Development of a Proposed Performance Standard for a Battery Storage System connected to a Domestic/ Small Commercial Solar PV system
ITP Data Analysis Report
Report Number: PP198127-AUME-MS02-TEC-03-R-01-A
Project Partners
Funding Partners
Revision History: Revision Date No 27/11/2018 1
Authored by
Reviewed by
Approved by
Dr. Shama Islam
Dr. Md Enamul Haque
2
Dr. Shama Islam
Dr. Sajeeb Saha Dr. Md Enamul Haque Dr Apel Mhamud Dr. Sajeeb Saha Dr. Md Enamul Haque Dr Apel Mhamud
18/12/2019
DNV GL Approval
Dr. Md Enamul Approved Haque 22/01/2020
The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for any information or advice contained herein.
DEAKIN UNIVERSITY
Deakin University CRICOS Provider Code: 00113B
Contents Acknowledgements ................................................................................................................................ 3 Figures… .................................................................................................................................................. 4 Tables… ................................................................................................................................................... 4 List of abbreviations................................................................................................................................ 5 Executive summary ................................................................................................................................. 6 1
2
3
4
Introduction .................................................................................................................... 7 1.1
Objective… .................................................................................................................... 7
1.2
Limitations and assumptions of the analysis ............................................................... 7
1.3
Battery chemistries and their relevance to ABPS ........................................................ 8
1.4
Conditions under the battery tested ......................................................................... 8
1.5
Definition of Battery Performance metrics considered in the analysis ................... 10
Daily Profile ................................................................................................................... 11 2.1
Introduction to the analysis ........................................................................................ 11
2.2
Power……………………………………………………………………………………………………………………11
2.3
Voltage…………………………. .........................................................................................12
2.4
Current………………………………………………………………………………………………………………….12
2.5
State of charge ........................................................................................................... 13
Charging Characteristics ................................................................................................. 15 3.1
Introduction to the analysis ........................................................................................ 15
3.2
Charging power........................................................................................................... 15
3.3
Charging voltage ........................................................................................................17
3.4
Charging current ........................................................................................................18
Discharging Characteristics ............................................................................................. 20 4.1
Introduction to the analysis ........................................................................................ 20
4.2
Discharging voltage.....................................................................................................20
4.3
Discharging current.....................................................................................................22
4.4
Battery temperature...................................................................................................24
4.5
Voltage regulation .....................................................................................................25
5
Impact of Weather Conditions ........................................................................................ 27 5.1
Introduction to the analysis ....................................................................................... 27
5.2
Deviation from average charging characteristics ...................................................... 27
5.3
6
5.2.1
Mean deviation in power‌ ..........................................................................27
5.2.2
Mean deviation in voltage ............................................................................. 28
Deviation from average discharging characteristics ................................................. 28 5.3.1
Mean deviation in voltage ............................................................................. 28
5.3.2
Mean deviation in current ............................................................................. 29
5.3.3
Energy throughput‌ .................................................................................... 30
Battery Performance with number of Cycles ................................................................. 32 6.1
Introduction to the analysis ....................................................................................... 32
6.2
Discharge capacity ..................................................................................................... 32
6.3
Discharge voltage........................................................................................................ 33
6.4
Efficiency ................................................................................................................... 35
7
Conclusions and Recommendations ..................................................................................... 36
8
Appendix A .................................................................................................................. 37
Acknowledgments The project Consortium (CSIRO, DNV GL, Smart Energy Council and Deakin University) wishes to acknowledge and thank the Australian Renewable Energy Agency (ARENA) and the Victorian Government for funding this work.
