Balancer Circuit for Lithium Polymer (LiPo) Cell

Figure 1 

The project was designed to produce a balancer for LiPo cells in order to prevent an excessive difference between stored charges.

The difference between the LiPo cells is emphasized by the repetitive process of charging and discharging the pack of rechargeable batteries in series. The cells with lower capacity will discharge deeply than higher ones but higher capacity cells will be overcharged during the charging. It is necessary to equalize each cell especially for NiMh and NiCd cell and the use of LiPo balancer avoids an over voltage of some cells.

The function of LiPo may be compared to a very accurate Zener diode where the system monitors the voltage with an equivalent resistance and a negligible current shunt when the voltage is below the threshold value. The balancer element will start to shunt current from the cell with an internal power resistor when the threshold is reached by the voltage.

The cell over voltage with respect to the threshold is proportional to the amount of current drooped by the element. The over temperature and heating for the LiPo balancer element is avoided by the power thermal control of the last element.

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