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Summary To investigate the thermal characteristics and uniformity of a lithium‐ion battery (LIB) pack, a second‐order Thevenin circuit model of single LIB was modeled and validated experimentally. A battery thermal management system (BTMS) with reciprocating liquid flow was established based on the validated equivalent circuit model. The effects of the reciprocation period, battery module coolant flow rate and ambient temperature on the temperature and the temperature imbalance of batteries were studied. The results illustrate that the temperature difference can be effectively reduced by 3°C when the reciprocating period is 590 seconds. The reciprocating coolant flow rate is 11.5% and 33.3% that of the unidirectional flow BTMS for cooling and heating when same thermal effects are to be achieved. Under the same ambient temperature condition, the maximum temperature and average temperature difference can be reduced by 1.67°C and 3.77°C, respectively, at best for the battery module investigated with a reciprocating liquid‐flow cooling system. The average temperature difference and heating power consumption could be reduced by 1.2°C and 14 kJ for reciprocating liquid flow heating system with period of 295 seconds when compared with unidirectional flow. As a result, the thermal characteristics and temperature uniformity can be effectively improved, and the parasitic power consumption can be significantly reduced through adoption of a reciprocating liquid flow BTMS.

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A lithium-ion (Li-ion) power battery and proton exchange membrane fuel cell (PEMFC) have an excellent electrical conversion performance, which makes them a hot spot in the new energy power research field. Electricity is generated inside the battery through an electrochemical reaction, during which time a large amount of heat is also generated. The ideal operating temperature of a Li-ion battery and PEMFC falls into a small range, but the operations require a high temperature uniformity. Efficient cooling technology can be used to timely discharge the heat generated and control the temperature field change inside the battery, thus ensuring the battery's efficient and stable performance. The micro heat pipe (MHP) is currently a more efficient passive cooling equipment with high heat fluxes and a high thermal conductivity through the phase change heat transfer of working fluid. The development and research status of an MHP and its application in the cooling of a Li-ion power battery and PEMFC are reviewed in this paper.

In this work, chopped carbon fibers (CCFs) with different lengths were added to graphite/polypropylene (PP) composite materials to achieve high conductivity and flexural strength performances, which are required for use in proton exchange membrane fuel cells. The effects of CCF length (2-4 mm), CCF content (0-5 wt.%), graphite type-natural flake graphite (NFG) and synthetic graphite (SG), and graphite particle size (18-106 µm) on the graphite/PP/CCFs composites are examined. The conductivities of the composites decrease significantly with increasing CCF length above 3 wt.%. CCFs improve the composite's strength, with a maximum strength of 45.8 MPa being achieved with 5 wt.% of 4 mm long CCFs. Composite with NFG exhibits superior conductivity to the one with SG but lacks flexural strength. The NFG particle size significantly affects the conductivity of the composite at high graphite contents, with a particle diameter of 75 µm resulting in maximum conductivity. An optimal composition with 38 µm/82 wt.% NFG and 2 mm/3 wt.% CCF, electrical conductivity, and flexural strength of 189.4 S/cm and 30.2 MPa, respectively, were achieved. Also, this composite exhibited interfacial contact resistance 2.52 mΩ ⋅ cm 2 and contact angles of 111 • , which showed favorable interfacial conductivity and hydrophobicity performances.

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