The most common method of cell balancing is to monitor the individual cell voltages and bleed energy away from the cells with the highest voltages. The energy is bled away through resistors and dissipated as heat. Other methods redirect the cell energy to charge other cells which wastes less energy but is more complex to accomplish.
While cell balancing will not turn a bad cell pack into a good one, it will keep good cells balanced and this will ensure the battery can be cycled many hundreds, sometimes thousands, of times. Reducing the charge voltage will also promote longer cycle life, albeit with a reduction in capacity per cycle.
Figure three shows a cell balancing diagram. Semiconductor specialist Texas Instruments explains that "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 field-effect transistor 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."
To keep pace with the size and demands of the global medical device market, it is vital that design engineers think carefully about their choice of battery, ensuring that their mission-critical devices continue to operate in the most demanding of environments.
About the author:
Neil Oliver is technical marketing manager of battery specialist Accutronics Ltd - www.accutronics.co.uk