Effective Modeling of Temperature Effects

on Lithium Polymer Cells

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Outline

• Introduction

• Cell model

• Model characterization

Experimental set-up

Test description

Parameter extraction

Thermal model

• Model validation

• Conclusions

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Introduction – Lithium Battery

Model TypeCapacity

[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 4.2 69 194 NA

Saft VL34P Li-ion 33 500 120 128 7378 NA

Kokam SLPB78216216H

LiPo 31 155 62 133 1334 >800

ThunderSky-LYP40AHA

LiFePO4 40 120 120 85 1707 >3000

Altairnano 50AhNano

LithiumTitanate

50 300 300 72 719 >4000

• Lithium batteries very promising for next generation hybrid and electric vehicles

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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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

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Electrical Cell model

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

Model Parameter Symbol DependenceCell capacity Ccapacity -

Self discharge Rself_discharge -

Ohmic resistence Rseries Icell, SOC, T

Long transient RC Rt_long / Ct_long Icell, SOC, T

Short transient RC Rt_short / Ct_short Icell, SOC, T

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Experimental set-up

• Standard set-up combined with a custom-designed temperature-controlled chamber

TTi LD300 80V-80A

TTi QL355TP 35V-5A

NI USB - 6008

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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

Temperature-controlled chamber

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Temperature effect on cell behavior

• 1C rate discharge at different cell temperatures

• As known, temperature significantly affects cell behavior decreasing the usable cell capacity

end-of-discharge voltage

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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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

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Cell model parameter extraction (cont’d)

• Model parameters derived from the cell voltage transient during the 5 min pauses

charge cycle discharge cyclepause

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Cell model parameter extraction (cont’d)

RseriesIcell

short Rt _ short Ct _ short

long Rt _ long Ct _ long

A Rt _ short Icell

B Rt _ long Icellpause (Icell = 0)Icell = Itest Icell = Itest

longshort

tt

occell BeAeVtV

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Cell model parameters

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

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Cell model parameters (cont’d)

• Let’s have a look at Rtot=Rseries+Rt_long+Rt_short

• As expected, Rtot increases at lower temperatures

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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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

dtR

TTQ

CTT

th

ambcell

thambcell

1

cellcelloc IVVQ

Param. Value

Cth 50 W/°C

Rth 14.7 °C/W

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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• Model implemented in Matlab/Simulink® using multidimensional LUT for model parameters

Model validation

Model 25 °C Model Model T

Max RMS Max RMS Max RMS

Abs error 398 mV 49 mV 181 mV 14 mV 181 mV 14 mV

% error 9.61 % 1.45 % 5.6 % 0.39 % 5.6 % 0.39 %

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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Model validation (cont’d)

Model 25 °C Model Model T

Max RMS Max RMS Max RMS

Abs error 521 mV 26 mV 478 mV 17 mV 486 mV 17 mV

% error 12.6 % 0.69 % 11.5 % 0.44 % 11.7 % 0.44 %

F. Baronti et al. - Effective modeling of temperature effects on lithium polymer cells

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• Experimental characterization of a1.5 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 iftemperature-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

Conclusions