Energy Storage 9. Power Converter Demo Assoc. prof. Hrvoje Pandžić Vedran Bobanac, PhD
Lecture Outline Rechargeable batteries basics Power converter for experimenting with rechargeable batteries Rechargeable batteries characteristics Experimental demonstrations 2
Rechargeable Batteries Common technologies: Lead acid Nickel based Nickel-cadmium (NiCd) Nickel-metal-hydride (NiMH) Lithium-ion (li-ion) Generally speaking all rechargeable battery technologies have similar characteristics Today s practical demonstration: Li-ion cell Lead acid battery pack 3
Rechargeable Batteries Main characteristics: voltage and capacity Capacity ampere-hours (Ah) or watt-hours (Wh) E.g. battery rated at 10 Ah delivers: Current of 10 A for 1 hour Current of 5 A for 2 hours, etc. Capacity degrades with time and usage C-rate = battery charging/discharging speed 1C corresponds to Ah rating, e.g.: 1C for a 10 Ah battery = 10 A 2C for a 10 Ah battery = 20 A 0.5C for a 10 Ah battery = 5 A 4
Rechargeable Batteries Battery price Usually expressed per unit energy $/kwh Price of a new li-ion battery is cca. 500-800 USD/kWh Specific energy Defines battery capacity per unit mass Wh/kg Energy density Defines battery capacity per unit volume Wh/l Specific power Maximum available power per unit mass W/kg Power density Maximum available power per unit volume W/l 5
Series/parallel configurations Series connection Increases voltage Parallel connection Increases maximal current Increases Ah-capacity Battery energy (Wh-capacity) Does not change with series/parallel configuration Depends on the total number of connected cell Nomenclature for series/parallel configurations, e.g.: 2s2p 3s2p 6
Series/parallel configurations Single cell U NOM = 3.6 V I MAX = 5 A (2C) C = 2.5 Ah C E 9 Wh Series connection U NOM = 7.2 V I MAX = 5 A (2C) C = 2.5 Ah C E 18 Wh Parallel connection U NOM = 3.6 V I MAX = 10 A (2C) C = 5 Ah C E 18 Wh Series/parallel connection U NOM = 7.2 V I MAX = 10 A (2C) C = 5 Ah C E 36 Wh 7
Custom Made Converter Custom made bidirectional AC/DC converter for battery charging/discharging Specifications: Nominal output power: 1 kw Output voltage: 0 20 VDC Output current: -50 to 50 ADC Input: 50 Hz, 230 VAC Input/output voltage/current measurements Analog signals 0 10 VDC Digital signals via isolated USB or RS-485 Remote battery voltage sensing (increased accuracy) 8
Custom Made Converter Three-stage topology Bidirectional grid inverter Resonant HF transformer Output bidirectional interleaved buck-boost converter Voltage/current measurements Voltage/current measurements/ setpoints Single-phase inverter HF transformer Buck-Boost 9
Custom Made Converter Communication and control NI LabVIEW Converter is connected to host PC Communication NI crio via Ethernet SCADA NI LabVIEW 10
Custom Made Converter 11
Custom Made Converter Voltage/current measurements Voltage/current measurements/ setpoints Configuration currently in use Single-phase inverter HF transformer Buck-Boost 12
Lithium-ion batteries The most widespread battery technology is lithium-ion (li-ion) Li-ion cell types Cylindrical 18650 21700 Prismatic Pouch Commonly Li-polymer 13
Experimental Research 18650 li-ion cells Tesla model S Laptop computers Power tools, etc. Samsung ICR18650-32A Chemistry: Lithium Cobalt Oxide (LiCoO 2 ) LCO or ICR Nominal voltage: 3.75 V Nominal capacity: 3.2 Ah Minimum capacity: 3.1 Ah 14
Datasheet ICR18650-32A 15
Charging Characteristic Constant-current/constant-voltage (CC/CV) charging 1. Current is constant while voltage rises to a predefined threshold 2. Voltage is constant while current gradually decreases 3. Full charge is reached after the current drops to some small value (typically 3-5% of the Ah rating) Adjustable parameters: Constant charging current Voltage threshold Cut-off current 16
Charging Characteristic Charging conditions: Constant current: 1.6 A (0.5C) Voltage threshold: 4.35 V Duration: 180 min Experimentally obtained 17
Charging Characteristic Demonstration (1) Experimentally obtained 18
Discharging Characteristic Current profile depends on the application Varying current various practical applications Constant current (CC) laboratory experiments Full discharge is reached after voltage drops to some predefined cut-off value Unlike charging duration, discharge durations are approximately consistent with the C-rate 0.5C cca. 2 hours 1C cca. 1 hour 2C cca. 30 minutes 19
Discharging Characteristic Discharging conditions: Constant current: 1.6 A (0.5C) Cut-off voltage: 2.75 V Experimentally obtained 20
