Abstract
In a battery management system (BMS), battery equalizer is used to achieve voltage consistency between series connected battery cells. Recently, serious inconsistency has been founded to exist in retired batteries, and traditional equalizers are slow or inefficient to handle the situation. The multicell-to-multicell (MC2MC) topology, which can directly transfer energy from consecutive strong cells to consecutive weak cells, is promising to solve the problem, but its performance is limited by the existing converter. Therefore, this paper proposes an enhanced MC2MC equalizer based on a novel bipolar-resonant LC converter (BRLCC), which supports flexible and efficient operation modes with stable balancing power, can greatly improve the balancing speed without much sacrificing the efficiency. Mathematical analysis and comparison with typical equalizers are provided to illustrate its high balancing speed and good efficiency. An experimental prototype for 8 cells is built, and the balancing powers under different operation modes are from 1.426 W to 12.559 W with balancing efficiencies from 84.84% to 91.68%.
Highlights
The inductance/capacitance and equivalent series resistance (ESR) of the inductor and capacitor are measured by a Tonghui TH2832 LCR meter with accuracy of 0.05%
In order to improve the active equalizers’ balancing speed and efficiency to face new challenge brought by retired batteries, this paper proposes a novel bipolar-resonant LC converter (BRLCC) for the
The BRLCC uses the bipolar resonance of the LC resonant tank to deliver energy and overcomes three main problems of existing switchedcapacitor converter (SCC) and half-bridge LC converter (HBLCC)
Summary
Available capacities (which are the integrals of the current) among cells tend to be various because of manufacturing tolerances, temperature difference across the pack, and differences in self-discharge rate [7,8]. This variety of capacity will lead to some cells’ overcharge during charging process or lead to some cells’ over-discharge during discharging process [9]. To avoid the destructive overcharge or over-discharge of lithiumion batteries [10], the pack normally has to end the charging/discharging process before it is fully charged/discharged, which means its capacity cannot be fully utilized [11,12]. This phenomenon can even get more serious after many charging and discharging cycles [13,14], and cannot be permanently guaranteed by an initial selection of equal battery cells [15]
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