The fixed threshold equalization strategy and the traditional fuzzy equalization strategy usually result in overvoltage or undervoltage protection to malfunction at the end of the battery pack charge and discharge
and it is difficult to be fully balanced
because a large difference between the batteries makes the balance insufficient. A variable universe double fuzzy controller(VDF)equalization strategy is thus proposed. The lithium battery simulation model is established according to the experimental data of charged and discharged NCR18650B ternary lithium battery. The state of charge(SOC)of the single lithium battery is used as the input of the first-level fuzzy controller
and its output multiplied by the lithium battery working current value is used to adjust the output domain of the second-level fuzzy controller; the difference between the SOC of a single lithium battery and the average SOC of the battery pack and its rate of change are used as the input of the second-level fuzzy controller
and its output is used as the equalizing current; the extreme difference of SOC between the single cells in the battery pack represents the inconsistency
and the equilibrium time is determined by the first convergence of the extreme difference to 0.05. The simulation results from Simulink show that the VDF equalization strategy effectively reduces the voltage protection malfunction. With constant charge and discharge current and in custom dynamic stress test(DST)
VDF equalization strategy is compared with the traditional fuzzy equalization strategy
and the results show that the equalization rate increases by above 30%
and as a large difference exists between the batteries
the consistency increases by above 20% after the end of the equalization.
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