Discharging Characteristic Demonstration (1) Experimentally obtained 21
Measuring Ampere-hours Coulomb counting Integration of the charge/discharge current Ampere = Coulomb / second Discharging fully charged battery Capacity C Ah = 0 T Idis τ dτ In practice we use trapezoidal integration: N C (Ah) I dis τ k 1 +I dis (τ k ) k=1 τ 2 k Discharging partially charged battery Remaining charge (battery SoC) 22
State-of-Charge (SoC) SoC measured against charging duration Series of partial charges applied (10 min steps) followed by immediate controlled full discharge Cut-off voltage: 2.75 V SoC Ah = N I dis τ k 1 +I dis (τ k ) k=1 τ 2 k Charging: I ch = 1.6 A (0.5C) I ch = 3.2 A (1C) Discharging: Experimentally obtained I dis = 1.6 A (0.5C) both cases 23
Charging Power and Energy Electrical power characteristic during full charge (0%-100% SoC) P = U I; [W=V A] Electrical energy: E = 0 t P τ dτ; [J=W h] Full charge energy: 12.6 Wh (0.5C), 12.9 Wh (1C) Experimentally obtained 24
Capacity Maximal number of Ampere-hours (Ah), or Watt-hours (Wh), that can be drawn from a battery on a single discharge Fully charged battery discharge to cut-off voltage C Ah = 0 T Idis τ dτ C E (Wh) = 0 T Pdis τ dτ Experimentally obtained 25
Rate Capacity Effect Higher discharge current = lower capacity Also known as Peukert s law Applied mostly to lead-acid batteries Experimentally obtained 100% capacity = 3.3 Ah 26
Rate Capacity Effect Black compound contains energy White compound no energy Reaction at electrode 1 creates black compound charging Reaction at electrode 2 creates white compound discharging Concentration of black compund at electrode 2 = battery voltage Source: Homan et al., A Comprehensive Model for Battery State of Charge Prediction, 2017. 27
Capacity Recovery Effect End of discharge predefined cut-off voltage Waiting period after end of discharge voltage recovers battery can be discharged further Experimentally obtained Demonstration (2) This effect is more expressed for higher discharge currents The higher the current during first discharge, the more Ah can be extracted on a second discharge (after the waiting period) This effect becomes insignificant for relatively low discharge currents 0.1C and lower 28
Capacity Recovery Effect Black compound contains energy White compound no energy Reaction at electrode 1 creates black compound charging Reaction at electrode 2 creates white compound discharging Concentration of black compund at electrode 2 = battery voltage Source: Homan et al., 2017. 29
Capacity Fade Effect All batteries gradually lose capacity with: Usage cycling (charging/discharging) Age Battery specifications are valid for a new and healthy battery Usual end-of-life threshold: 80% of the initial capacity Li-ion typically delivers 300-1000 cycles Source: batteryuniversity.com 30
Capacity Fade Effect Capacity measurements of a Samsung INR18650 1.5 Ah li-ion battery cell at various C-rates Cycling: Full CC/CV charge Full CC discharge Voltage limits: 2.5 4.2 V Cut-off current: 100 ma Source: Sarker et al., Optimal operation, 2017. Capacity measured every 10 cycles at ±0.5C 31
Internal Resistance Every battery has internal resistance from: Electrodes Electrolyte Connections, wiring etc. Causes voltage drop when charge/discharge current is applied Ohm s law: U = I R Open circuit voltage (OCV) no current flow Closed circuit voltage (CCV) current flow Charging raises CCV Discharging lowers CCV Rubber band effect Demonstration (1) 32
Internal Resistance Voltage drop due to internal resistance Experimentally obtained 33
Internal Resistance Pure resistance R Impedance Z Impedance = resistance + reactance Z = R + jx Reactance: Capacitive (capacitor) Inductive (coil) Measuring internal resistance DC methods measure R AC methods measure Z Randles model of a lead acid battery Source: batteryuniversity.com 34
Internal Resistance Charging characteristic: I DC load method R = U I U Experimentally obtained Demonstration (1) 35
Battery Efficiency Cause of energy losses internal resistance heat dissipation Types of efficiencies (1): Coulombic Voltaic Energy (includes both coulombic and voltaic) Types of efficiencies (2): Charging Discharging Overall (charging + discharging) 36
Battery Efficiency Efficiency is predominantly dependent on the charging/discharging current Experimentally obtained 37
Determining SoC State-of-Charge (SoC) (%) = measure for the amount of energy left in a battery Fully charged battery has 100% SoC Fully depleted battery has 0% SoC Determining SoC is not straightforward 38
