EE152 Green Electronics Batteries 11/5/13 Prof. William Dally Computer Systems Laboratory Stanford University
Course Logistics Tutorial on Lab 6 during Thursday lecture Homework 5 due today Homework 6 out today Quiz 2 next Thursday 11/14
AC Input/Output Summary AC is just slowly changing DC But need to store energy during the nulls Power factor = P real /P apparent 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
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?
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
Batteries
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
Energy Density Device Energy Density MJ/kg Gasoline 44 Lithium Ion Battery 1.7 Lead Acid Battery 0.15
Two Dimensions of Energy Storage igure 3. f o o of o
Energy Density vs Battery Chemistry 250 Lithium Polymer Prismatic 200 Lithium Phosphate WattHours/Kilogram 150 100 50 Nickel Cadmium Lead Acid Cylindrical Prismatic Nickel Metal Hydride Cylindrical Prismatic Lithium Ion Cylindrical Aluminum Cans Prismatic 50 100 150 200 250 300 350 400 450 WattHours/Litre
Photo of Pack
18650 Cell
Tesla Pack
Battery Model L B R B + V BS V B - V BS depends on state of charge and temperature
Panasonic 18650 Dimensions(Typ.) H 64.93mm of D 18.2mm Bare Cell d 7.9mm Discharged State after Assembling Rated Capacity (at 20 ) Nominal Capacity (at 25 ) Nominal Voltage Charging Method Charging Voltage Charging Current Charging Time Ambient Temperature Weight (Max.) Dimensions (Max.) Maximum size without tube Cell Type NCR18650B Volumetric Energy Density Gravimetric Energy Density Specifications Min.3200mAh Min.3250mAh 3350mAh 3.6V Constant Current -Constant Voltage 4.2V Std.1625mA 4.0hrs. Charge +10 +45 Discharge -20 +60 Storage -20 +50 47.5g (D) (H) 18.25mm 65.10mm 676Wh/l 243Wh/kg 2G23X0KYKU
4.5 Discharge Temperature Rate Characteristics Characteristics for for NCR18650B NCR18650B1S cell-1 Charge:CC-CV:1.625A-4.2V Charge:CC-CV:1.625A-4.20V(65.0mA cut) cut) Temp:25 1.-20 2.-10 3.0 4.25 5.40 6.45 7.60 Discharge:CC:Variable Discharge:CC:3.25A(E.V.:2.50V) Current (E.V.:2.50V) 4.0 Cell Voltage / V 3.5 3.0 2.5 2.0CA 1.0CA 0.5CA 0.2CA 2.0 0 500 1000 1500 2000 2500 3000 3500 4000 Discharge Capacity / mah G23X0KYKU
Discharge Discharge Temperature Characteristics for for NCR18650B1S NCR18650B 4.5 cell-1 1.-20 2.-10 3.0 4.25 5.40 6.45 7.60 Charge:CC-CV:1.625A-4.20V(65.0mA cut) Discharge:CC:3.25A(E.V.:2.50V) 4.0 Cell Voltage / V 3.5 3.0 2.5 40 25 0-10 -20 2.0 0 500 1000 1500 2000 2500 3000 3500 4000 Discharge Capacity / mah
Charge Characteristics for for NCR18650B1S 4.5 No.1 Cell Voltage Charge:CC-CV:1.625A-4.20V(65.0mA cut) 5000 Cell Voltage / V 4.0 3.5 3.0 2.5 Current Capacity 45 25 0 4000 3000 2000 1000 Current / ma 4000 3000 2000 1000 Capacity / mah 2.0 0 60 120 180 240 Charge Time / min 0
Lead-Acid Charge Cycle I B Battery Voltage Battery Current I C I T V C V F V T trickle bulk charge completion float t 0 t B t C t F Time
High Charge Voltage Reduces Life
Deep Discharge Reduces Life
High Temperature Reduces Life Particularly at high SOC
Lead-Acid Charge Cycle I B Battery Voltage Battery Current I C I T V C V F V T trickle bulk charge completion float t 0 t B t C t F Time
Layered Control V bat State Machine State I bat V or I Control I max I M1 PWM Current Control G
Lead-Acid Charging States V Max >V V C Reset, V<0, V>V Max V B >V V T Off V T >V 0 Trick V V T Bulk V V C Comp I<I F Float Off I = I T I = I B V = V C V = V F
Charging Power Path CT F 1 D 1 V i 200:1 V ac input stage 400V C i 1mF G M 1 D 2 V x GND switched inductor L 1 50 H C o 1mF V o output filter R CS.005 V bat
Cells One Cell ~2.0V for lead-acid ~3.2V for LiFePO 4 ~3.7V for LiCo Capacity depends on volume 0.4Ah to 200Ah or more 1.28Wh to 640Wh
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)
Series Parallel vs Parallel Series Which is preferred (assuming same Volts and Ah)?
Charge control CC, CV profile Battery Management Tasks 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
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 V max Simple resistive balancer or flyback to recycle energy
When Things go Wrong
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
Grounding
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
Avoid loops Ground Loops
Why Ground Loops are Bad
Consider The PV Lab Power Path
A Poor Way to Ground This Why?
Grounding Resistor Introduces a Loop
Single-point ground Avoid loops Grounding Summary
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 Debugging 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
Future Lectures Tutorial for Lab 6 Quiz 2 Review Guest Lectures Tesla, Renovo, Enphase Wrapup Project Presentations