Effective Modeling of Temperature Effects on Lithium Polymer Cells F. Baronti, G. Fantechi, E. Leonardi, R. Roncella, R. Saletti University of Pisa, Italy Dept. of Information Engineering: Electronics, Computer Science and Telecommunications

Outline Introduction Cell model Model characterization Experimental set-up Test description Parameter extraction Thermal model Model validation Conclusions

Introduction – Lithium Battery Lithium batteries very promising for next generation hybrid and electric vehicles Model Type Capacity [Ah] Max continuous current [A] Energy density [Wh/kg] Power density [W/kg] Cycle Life Charge Discharge Saft VH AA 1500 Ni-MH 1.5 4.2 69 194 NA Saft VL34P Li-ion 33 500 120 128 7378 Kokam SLPB78216216H LiPo 31 155 62 133 1334 >800 ThunderSky-LYP40AHA LiFePO4 40 85 1707 >3000 Altairnano 50Ah Nano Lithium Titanate 50 300 72 719 >4000

Introduction - Lithium Battery (cont’d) Battery management systems (BMSs) needed for safe and reliable operation of the vehicle battery Monitor cell voltage and temperature Evaluate state-of-charge (SOC) and state-of-health (SOH) Accurate cell model to be embedded in the BMS for SOC and SOH estimate to be used for BMS design and simulation In this work, a scaled model of LiPo cell is considered Kokam SLPB723870H4 1.5 Ah, 1.5 A max charge current, 30 A max continuous discharge current

Electrical Cell model Model Parameter Symbol Dependence Cell capacity Ccapacity - Self discharge Rself_discharge Ohmic resistence Rseries Icell, SOC, T Long transient RC Rt_long / Ct_long Short transient RC Rt_short / Ct_short M. Chen et al. “Accurate Electrical Battery Model Capable of predicting Runtime and I-V Performance,” IEEE Trans. Energy Convers., vol. 21, no. 2, June 2006

Experimental set-up Standard set-up combined with a custom-designed temperature-controlled chamber NI USB - 6008 TTi LD300 80V-80A TTi QL355TP 35V-5A

Temperature-controlled chamber Cell encased in two symmetrical halves. Each of them contains: Array of digital temperature sensors 82 W Peltier TEC

Temperature effect on cell behavior 1C rate discharge at different cell temperatures As known, temperature significantly affects cell behavior decreasing the usable cell capacity Actual capacity is about 1.4 Ah while the nominal one is 1.5 Ah end-of-discharge voltage

Cell model parameter extraction Test procedure: Test conditions: Charge current (Itest): 0.5C, 1C rates Discharge current (Itest): 0.5C, 1C, 5C, 10C, 20C rates Cell temperature: 10, 25, 35 °C C - Used to signify a charge or discharge rate equal to the capacity of a battery divided by 1 hour. Init phase: 1C charge 1 h pause 1C discharge Pause phase: 1 h pause Test phase: charge/discharge cycle current and temperature of interest 5 min pause after 1% SOC variation and every 9% SOC variation

Cell model parameter extraction (cont’d) Model parameters derived from the cell voltage transient during the 5 min pauses charge cycle pause discharge cycle

Cell model parameter extraction (cont’d) Icell = Itest pause (Icell = 0) Icell = Itest

Cell model parameters Open Circuit Voltage (OCV) Extracted from 1C charge/discharge Small dependence on cell temperature

Cell model parameters (cont’d) Let’s have a look at Rtot=Rseries+Rt_long+Rt_short As expected, Rtot increases at lower temperatures

Thermal Model The electrical model has been improved with a first order thermal model Cth and Rth estimated by observing the thermal evolution of the cell Param. Value Cth 50 W/°C Rth 14.7 °C/W

Model validation Model implemented in Matlab/Simulink® using multidimensional LUT for model parameters Model 25 °C Model Model T Max RMS Abs error 398 mV 49 mV 181 mV 14 mV % error 9.61 % 1.45 % 5.6 % 0.39 %

Model validation (cont’d) Model 25 °C Model Model T Max RMS Abs error 521 mV 26 mV 478 mV 17 mV 486 mV % error 12.6 % 0.69 % 11.5 % 0.44 % 11.7 %

Conclusions Experimental characterization of a 1.5 Ah Kokam LiPo cell Accurate modeling including thermal effects Model implemented in Matlab/Simulink® Very good matching between experimental and simulated dynamic cell behaviors Model accuracy significantly improves if temperature-dependent parameters are used Starting point for the design and simulation of the battery pack and BMS using 31 Ah Kokam LiPo cells targeting a Fuel Cell HEV