Passive balancing battery management system using MOSFET internal resistance as balancing resistor

Balancing process is crucial to keeping battery lifespan and protecting the battery cell in series connection. Commonly, passive balancing is widely used in BMS because cheap and simple to implement. The passive balancing used passive element (resistor) to dispose of the excess charge of battery. The use of this resistor is quite effective for small battery capacity. For large battery capacity used in EV and HEV, a big power resistor is needed so that the balancing time of the battery is faster. With a large power resistor for each battery cell will result in BMS hardware becomes large. In this paper was designed a passive balancing BMS that combines the power resistor and MOSFET internal resistance as a balancing resistor. The use of MOSFET internal resistance will save space on BMS hardware compared with power resistor with greater balancing current capability. The proposed method was applied to 1P15S battery pack consisting of 15 cells LiFePO4 battery with a capacity of 200 Ah. The preliminary experimental result shows that using internal resistance of the MOSFET as a balancing resistor was feasible and effectively done. The balancing current of the battery is determined by the internal resistance value of the MOSFET.