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Figures Fig. 2.2.1 Daily power consumption profiles for different batteries..................................................... 11 Fig. 2.3.1 Daily voltage profiles for different batteries ......................................................................... 12 Fig. 2.4.1 Daily current profiles for different batteries ........................................................................ 13 Fig. 2.5.1 Daily state of charge profiles for different batteries ............................................................. 13 Fig. 3.2.1 Charging power vs (a) Charging time, (b) State of charge ..................................................... 16 Fig. 3.3.1 Charging voltage vs (a) Charging time, (b) State of charge ................................................... 17 Fig. 3.4.1 Charging current vs charging time......................................................................................... 19 Fig. 4.2.1 Discharging voltage vs discharging time ................................................................................ 20 Fig. 4.2.2 Discharging voltage vs state of charge for batteries installed in (a) phase 1, (b) phase 2..... 21 Fig. 4.3.1 Discharging current vs discharging time ................................................................................ 22 Fig. 4.3.2 Discharging current vs state of charge for batteries installed in (a) phase 1, (b) phase 2 .... 23 Fig. 4.4.1 Battery temperature vs (a) discharge duration, (b) state of charge ...................................... 24 Fig. 4.5 Voltage regulation for different batteries ................................................................................ 26 Fig. 5.2.1 Mean deviation of charging power in different weather conditions from the average charging power over 2 years ................................................................................................................. 27 Fig. 5.2.2 Mean deviation of charging voltage in different weather conditions from the average charging voltage over 2 years ............................................................................................................... 28 Fig. 5.3.1 Mean deviation of discharging voltage in different weather conditions from the average discharging voltage over 2 years........................................................................................................... 29 Fig. 5.3.2 Mean deviation of discharging current in different weather conditions from the average discharging current over 2 years........................................................................................................... 30 Fig. 5.3.3 Average energy throughput for different batteries in different weather conditions.‌‌‌..31 Fig. 6.2.1 Discharge Capacity vs number of cycles for different batteries (a) raw data (b) linear fit .... 32 Fig. 6.3.1: Discharge voltage vs number of cycles for different batteries (a) raw data (b) linear fit ..... 34 Fig. 6.4.1: Efficiency vs number of cycles for different batteries (a) raw data (b) linear fit .................. 35
Tables Table 1.3.1 Battery Chemistries ............................................................................................................ 11 Table 1.4.1 Installed capacity of the tested batteries ........................................................................... 11 Table 1.4.2 Test conditions for different battery types ........................................................................ 12 Table 8.1: Summary of battery characteristics derived from the analysis ............................................ 37
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List of abbreviations ABPS
Australian Battery Performance Standard
AC
Alternating current
ARENA
Australian Renewable Energy Agency
BMS
Battery Management System
DC
Direct current
DoD
Depth of discharge
ITP
IT Power (Australia) Pty Ltd, trading as ITP Renewables
kW
kilowatt
kWh
Kilowatt hour
LA
Lead Acid
LC
Advanced Lead Acid
LFP
Lithium Iron Phosphate
NMC
Nickel Manganese Cobalt
NCA
Nickel Cobalt Aluminium Oxide
LMO
Lithium Manganese Oxide
LTO
Lithium Titanate
PV
Photovoltaic
SI
Sodium Ion
SoC
State of Charge
ZBF
Zinc Bromide Flow
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Executive Summary In this analysis, a number of batteries with different chemistries installed during phase 1 and phase 2 of the ITP trial have been investigated. These battery chemistries comprise lithiumion batteries, namely Nickel Manganese Cobalt and Lithium Iron Phosphate, Advanced Lead Acid, and Lead Acid batteries. The data is available for two years from June 2016 to May 2018, which includes information on battery voltage, current, power, and state of charge during charging, as well as discharging conditions for a range of batteries. The purpose of this analysis is to compare performance metrics across different battery chemistries and investigate the correlation between the measurable parameters and the state of charge of a battery. The analysis investigates the current, voltage and power characteristics for different batteries during charging and discharging conditions, both as functions of time and state of charge. The impact of weather conditions on charging and discharging characteristics, as well as energy throughput is considered. Moreover, the degradation of available battery capacity and terminal voltage is evaluated with the number of cycles for different batteries. The analysis will be highly useful for ABPS project, specifically for the battery capacity estimation part of the work. Since the battery state of charge can be related to external measurable parameters like voltage and current, as depicted in this analysis, the correlation will be useful for estimating the state of charge for a range of battery chemistries. Moreover, the findings will allow stakeholders to develop a more detailed and accurate battery model. As a result, the analysis can extend in future to include diverse load conditions, as well as network disturbances for a more realistic evaluation of batteries.
