Investigation on the heat generation and heat sources of cylindrical NCM811 lithium-ion batteries

High temperature may cause the degradation of capacity and lifespan for the batteries, and even possibly triggers the thermal runaway. Therefore, in order to safe use of batteries, investigation on the heat generation characteristics of lithium-ion batteries is essential to guard the design of thermal management system (TMS). In this work, the heat generation rate (HGR) and heat sources of the 18650 NCM811 battery is investigated by both the isothermal calorimetry test and calculation using the modified Bernardi model, respectively. It is revealed that during charging or discharging, the irreversible heat increases rapidly at low SOC due to large internal resistance, while the reversible entropic heat becomes more important at about 40–60 % SOC due to high entropy coefficient therein. Moreover, the ratio between heat sources has been revealed. The irreversible heat is dominated among the heat sources during charging/discharging with a proportion of 76.33–96.65 %. Wherein, the ohmic heat contributes the most heat generation. Meanwhile, the reversible heat accounts for the smallest proportion of 3.25–23.67 %. It should be noted that current is the main influence factor to the HGR of the lithium-ion battery. With increasing the current, the irreversible HGR, especially ohmic heat, increases much faster than that of reversible heat, resulting in higher ratio. The calculation data are validated to be well consistent with experimental results, giving insights to the investigation of the heat generation of the lithium ion battery.