EE152 Green Electronics - web.stanford.eduweb.stanford.edu/class/ee152/docs/Batteries_110513.pdf ·...
Transcript of EE152 Green Electronics - web.stanford.eduweb.stanford.edu/class/ee152/docs/Batteries_110513.pdf ·...
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EE152 Green Electronics
Batteries 11/5/13
Prof. William Dally Computer Systems Laboratory
Stanford University
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Course Logistics • Tutorial on Lab 6 during Thursday lecture • Homework 5 due today • Homework 6 out today • Quiz 2 next Thursday 11/14
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AC Input/Output Summary • AC is just slowly changing DC
– But need to store energy during the “nulls”
• Power factor = Preal/Papparent – Want power factor very close to 1 – Requires current proportional to voltage
• PF correcting input stage – Controls input current – sine x error
• Grid connected inverter – Controls output current
• Independent inverter – Control output voltage – Use a full-bridge to generate a PWM Sine Wave
• Pulse width proportional to sin(x)
– LC Filter to reject high frequencies
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Anti-Islanding • Grid-connected inverters need to turn off when the grid
goes down. • Safety issue for firemen, linemen, etc… • How do you detect when the grid goes down?
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Anti-Islanding • Line monitoring
– Voltage limits, frequency limits. – Rate of change of frequency – Rapid phase shift
• Active detection – Impedance measurement – Forced phase shift/frequency shift
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Batteries
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Batteries • Many Green Electronic systems require energy
storage • Batteries are widely used to store energy in chemical
bonds • Model as dependent voltage source • Care required in charging and discharging
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Energy Density
Device Energy Density MJ/kg
Gasoline 44 Lithium Ion Battery 1.7 Lead Acid Battery 0.15
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Two Dimensions of Energy Storage
electrodes exhibit power characteristics similar tothose of supercapacitors. Moreover, there are alsohybrids such as metal/air batteries (or, in otherwords, metal/air fuel cells), which contain a batteryelectrode (metal anode) and a fuel cell electrode (aircathode). Finally, Figure 3 also shows that no singleelectrochemical power source can match the charac-teristics of the internal combustion engine. Highpower and high energy (and thus a competitivebehavior in comparison to combustion engines andturbines) can best be achieved when the availableelectrochemical power systems are combined. In suchhybrid electrochemical power schemes, batteries and/or supercapacitors would provide high power and thefuel cells would deliver high energy.
Figure 4 shows the theoretical specific energies[(kW h)/t] and energy densities [(kW h)/m3)] ofvarious rechargeable battery systems in comparisonto fuels, such as gasoline, natural gas, and hydrogen.The inferiority of batteries is evident. Figure 5,showing driving ranges of battery-powered cars incomparison to a cars powered by a modern combus-tion engine, gives an impressive example of why fuel
cells, and not batteries, are considered for replace-ment of combustion engines. The theoretical valuesin Figure 4 are an indication for the maximum energycontent of certain chemistries. However, the practicalvalues differ and are significantly lower than thetheoretical values. As a rule of thumb, the practicalenergy content of a rechargeable battery is 25% ofits theoretical value, whereas a primary batterysystem can yield >50% of its theoretical value indelivered energy. In the future, fuel cells might beable to convert the used fuels into electrical energywith efficiencies of >70%. The difference between thetheoretical and practical energy storage capabilitiesis related to several factors, including (1) inert partsof the system such as conductive diluents, currentcollectors, containers, etc., that are necessary for itsoperation, (2) internal resistances within the elec-trodes and electrolyte and between other cell/batterycomponents, resulting in internal losses, and (3)limited utilization of the active masses, as, forexample, parts of the fuel in a fuel cell leave the cellwithout reaction or as, for example, passivation ofelectrodes makes them (partially) electrochemicallyinactive. However, as batteries and fuel cells are notsubject to the Carnot cycle limitations, they mayoperate with much higher efficiencies than combus-tion engines and related devices.
1.2. DefinitionsThe following definitions are used during the
course of discussions on batteries, fuel cells, andelectrochemical capacitors.A battery is one or more electrically connected
electrochemical cells having terminals/contacts tosupply electrical energy.A primary battery is a cell, or group of cells, for
the generation of electrical energy intended to beused until exhausted and then discarded. Primarybatteries are assembled in the charged state; dis-charge is the primary process during operation.A secondary battery is a cell or group of cells for
the generation of electrical energy in which the cell,after being discharged, may be restored to its originalcharged condition by an electric current flowing inthe direction opposite to the flow of current when thecell was discharged. Other terms for this type ofbattery are rechargeable battery or accumulator. Assecondary batteries are ususally assembled in the
Figure 3. Simplified Ragone plot of the energy storagedomains for the various electrochemical energy conversionsystems compared to an internal combustion engine andturbines and conventional capacitors.
Figure 4. Theoretical specific energies [(kW h)/tonne] andenergy densities [(kW h)/m3] of various rechargeable bat-tery systems compared to fuels, such as gasoline, naturalgas, and hydrogen.
Figure 5. Comparison of the driving ranges for a vehiclepowered by various battery systems or a gasoline-poweredcombustion engine.
Editorial Chemical Reviews, 2004, Vol. 104, No. 10 4247
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Energy Density vs Battery Chemistry
250
200
150
100
50
50 100 150 200 250 300 350 400 450
WattHours/Litre
WattH
ours/K
ilogram
Lead Acid
Nickel Cadmium
Cylindrical
Cylindrical
Cylindrical
Prismatic
Prismatic
Prismatic
Prismatic
Aluminum Cans
Nickel Metal Hydride
Lithium Ion
Lithium Phosphate
Lithium Polymer
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Photo of Pack
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18650 Cell
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Tesla Pack
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Battery Model
LB RB
VBS VB
+
-
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Panasonic 18650 /LWKLXP�LRQ5HFKDUJHDEOH��EDWWHU\
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Rated Capacity (at 20䉝)
Nominal Voltage
Charging Method
Charging Voltage
Nominal Capacity (at 25䉝)
Cell Type NCR18650BSpecifications
2G23X0KYKU
Std.1625mA4.0hrs.
