Reducing cell to cell temperature gradients

Reducing Cell-to-Cell Temperature Gradient Related Inhomogeneous Aging

In state-of-the-art indirect liquid cooling systems, temperature variations within a battery electric vehicle (BEV) lithium-ion battery (LIB) module are unavoidable:

  • LIB cells generate heat during operation
  • The generated heat is absorbed by the coolant
  • Coolant temperature increases downstream of the coolant channel
  • The temperature difference between LIB cells and coolant decreases downstream of the coolant channel
  • LIB cells located downstream experience higher operating temperatures

A cell-to-cell temperature difference of up to 5 K is generally considered acceptable within existing battery thermal management system (BTMS) concepts. However, the operating temperature of LIB cells plays a crucial role in their degradation behaviour. Operating at different temperatures than the optimum leads to:

  • High temperatures: Accelerated aging due to the dominant aging effect of solid electrolyte interphase (SEI) layer growth
  • Low temperatures: Accelerated aging due to the dominant aging effect of lithium plating

In conventional indirect liquid cooling BTMS concepts, this effect leads to inhomogeneous aging over the life cycle of the BEV, especially when thermally demanding load cycles such as fast charging are repeatedly used.

Figure 1: Coolant and LIB temperature increase downstream the cooling channel.
Figure 2: Temperature-dependent aging rate. Adapted from Waldmann et al. [1].

To address this issue, a switchable thermal management system for the traction battery is proposed. In this concept, the coolant flow direction is reversed after each complete load cycle (charging and discharging) by integrating a 4/2-way valve into the existing BTMS architecture. Consequently, in subsequent load cycles, the cells that previously operated at the coldest temperatures become the hottest and vice versa, while the centrally located cells maintain their temperature.

Figure 3: Schematic of the conventional and switchable thermal management concepts. Adapted from [2].

While previous studies have investigated altering the flow direction, they mainly focus on reducing the temperature gradient within the module during a single charging or discharging process. However, this can lead to temporary coolant standstill and/or require complex changes to the BTMS architecture. In contrast, the proposed concept only requires one additional 4/2-way valve (e.g., which can be directly placed or integrated into the octovalve). The 4/2-way valve should be activated during vehicle standstill, either after charging or driving, with a sensor detecting each applied fast charging event.

To investigate the impact of the proposed BTMS concept on cumulative, temperature-related aging inhomogeneities within LIB modules, a holistic thermal modeling approach was developed. This approach yields a validated 1D electro-thermal-fluid-aging model suitable for life-cycle simulations at the module level, using an Arrhenius-based aging model.

The simulation results for a conventional unidirectional BTMS (based on the Tesla Model 3 Long Range 2019 version) reveal:

  • The highest aging rate occurs during fast charging (combined effect of high C-rate and high operating temperature)
  • 1.68 % state-of-health (SOH) difference when the first cell reaches end-of-life (EOL)
  • 15.5 % difference in total ampere-hour throughput between front and rear cells until their respective EOL is reached

Compared to the conventional system, the switchable BTMS concept shows promising results:

  • The hottest cells become the coldest in the subsequent load cycle, and vice versa
  • Centrally located cells maintain their operating temperatures during all load cycles
  • 0.02 % SOH difference when the first cell reaches EOL
  • 0.18 % difference in total ampere-hour throughput between front and rear cells until their respective EOL is reached
Figure 4: Comparison of cell (brick) temperature behavior during the first four load cycles and SOH behaviour over the entire life-cycle until the first cell (brick) reaches the EOL criterion. Adapted from [2].
  • The total charge throughput increases by 7.2 % until the first cell reaches EOL compared to the conventional unidirectional BTMS concept

This 7.2 % increase in charge throughput leads to a corresponding 7.2 % increase (≈ 20000 km) in vehicle mileage until the first cell reaches EOL. Further investigation of the applied switching interval shows that the interval can be extended up to 10 load cycles while maintaining similar benefits of the concept.

More details on this concept and the applied methodology for the investigation can be found in the published version [2] or the preprint version [3].

References:

  1. Waldmann, Thomas; Wilka, Marcel; Kasper, Michael; Fleischhammer, Meike; Wohlfahrt-Mehrens, Margret: Temperature dependent ageing mechanisms in Lithium-ion batteries–A Post-Mortem study. In: Journal of Power Sources 262 (2014), S. 129–135.
  2. Auch, Marcus; Weyershauser , Konstantin; Kuthada, Timo; Wagner, Andreas: Reducing Temperature-related Aging Inhomogeneities in Battery Modules Using a Switchable Thermal Management System. SAE Technical Paper (2025).
  3. Auch, Marcus; Weyershauser , Konstantin; Kuthada, Timo; Wagner, Andreas: Reducing Temperature-related Aging Inhomogeneities in Battery Modules Using a Switchable Thermal Management System. TechRxiv (2024).

See also: Cell Temperature, Cell Temperature Sensing, Thermal Management System

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