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Chapter 1 Introduction 1.1 Objective: In this report, a comprehensive analysis is performed for the battery data available from ITP Renewables over the time-period June 2016 to May 2018. The analysis is conducted to provide useful insights into charging-discharging characteristics of a range of battery chemistries under practical load conditions. These insights will be used to define threshold criteria for benchmarking and comparing battery performance required for the ABPS project. Moreover, the capacity degradation after a number of cycles will be evaluated to predict battery performance over its lifetime. The findings of this report will facilitate the development of battery models that conform to practical battery systems and a better estimate of battery conditions. Overall, the data analysis outcome will be used as a valuable input to the stage 2 of the Australian Battery Performance Standard process. 1.2 Limitations and assumptions of the analysis: The analysis considers state of charge, voltage, current, power, and temperature data for a range of batteries with different chemistries. Similar to any practical dataset, the available data is not free from missing values or measurement noises. This may impact the accuracy of all data points, and to mitigate this, curve fitting has been employed whenever necessary. Some batteries have been installed in phase 2 of the ITP Renewables project, and for these batteries, data is available only for a 5 month period. Thus, this data is often not sufficient to come up with conclusive findings, especially regarding capacity and efficiency degradation. That is why the Zinc Bromide Flow (ZBF) and Sodium Ion (SI) batteries are excluded from the analysis. The batteries are cycled three times per day rather than once a day, which may not be the perfect representation of the real world scenario. However, the cycle rate is maintained within manufacturer stated limitations. Moreover, the data has been collected from the battery management system, and not measured directly across the battery terminals. Thus there could be some mismatch in the actual parameter and values obtained from the battery management system. The data is obtained to monitor the system performance and not the battery performance explicitly. Thus, the limitations in hardware and BMS are expected to be present in the dataset. The Lead Acid (LA) battery experienced sulfation, so the state of charge estimation may not be reliable. There have been issues with the state of charge estimation of the first Lithium Iron Phosphate (LFP1) battery, so the conclusions may not be accurate. Also, the analysis considers the DC side of the batteries, any variations in AC side performance due to a change in loading conditions, AC supply variations (e.g. power factor, voltage) has not been considered. However, it can be noted that any degradation in the DC side performance will influence the AC side performance to a large extent and therefore, should be given a due consideration. And lastly, the characteristics obtained from the data analysis may be biased by any imperfection of the individual battery tested and may not reflect the average representation or characteristics of the same battery type. Some of the batteries used in the analysis had faulty cells and ITP reported that there had been issues
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with the SoC estimation of certain batteries. Moreover, there could be imperfections on the control hardware and not necessarily with the battery packs. 1.3 Battery chemistries and their relevance to ABPS: In this analysis, the following battery chemistries have been considered. Table 1.3.1 Battery Chemistries
Battery Chemistry Nickel Manganese Cobalt Lithium Iron Phosphate Advanced Lead Acid Lead Acid
Acronym NMC LFP LC LA
Number of Units 2 4 1 1
These batteries have been installed in two different phases by ITP, where the NMC, LC, LA and 2 LFP batteries have been installed in phase 1 and 2 LFP batteries have been installed in phase 21. The ABPS project aims to develop battery performance standards applicable to domestic/commercial solar PV system connected to a battery energy storage system. As indicated above, ITP batteries represent a great selection of current Australian market choice. Analysis of such battery systems designed for PV connection helps to characterise its performance, as well as impact in relation to Australian weather and domestic load demand conditions. It is identified that NMC, LFP, LA and LC batteries are commonly employed battery chemistries within the Australian market where as ZBF and Sodium Ion battery chemistries have gained significant market attention recently2. Thus, it is important to consider these battery chemistries in the ABPS project. Note that the ZBF and Sodium Ion batteries could not be incorporated in this analysis, since they were not commissioned until May 2018. So, the ITP data analysis incorporates the battery types mentioned in Table 1.3.1. 1.4 Conditions under which the batteries were tested: The batteries analysed in this report are indicated as NMC1, LFP1, LC1, LA1, LFP2, LFP3, LFP4, and LFP5. Here the first part of the name indicates the acronym for the battery chemistry and the last part indicates the number of batteries with a specific chemistry. The manufacturer details have not been mentioned due to confidentiality reasons. The installed capacity for each battery type is outlined in the following table. Table 1.4.1 Installed capacity of the tested batteries
Battery Type NMC1 LFP1 LC1 NMC2 LA1 LFP2 LFP3 1 2
Installed capacity (kWh) 9.6 10.24 14.8 8.3 15.84 9.6 9.6
ITP Renewables, “Battery Test Centre Report 3”, Nov. 2017, online: www.batterytestcentre.com.au Smart Energy Council, “Battery Finder”, online: https://www.smartenergy.org.au/batteryfinder
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Battery Type Installed capacity (kWh) LFP4 9 LFP5 10.24 The test conditions and data availability for each battery type is outlined in Table 1.4.2.
