........................................................................ 3
4 Improper Cell-Balancing Configuration Which Results in Damage to the IC ............... 6
5 Proper Cell-Balancing Configuration Which Avoids Damage to the IC ...................... 7
List of Tables
1 Stand-alone Balancing Algorithm ................................................................. 4
2 Damaging Input Voltages With Improper Cell Balancing (0,1,2,3,5,6,7,8 balancing) ...... 6
3 Example Cell Voltages With Proper Balancing ................................................. 8
Cells are usually matched during the manufacturing of a battery pack. Over time, an imbalance in the state
of charge may develop between cells and reduce the overall capacity of the pack. Cell balancing that
equalizes the cells allows the pack to operate longer.
The bq77PL900 provides a bypass method of balancing. Some of the current which would charge the cell
is diverted through a parallel path. Cell balancing operates during on a 50-ms nominal period. Voltage
monitoring occurs during approximately 10 ms of this interval, a balancing FET internal to the IC is
switched on during the remaining 40 ms of the period to provide a bypass path. The FET is switched off
again at the end of the balancing interval to measure the cell voltage.
SLUA463 – July 2008 Cell Balancing With the bq77PL900 1
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