Energy Storage (Battery) Systems

Transcription

1 Energy Storage (Battery) Systems Overview of performance metrics Introduction to Li Ion battery cell technology Electrochemistry Fabrication Battery cell electrical circuit model Battery systems: construction and modeling Battery management system (BMS) Functions and circuit implementation Cell balancing Simulation examples 1

2 Battery System in the Electrified Drivetrain Conventional Battery System Electric drive vehicle example n cells (+protection) in series Battery Management System (BMS) + V bat DC bus DC-DC converter + V DC Electric drive propulsion components Control bus Vehicle controller Many battery cells connected in parallel and in series Singe high voltage DC DC converter regulates bus voltage BMS provides protection, battery health monitoring, charge balancing among series cells, and communicates information to vehicle controller 2

3 Battery Performance Metrics Energy Available energy storage between charging cycles A*hr rating Specific energy, Wh/kg, energy density Wh/L Power Instantaneous power available C rating: peak discharge current Specific power, W/kg, W/L Cost Initial investment Total energy cost over life of battery Safety Hazardous chemical content Outgassing Risk of fire from damage or heating Lifetime Number of charge / discharge cycles to 80% capacity Dependence on % discharge and peak currents 3

4 Specific Energy vs. Specific Power Trade Offs 4

5 Energy Density and Specific Energy Gravimetric energy density (specific energy) Volumetric energy density For comparison, energy density and specific energy of gasoline are orders of magnitude higher: 9700 Wh/L, Wh/kg 5

6 Battery cycle life comparison 6

7 Comparison of Battery Technologies Many competing technologies, no clear winners 7

8 Introduction to cell electrochemistry Oxidation reduction Oxidation is loss (OIL) of a valance electron; reducing agents have surplus of valenceshell electrons, which they donate in a redox reaction, becoming oxidized Reduction is gain (RIG) of a valence electron; oxidizing agents have a deficit of valence shell electrons and accept electrons in a redox reaction, becoming reduced Reference: 8

9 Redox based battery cell Electrolyte (ionic conductor) Cations (positive) Anions (negative) + _ half cell Positive electrode, cathode separator half cell Negative electrode, anode 9

10 Current flow charge discharge Redox based battery cell Electrolyte (ionic conductor) Cations (positive) Anions (negative) + _ half cell Positive electrode, cathode CATHODE during discharge accepts electrons ; is reduced During charge gives up electrons; is oxidized separator half cell Negative electrode, anode ANODE during discharge; gives up electrons to external circuit; is oxidized; During charge accepts electrons; is reduced OIL = oxidation is loss of electrons RIG = reduction is gain of electrons 10

11 Strengths of Oxidizing and Reducing Agents The values in the table are reduction potentials, Lithium is the strongest reducing agent The strongest oxidizing agent is Fluorine The highest potentially possible cell voltage (3.04V V = 5.91V) would combine the top and the bottom reaction; but no known electrolyte can withstand that voltage without decomposing 11

12 Example of a standard redox based battery cell Lead Acid battery cell Lead dioxide PbO 2 Porous lead Pb ev Sulfuric acid ev H 2 SO 4 + H 2 O Open circuit cell voltage (Nernst equation): 1.685V V + Vt ln((electrolyte concentration)/1 mol) Vt = thermal voltage = kt/q = 26 mv at room temperature SOC directly determined by acid concentration (6 mol at 100%, 2 mol at 0%) Energy density: Wh/kg, Wh/l Cost: $( )/Wh 12

13 Nickel Metal Hydride: NiMH Metal alloy MH Nickel oxyhydroxide NiOOH + MH + OH > M + H 2 O + e 0.83 ev Potassium hydroxide KOH + H 2 O NiOOH + H 2 O + e > Ni(OH) 2 + OH 0.52 ev Open circuit cell voltage: 0.83V V + V t ln(electr.conc/1 mol) 1.4 V SOC directly determined by electrolyte concentration (6 mol at 100%) Energy density: 70 Wh/kg, 170 Wh/l Cost: $(0.5 1)/Wh Not a standard redox based cell Metallic alloy ( hydrate ) has the ability to absorb hydrogen Electrolyte transports hydrogen between the electrodes but does not participate in the reactions 13

14 Example: 2004 Prius battery 19.6mm(W) 106mm(H) 285mm(L) NiMH Module 6 cell (7.2 V) NiMH modules, 6.5 Ah at C/2 46 Wh/kg 1.3 kw/kg Battery pack 28 modules V DC = 202 V E bat = 1.3 kwh Pack weight: 30 kg SOC min = 35% SOC max = 75% $3K retail replacement cost 14

15 Lithium Ion Chemistry A. Pesaran (NREL), Battery Choices for Different Plug in HEV Configurations, Plug in HEV Forum, July 12,

16 Li ion chemistry cells Not standard redox based cells Intercalation = insertion of Li ions into electrode crystalline lattice 16

17 Li ion advantages and disadvantages Advantages Higher energy density, Wh/kg, Wh/l High power density, can be optimized for energy or power Higher voltage, approx. 3.2 V to 3.8 V Low self discharge rate, retain charge for months No liquid electrolyte Relatively long cycle life (1,000 3,000 deep cycles) Disadvantages More complex to manufacture, more expensive (0.5 1 $/Wh) Safety concerns: require circuitry to protect against overcharging or over discharging 17

18 Cell Equivalent Circuit Models Objective: Dynamic circuit model capable of predicting cell voltage in response to charge/discharge current, temperature Further key techniques discussed in [Plett 2004 Part 2] and [Plett 2004 Part 3] Model parameters found using least square estimation or Kalman filter techniques based on experimental test data Run time estimation of state of charge (SOC) Reference: [Plett ] G. Plett, Extended Kalman Filtering for Battery Management Systems of LiPB Based HEV Battery Packs Part 2: Modeling and Identification, Journal of Power Sources, Vol. 134, No. 2, August 2004, pp

19 Pulsed current tests [Plett ] 19

20 Model A State of Charge (SOC), Open Circuit Voltage 20