Charge +10䡚+45䉝Discharge -20䡚+60䉝Storage -20䡚+50䉝
47.5g
(D) 18.25mm
(H) 65.10mm
676Wh/l243Wh/kgGravimetric Energy Density
Charging CurrentCharging Time
Ambient Temperature
Weight (Max.)
Dimensions (Max.)
Volumetric Energy Density
H 64.93mmD 18.2mmd 7.9mm
Dimensions(Typ.)of
Bare CellMaximum size without tube
Discharged State after Assembling
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Discharge Temperature Characteristics for NCR18650B1Scell-1 Charge:CC-CV:1.625A-4.20V(65.0mA cut)
Discharge:CC:3.25A(E.V.:2.50V)1.-20Υ 2.-10Υ 3.0Υ 4.25Υ 5.40Υ 6.45Υ 7.60Υ
Discharge Rate Characteristics for NCR18650BCharge:CC-CV:1.625A-4.2V (65.0mA cut)Discharge:CC:Variable Current (E.V.:2.50V)Temp:25Υ
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VDischarge Temperature Characteristics for NCR18650B1S
cell-1 Charge:CC-CV:1.625A-4.20V(65.0mA cut)Discharge:CC:3.25A(E.V.:2.50V)1.-20Υ 2.-10Υ 3.0Υ 4.25Υ 5.40Υ 6.45Υ 7.60Υ
Discharge Temperature Characteristics for NCR18650B
2G23X0KYKU
0 500 1000 1500 2000 2500 3000 3500 4000Discharge Capacity / mAh
2.0
2.5
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Charge Characteristics for NCR18650B1S
No.1 Charge:CC-CV:1.625A-4.20V(65.0mA cut)
Cell Voltage
Charge Characteristics for NCR18650B
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Lead-Acid Charge Cycle
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High Charge Voltage Reduces Life
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Deep Discharge Reduces Life
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High Temperature Reduces Life Particularly at high SOC
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Lead-Acid Charge Cycle
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VF
t0 tB tC tF
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ry C
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Time
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Layered Control
State Machine
V or IControl
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PWMCurrentControl
Imax
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Lead-Acid Charging States
Off Trick Bulk Comp Float
Off I = IT I = IB V = VC V = VF
VT>V≥0 V≥VT
VB>V≥VT
V≥VC
VMax>V≥VC
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Charging Power Path
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Cells
One Cell ~2.0V for lead-acid ~3.2V for LiFePO4 ~3.7V for LiCo Capacity depends on volume 0.4Ah to 200Ah or more 1.28Wh to 640Wh
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Series Connection Increases Voltage Ah remains the same
100 LiFePO4 cells is about 320V (280-360) Capacity (in Ah) the same as one cell 128Wh (0.4Ah cells) to 64kWh (200Ah cells)
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Series Parallel vs Parallel Series
Which is preferred (assuming same Volts and Ah)?
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Battery Management Tasks • Charge control
– CC, CV profile – Cell balancing – Temperature monitoring/control
• SOC (state of charge) estimation – Fuel gauge – Integrate power – Estimate from voltage, current, and temperature
• Lifetime extension – Avoid deep discharge – Avoid high-charge, high-temperature storage
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Cell Balancing
Maximum cell voltage must not be exceeded during charging Voltage of each cell must be monitored Current must be “bled off” of high-voltage cells before they exceed Vmax Simple resistive balancer or flyback to recycle energy
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When Things go Wrong
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Battery Summary • Batteries store energy in chemical bonds • Model as dependent voltage source with series R and L • Terminal voltage is a function of charge state Q
– Also a function of temperature and charge rate
• Area of charge-discharge curve is loss in battery • Cells connected in series and parallel to build large batteries
(series connection of parallel cells) • Cells must be balanced to avoid overcharge • Battery management
– Charge control – Fuel gauge – Lifetime extension
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Grounding
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Ground • Exactly one point in your circuit is GND
– An arbitrary point that we refer to as having 0 Volts potential – All other voltages are referenced to this point
• Connections to GND should be made in a Spider not a Daisy Chain
Good Bad
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Ground Loops • Avoid loops
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Why Ground Loops are Bad
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Consider The PV Lab Power Path
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A Poor Way to Ground This Why?
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Grounding Resistor Introduces a Loop
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Grounding Summary • Single-point ground • Avoid loops
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Debugging
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Debugging • Get one thing working at a time
– Work from input of circuit to output – Work with simple stimulus, then more complex
• Form a hypothesis and test it – Don’t just randomly change things – Work from the schematic – Calculate voltage on each node and then verify it.
• If a chip isn’t doing what it should – Check every pin of the chip – Vdd, GND, all inputs, all outputs
• For current monitor – Check voltage across sense resistor – Check “bias” voltage for op-amp (0.45V) – Check that op-amp inputs have the expected voltage – Verify output voltage – Check with DC input first, then with operating MPPT – Check AC common-mode input voltage
• Use a scope – Signals that look good at DC (on a multimeter) may have big AC problems – Signals that look good on a logic analyzer (digitized) may have big analog problems
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Future Lectures • Tutorial for Lab 6 • Quiz 2 Review • Guest Lectures Tesla, Renovo, Enphase • Wrapup • Project Presentations