Experiment on the Thermal Issues of High Power and High Energy Parallel-serial LiFePO4 Lithium-Ion Battery Pack

 

Tong Meng^*,a,b Ouyang Minggao ^a, Lu Languang ^a, Wang Ying ^a, Shao Jingyue ^a, Deng longyang ^a, Li Zhe ^a, Yan Jun ^a, Tang Tao ^a,Qingfeng Lin ^a,Yongping Wu ^b, Jian Ma ^b, Shengjie Jiao ^b

a. Department of Automobile Engineering, Tsinghua University, Beijing, China

b. Department of Construction Mechanical, Chang An University, Xi’an, China

Abstract

A LiFePO₄ based Lithium-Ion (LFP) battery pack in parallel-series configuration with 208 11Ah cells has been tested to find the potential thermal issues, which are important to its performance, safety, and durability in battery electric vehicle (BEV) application.

While temperature rise is only moderate for most cells in charge/discharge at C/3 or C/2 rate, overheating and uneven temperature distribution phenomenon in some cells are observed in discharging process. Quite a few of cells are seriously overheated in the initial discharging process, even with only a 30Ah discharge at C/3 rate.

Temperature distribution and overheating is relevant to current distribution, internal resistance, electrical contacts resistance, and SOCs or discharging progress imbalances between cells in parallel or in series. Overdischarge and voltage reverse induced by SOCs imbalances, contact resistance difference, and SEI film effects are the major factors contributed to the overheating issues.

Current waveform distortion phenomenon in single cells Experiment shows an agreement to existence of random breakdown of SEI film in LFP battery, which may greatly influence the current distribution among cells in parallel.

The merits of the Lower internal resistance of the powerful LFP Battery pack inversely deteriorate the thermal issues due to increasing the negative influence of the contact resistance imbalance and SEI’s random breaking.

Balancing the modules (sub-stack) and group, adjusting the contact force apparently lessen the generated heat and overheating phenomenon.

Measures, like equalization of all the series in the pack and monitoring temperatures of cells, are required to avoid the partially abuse of cells. Reasonable mechanical structure and electrical contacts design and assembly are equally important for the LFP Battery pack too.

Keywords: lithium iron phosphate (LFP) battery; parallel-series connected battery pack; imbalance; overheat; SEI

summitted paper, welcome to give review to me. Email: drtongmeng【at】gmail.com

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