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LEAD ACID BATTERY MODELING IEEE ESSB Summer 2016 Meeting

LEAD ACID BATTERY MODELING IEEE ESSB Summer 2016 Meeting. Frank X. Garcia 12 June 2016. Presentation Objectives. Explain lead acid cell operation at the atomic level Present a Randles circuit model approximation of a lead acid cell

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LEAD ACID BATTERY MODELING IEEE ESSB Summer 2016 Meeting

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  1. LEAD ACID BATTERY MODELINGIEEE ESSB Summer 2016 Meeting Frank X. Garcia 12 June 2016

  2. Presentation Objectives • Explain lead acid cell operation at the atomic level • Present a Randles circuit model approximation of a lead acid cell • Examine computer simulations trending using Randles circuit model

  3. Outline • Battery Overview • Components • Electrochemistry • Double layer capacitance • State of health parameters • Randles circuit model • Single cell approximation • Failure modes analysis • Computer Simulations

  4. Battery Components • A battery is an energy storage device • Converts chemical energy into electrical energy during a discharge, and can also • Store electrical energy during a recharge • The major components of a lead acid battery include: • CATHODE-terminal post connected to the positive plates • ANODE- terminal post connected to the negative plates • NEGATIVE PLATES - lead grids filled with pure, spongy Pb • POSITIVE PLATES -lead grids filled with PbO2 • ELECTROLYTE- aqueous conductor of ions between the negative and positive plates, H2SO4 with H20 • (38% concentration of sulfuric acid) • SEPARATOR- non-conductive material that separates the negative and positive plates and prevents them from shorting

  5. Electrochemistry New electrodes inserted into electrolyte -Anode Cathode + -Anode Cathode + Pb0 PbO2 H2O H2O H+ H+ SO4-2 H+ H+ H+ H2O SO4-2 - - H+ H+ SO4-2 SO4-2 H+ - - H2O H+ H+ H+ H+ Pb+2 SO4-2 H2O H2O SO4-2 Pb+2 Reduction Oxidation

  6. Electrochemistry Reduction Pb+4 +2O-2(s) +4H+(aq) + SO4-2(aq) + 2e- PbSO4(s) +2H2O(aq) Energy released E0 = 1.69 eV Vcell= 1.69V – (-0.36V) = 2.05V -Anode Cathode + @ T = 25C H2O H+ H2O - - H+ SO4-2 - - H2O H+ H+ Pb+2 SO4-2 H2O SO4-2 Pb+2 Double Layer Capacitances Diffusion Layer s* 1.69V * Discussed Later 0V Oxidation Pb(s) + SO4-2(aq)PbSO4(s) + 2e- Energy released E0 = -0.36 eV -0.36 V

  7. Electrochemistry Charge Cycle: Electrochemical process is reversed Discharge Cycle Vcell - + - I Iload H2O - - H2O - - H2O H+ H+ SO4-2 Pb+2 SO4-2 SO4-2 H+ H+ Pb+2 H2O Reduction Oxidation

  8. Double Layer Capacitance • Ions adsorbed to the surface of the electrode held by electrostatic force • Positive ions too large to penetrate electrode metal surface • Only electrons can travel through the metal conduction bands • Solvated ions encapsulated by water molecules can migrate through diffusion layer • A second layer of solvated ions create the outer layer forming capacitive double layer • Double layer has an ionic density gradientwhich allow Ionic migration in the diffusion layer • Bulk region maintains an equal concentration of electrolyte

  9. State of Health Parameters • Float voltage • Set to recommended range for optimal battery life • Life expectancy decreases as float voltage increases • Float current • A high float current indicates aging battery • Negative post temperature • Indefinite 10oC temperature rise decrease battery life by 50% • Electrolyte Specific Gravity • Sulfuric acid (H2SO4) concentration can indicate state of charge • Fully charged: 1.26 to 1.3 specific gravity • Admittance/Impedance/Resistance Trending • Admittance will decrease as the battery ages • Impedance and Resistance will increase as battery ages

  10. Complete Randles Circuit Model

  11. Randles Circuit Model Simplification Step 1 Ignore Cbulk since its magnitude is much larger than Cdl- and Cd+ Step 2 Add the series resistances Rm+, Rm- and Rbulk Step 3 Add the 2 voltage potentials - 1.69V +0.36V = 2.05V

  12. Randles Circuit Model Simplification Note 1 Most models in the literature combine the half-cell circuits resulting in Cdl_eq, Rct_eq and Zdiff_eq Note 2 Many models in the literature often ignore Zdiff

  13. Failure Modes Analysis

  14. Randles Circuit Parameters Progression Randles circuit parameters progression over the cell lifetime or discharge

  15. Computer Simulations • Strategy • Examine degradation of a cell using complete Randles Circuit Model • Simulate Nyquist plots of baseline, +20% and +40% impedance due to cell aging • Synthesize component values to determine parametric changes • Circuit Model (one cell)

  16. Computer Simulations • Nyquist Plots • Baseline impedance: 1 • +20% Impedance: 2 • Component Synthesis 1 2 Zim 5kHz 0.001 Hz Zre * C – constant of the Warburg element,  - constant phase of the Warburg element

  17. Computer Simulations • Nyquist Plots • Baseline impedance: 1 • +40% Impedance: 2 • Component Synthesis 1 2 Zim 5kHz 0.001 Hz Zre * C – constant of the Warburg element,  - constant phase of the Warburg element

  18. Summary • Randles circuit model approximates electrochemistry of a lead acid cell • Simplified Randles circuit model reduces analysis accuracy • Trending in Randles element values add visibility to battery state of health • Baseline immitance using Discrete Frequency Immitance Spectroscopy • Synthesize Randles circuit model battery elements • Analyze battery degradation by comparing element value changes from a known reference

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