The Effects of Thermal Gradients in Automotive Battery Packs Balancing Strategy Dr Alastair Hales Mechanical Engineering Imperial College London
ሶ Thermal Performance of Lithium-Ion Batteries Temperature effects Thermal gradients in Thermal gradients in Thermal in your battery your pack your battery management -13 °C methods -5 °C -200 5 °C -9 15 °C 25 °C -6 35 °C 45 °C -150 -3 55 °C -Z imag [m ] 0 -100 0 3 6 9 12 15 1 Hz -50 10 Hz Decreasing temperature 0.1 Hz 0 0 50 100 150 200 Z real [m ] The Cell Cooling Application and Modelling thermal Battery thermal Coefficient innovation gradients performance 𝑹 𝒋 ∆𝑼 𝒋 ൘ 𝑫𝑫𝑫 𝒋 =
How does temperature affect your battery? • Temperature affects impedance -13 °C -5 °C exponentially -200 5 °C -9 15 °C • Non-linear temperature dependence on charge 25 °C -6 transfer resistance 35 °C 45 °C -150 -3 55 °C • Two common thermal management methods -Z imag [m ] 0 for pouch cells -100 0 3 6 9 12 15 • Surface cooling 1 Hz • Tab cooling -50 10 Hz • Decreasing Thermal gradients within battery packs temperature 0.1 Hz are inevitable 0 0 50 100 150 200 Z real [m ] Troxler, Y. et al. Journal of Power Sources, Vol 247, Pages 1018-1025 (2014)
How do thermal gradients affect your pack? • Pack-level thermal gradients induce current inhomogeneities • Uneven cell-to-cell loading • Suboptimal pack operation • Uneven current distribution leads to non-uniform cell heat generation rates • Thermal gradients affect parallel cells at pack-level in the same way they affect parallel layers at cell-level Wu, B. et al. Journal of Power Sources, Vol 243, Pages 544-554 (2013)
Are there good and bad thermal gradients? Tab cooling • Some thermal gradient within a layer • Very low layer-to-layer thermal gradients • Very low layer-to-layer current inhomogeneities • Each layer behaves the same • Each layer is loaded evenly over a dynamic drive cycle Surface cooling • Low thermal gradients within a layer • Significant layer-to-layer thermal gradients • Significant layer-to-layer current inhomogeneities • Layers behave differently to one another • Layers are loaded unevenly over a dynamic drive cycle Hunt, I. et al. Journal of the Electrochemical Society, Vol 163, Pages A1846-A1852 (2016)
Are there good and bad thermal gradients? Tab cooling • Some thermal gradient within a layer • Very low layer-to-layer thermal gradients • Very low layer-to-layer current inhomogeneities Yes • Each layer behaves the same • Each layer is loaded evenly over a dynamic drive cycle Surface cooling • Low thermal gradients within a layer • Significant layer-to-layer thermal gradients • Significant layer-to-layer current inhomogeneities • Layers behave differently to one another • Layers are loaded unevenly over a dynamic drive cycle Hunt, I. et al. Journal of the Electrochemical Society, Vol 163, Pages A1846-A1852 (2016)
Tab cooling vs. surface cooling • Surface cooling causes accelerated degradation, compared to tab cooling • Tab cooling triples the usable life of an EV battery https://www.youtube.com/watch?v=_jd8REVB-c8 Hunt, I. et al. Journal of the Electrochemical Society, Vol 163, Pages A1846-A1852 (2016) Ardani, M. I. I., et al. Energy Vol 144, Pages 81 – 97 (2018)
Tab cooling vs. surface cooling: ECM Surface Cooled Tab Cooled Simulation domain • Developed temperature profile over a 6C discharge • Tab cooling, following discharge: ∆𝑼 𝒏𝒃𝒚 = 𝟗. 𝟏°𝑫 ∆𝑼 𝒏𝒃𝒚 = 𝟐. 𝟓°𝑫 • 82.5% reduction in temperature distribution • 80% reduction in current distribution ∆𝑱 𝒏𝒃𝒚 = 𝟏. 𝟕𝑫 ∆𝑱 𝒏𝒃𝒚 = 𝟒. 𝟏𝑫 • 93% reduction in depth of discharge distribution ∆𝑻𝒑𝑫 𝒏𝒃𝒚 = 𝟏. 𝟔𝟕% ∆𝑻𝒑𝑫 𝒏𝒃𝒚 = 𝟗. 𝟏𝟏% Zhao, Y. et al. Journal of the Electrochemical Society, Vol 165, Pages A3169-A3178
ሶ Why is tab cooling so effective? • Non-isotropic thermal conductivity 𝑹 = 𝑽 𝑩 ∆𝑼 in a cell Tab Cooled • 𝑽 = Τ 𝒍 𝒚 • k eff for tab cooling is 2 orders of magnitude greater Surface Cooled Hunt, I. et al. Journal of the Electrochemical Society, Vol 163, Pages A1846-A1852 Zhao, Y. et al. Journal of the Electrochemical Society, Vol 165, Pages A3169-A3178
ሶ Why is tab cooling so effective? • LIB A ( 5Ah Kokam SLPB11543140H5 ), power cell: 𝑹 = 𝑽 𝑩 ∆𝑼 • LIB B ( 7.5Ah Kokam SLPB75106100 ), energy cell: LIB A LIB B k eff thru-layer 65.2 W/mK 36.7 W/mK (tab cooling) k eff layer-to-layer 0.91 W/mK 0.64 W/mK (surface cooling)
