Abstract
As the number of parallel battery connections in an energy storage system is increased to extend the energy capacity and second-life batteries are actively adopted, the battery is more prone to cell inconsistency issues. The difference in the internal impedance and the mismatched state-of-charge accelerates the self-balancing effect between the parallel branches to reduce cell utilization and eventually results in harmful effects, both to the lifetime and to the safety of the batteries. However, conventional methods only partially mitigate the parallel inconsistency issue. This paper proposes a dynamic resistance equalizer for parallel-connected battery configurations to improve equalization performance. The optimal design procedure is also presented to minimize the power loss and equalization time. The overall performance is experimentally verified by a sequence of tests for a Li-ion battery in a 2S-4P configuration. The experimental results show that the proposed method dissipates less external power loss than the fixed resistor equalizer and less internal loss than the conventional sequencing method. When both total loss and balancing performance are considered together, as the number of series connections increases, the merits of the proposed method stand out. This is verified by additional hardware-in-the-loop tests, presenting a fascinating feature for most practical battery applications.
Highlights
In an effort to prevent climate change, the transportation industry is becoming increasingly electrified [1,2]
This paper proposes a novel dynamic resistance equalizer for parallel-connected battery configurations
Based on the SOC status of battery cells, the switches are controlled to modulate the impedance of the parallel branches while adjusting the branch current
Summary
In an effort to prevent climate change, the transportation industry is becoming increasingly electrified [1,2]. The battery packs of electric vehicles (EV) have a limited lifespan and usually need to be replaced when the state-of-health decreases to 70–80% [3]. The second-life battery energy storage system (SL-BESS) is a promising solution to re-use the retired battery packs [6]. The difference in the internal impedance, capacity, and electrical characteristics between cells—so-called cell-inconsistency—becomes more serious in the second-life battery application. In most energy storage systems, like EV or SL-BESS, battery cells are connected in series to reach the operating voltage specification. Due to the imbalance in the electrochemical impedance, cell-inconsistency issues arise, and various cell-balancing techniques for the series-connected battery cells have been developed, which can be classified into passive and active techniques [7,8]
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