Methods for determining SoC Laboratory method Ampere-hours measurement during controlled discharge The most accurate method Time consuming Not appropriate for real-time implementation Open circuit voltage (OCV) measurement Simple usage of look-up table Battery should rest up to 24 h before the measurement Not appropriate for real-time implementation Closed circuit voltage (CCV), or terminal voltage, measurement Requires some kind of battery model not always simple to obtain 39
Methods for determining SoC Coulomb counting SoC t = SoC t 1 + 100 C t t 1 I τ dτ Prone to current measurement error Losses must be taken into account SoC t = SoC t 1 + η CH 1 d 100 η DIS d 100 C t t 1 I DIS τ dτ Demonstration C C capacity I current η efficiency d = 0 charging d = 1 discharging t t 1 I CH τ dτ Problems: Capacity fade Efficiencies vary with charge/discharge C-rates Coulomb counting is the basis for most SoC estimating algorithms used in: EVs, laptops, consumer products etc. 40
Methods for determining SoC Impedance spectroscopy Correlating cell impedance (over a wide range of AC frequencies) with SoC Hydrometer (for lead acid batteries) Correlating electrolyte s specific gravity to SoC Kalman filtering Well-established real-time estimation method in various fields and applications Estimates states of the dynamic system based on available measurements and model of the system Often combined with models that are based on coulomb counting and OCV/CCV Neural networks Fuzzy logic... 41
Methods for determining SoC SoC estimation example Source: https://www.sheffield.ac.uk/creesa/research/state-charge 42
Determining SoH State-of-Health (SoH) (%) = measure for the overall battery condition New healthy battery has 100% SoH Determining SoH is not straightforward SoH indicators: Capacity Higher is better Internal resistance Lower is better Self-discharge Lower is better E.g. End-of-life threshold capacity drops to 80% of the initial value 43
Battery Management System (BMS) Monitors and controls the battery BMS functions may include Charging/discharging control Overcurrent protection Overvoltage protection Undervoltage protection SoC assessment and visualization SoH assessment Cell balancing Temperature and cooling mechanism control 44
Battery Management System (BMS) Source: https://www.utdallas.edu/essl/projects/battery-management-systems-investigator-m-pyne/ 45
Depth-of-Discharge (DoD) DoD is an antonym for SoC SoC + DoD = 100 % A deep discharge stresses the battery more than a partial discharge It is better not to fully discharge the battery apply charge more often Exceptions occasional full discharge is recommended for: Recalibration of the SoC estimation algorithm (coulomb counting accumulates error) Control of memory effect (only nickel-based batteries) 46
Memory Effect Only nickel-based batteries are prone to this effect Occurs when battery is: Overcharged (e.g. left in charger for days) Partially discharged repeatedly Crystalline formation on the electrodes reduces battery capacity Available capacity is in correlation with recent (partial) charge/discharge cycles Solution: Apply full charge/discharge every 1-3 months Maintenance known as exercise Breaks the crystalline formation Source: batteryuniversity.com 47
Temperature Battery type Lead acid Charge temperature Discharge temperature 20 C to 50 C 20 C to 50 C Charge advisory Charge at 0.3C or less below freezing. Lower V-threshold by 3mV/ C when hot. NiCd, NiMH 0 C to 45 C 20 C to 65 C Charge at 0.1C between 18 C and 0 C. Charge at 0.3C between 0 C and 5 C. Charge acceptance at 45 C is 70%. Charge acceptance at 60 C is 45%. Li-ion 0 C to 45 C 20 C to 60 C No charge permitted below freezing. Good charge/discharge performance at higher temperature but shorter life. Source: batteryuniversity.com 48
Practical Advices Battery s working life can be prolonged by: Avoiding high charge/discharge currents (C-rates) Avoding deep discharges Keeping batteries at moderate temperatures Especially avoid battery operation at high SoC (near 100%) and at elevated temperatures Li-ion batteries Last the longest when operated between 30% and 80% SoC Should be stored in a cool place at partial charge 49
Regatron Converter Professional controllable bidirectional converter made by Swiss company Regatron Specifications: Rated lineside AC voltage: 400 V, 50 Hz, three-phase Power span on the DC side: 0 20 kw Voltage span on the DC side: 0 400 V Current span on the DC side: 0 63 A 50
Regatron Converter Demonstration 51
BT-HSE-150-12 52