ሶ Why is tab cooling not universal? • Cross-sectional area through which cooling 𝑹 = 𝑽 𝑩 ∆𝑼 must occur • The tabs are a significant thermal bottleneck • Cells are designed for to optimise energy or power density Which surface would you pick to remove heat from your cell? • Surface cooling is ‘easiest’ LIB A LIB B 14 mm 2 2.78 mm 2 Area, both tabs Area, single 4,520 mm 2 9,084 mm 2 surface Hales, A. et al. Paper under review
How do we improve cell thermal management? • Cell redesign to optimise internal thermal pathways Energy Power • Quantify a cell’s heat rejection Performance Performance capabilities Relevant Datasheet Information: LIB A 1. A metric for cell designers to enhance Capacity (Ah) 5 2. A standard against which all cells may Energy Density (Wh/kg) 140 be compared Rated Charge Rate (C-Rate) 2 3. A tool for battery pack design Rated Continuous Discharge Rate (C-Rate) 30 engineers to use in the initial design stages Rated Pulse Discharge Rate (C-Rate) 50 Hales, A. et al. Paper under review
How do we improve cell thermal management? Thermal • Cell redesign to optimise internal Performance thermal pathways Energy Power • Quantify a cell’s heat rejection Performance Performance capabilities Relevant Datasheet Information: LIB A 1. A metric for cell designers to enhance Capacity (Ah) 5 2. A standard against which all cells may Energy Density (Wh/kg) 140 be compared Rated Charge Rate (C-Rate) 2 3. A tool for battery pack design Rated Continuous Discharge Rate (C-Rate) 30 engineers to use in the initial design stages Rated Pulse Discharge Rate (C-Rate) 50 Cell Cooling Coefficient (W/K) ?? Hales, A. et al. Paper under review
Why do you need the Cell Cooling Coefficient? Parameter LIB A LIB B • Many cell parameters affect heat rejection Cell length/ mm 113.0 89.5 Cell width/ mm 40.0 101.5 • Decoupling highly complex and rarely conducted Cell thickness/ mm 11.3 7.4 Negative tab width/ mm 20.0 7.0 • Subsequent knowledge gap Negative tab thickness/ mm 0.3 0.2 Positive tab width/ mm 20.0 6.9 Positive tab thickness (cell side of weld)/ mm 0.4 0.2 Positive tab thickness (at weld)/ mm 0.6 0.4 • Heat rejection from cells is not Positive tab thickness (tab side of weld)/ mm 0.2 0.2 Negative tab internal length 13.0 10.0 quantified Positive tab internal length 13.0 10.0 Tab locations (on the cell) Opposite ends Same end Negative Tab Position (width dimension) Central 4.5mm offset Positive Tab Position (width dimension) Central 30.9mm offset Negative Tab Position (Thickness) Central Fully offset Positive Tab Position (Thickness) Central Fully offset What temperature gradient do you need to remove 1W of Component Negative CC Positive CC Separator Anode Cathode Casing LIB A k/ W.m -1 K -1 398 238 0.34 1.58 1.04 238 heat from your cell? Volumetric 9.38% 9.38% 21.42% 33.93% 25.89% 2.75% proportion of cell LIB B k/ W.m -1 K -1 398 238 0.33 1.045 0.44 238 Volumetric 4.53% 4.66% 11.72% 45.46% 33.62% 3.77% proportion of cell Hales, A. et al. Paper under review
ሶ Cell Cooling Coefficient 1. The rate of heat rejection for a given thermal gradient 2. A constant for a certain cell and thermal management method 3. A standard against which any two cells may be compared 𝑹 𝒋 ∆𝑼 𝒋 LIB A LIB B ൘ 𝑫𝑫𝑫 𝒋 = CCC tab (W/K) 0.332 0.204 Hales, A. et al. Paper under review
ሶ How do you use the Cell Cooling Coefficient? 𝑹 𝒉𝒇𝒐 ∆𝑼 𝒅𝒇𝒎𝒎 𝒏𝒃𝒚 𝒖𝒑 𝒖𝒃𝒄𝒕 = • A worked example: 𝑫𝑫𝑫 𝒖𝒃𝒄 • A 15Ah battery pack to be designed • Must be capable of a 4C discharge ∆T LIB A = 15.0 o C ∆T LIB B = 40.6 o C • Entire pack must be kept below 40 o C • The ambient air temperature is 20 o C T tab max = 25.0 o C T tab max = -0.6 o C Cell Datasheet Information CCC tab Max. discharge Capacity Cell 5 o C above ambient 20.6 o C below ambient (W/K) C-Rate (Ah) LIB A 0.332 30 5 LIB B 0.204 5 7.5 Down-selection: • • LIB A is suitable for application Averaged over a 4C discharge : • LIB B is entirely unsuitable for application, • LIB A generates 4.97W of heat despite all existing datasheet indicators • LIB B generates 8.28W of heat suggesting otherwise Hales, A. et al. Paper under review
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