Battery type NMC1 LFP1 LC1 NMC2 LA1 LFP2 LFP3 LFP4 LFP5
Table 1.4.2 Test conditions for different battery types
Time Period 2 years 2 years 2 years 4 months 2 years 2 years 5 months 5 months 5 months
Minimum SoC after Discharge 10% 10% 30% 10% 50% 10% 8% 20% 10%
Note that NMC2 battery reached end of life for the application in October, 2016. So, the analysis results for this battery is applicable for only the first four months. Within that time, the battery capacity did not degrade to less than 98%. Similarly, the LFP3, LFP4 and LFP5 batteries were commissioned in 2018, so only 5 months of data was available for this analysis. Each battery pack is first charged over several hours (to emulate daytime charging from the PV), followed by a short rest period, then discharged over a few hours (to emulate the late afternoon, early evening period) followed by another short rest period. Overall, there are three charge/discharge cycles for each battery on a daily basis. The ambient temperature in the battery testing room is varied on a daily basis, and varies throughout the year. ITP implements two temperature regimes in each day. From November-April, the batteries undergo two cycles at high temperature and the third at low temperature. During other months, the batteries undergo two cycles at low temperature and the third at high temperature. The timing of the high and low temperature cycles is aligned with the outdoor temperature variations, so that transitions between high and low temperature set-points can be assisted by outdoor air. On the last day of each month, the batteries operate at 25 °C as this is the reference temperature at which most manufacturers provide the capacity of their batteries1. The load conditions applied to each of the batteries are the same. The charging and discharging data for each battery has been averaged for 4 different weather conditions, winter (June-August), spring (September-November), summer (December-February) and autumn (March-May). The average characteristics are then obtained for each battery for charging and discharging. The batteries are kept in ambient temperature conditions and thus the battery temperature follows the ambient temperature during test conditions. However, the battery charging-discharging performance varies slightly with different weather conditions. This is because the room temperature is varied to emulate different weather conditions in different months.
1
ITP Renewables, “Battery Test Centre Report 3�, Nov. 2017, online: www.batterytestcentre.com.au
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1.5 Definition of Battery Performance metrics considered in the analysis Battery: A unit consisting of one or more energy storage cells connected in series, parallel or series parallel arrangement. Battery Management System: The Battery Management System (BMS) is an electronic system associated with a battery or battery system, which monitors and/or manages in a safe manner its electric and thermal state by controlling its environment. Generally, the BMS monitors and balances cells to ensure that the cell voltage and temperature are maintained within the pre-defined thresholds. BMS also provides communications between the battery system and the PCE and connected devices Capacity: The quantity of electricity that a fully charged battery is able to deliver under specified conditions State of charge: The amount of capacity that remains in a battery or battery system expressed as a percentage of the rated capacity of the battery or battery system Depth of discharge: The extent to which a battery is discharged Charging characteristics: These characteristics define how the battery voltage, current and power changes throughout the charging period. Charging operation during which a battery receives electric energy, and then converted to chemical energy, from an external circuit is defined as the charging period. Throughout this period, the battery state of charge increases. Discharging characteristics: These characteristics define how the battery voltage, current and power changes throughout the discharging period. The discharging period is defined as the time period for which the battery is delivering power to a load. Throughout this period, the battery state of charge decreases. That is, during discharge, a battery delivers current to an external circuit by the conversion of chemical energy to electric energy Voltage regulation: The rate at which battery terminal voltage decreases when battery delivers current to a load Energy throughput: The cumulative energy available from a battery throughout its normal service life Capacity: The amount of energy a battery can deliver when the battery is fully charged at a given set of discharging conditions, e.g., discharge current or temperature Cycle life: Number of cycles during charging and discharging before a battery reaches 80% of the rated capacity. This indicates the total period of useful life of a cell or a battery in operation. Round trip efficiency: The ratio of the total energy delivered during discharging period and total energy extracted during the charging period, when a battery is charged and discharged between two pre-defined state of charge conditions 10
Chapter 2 Daily Profile: 2.1 Introduction to the analysis: In this section, the daily profiles of power, voltage, current and state of charge for different batteries have been investigated. The aforementioned parameters are averaged for each hour in different days and hourly patterns were observed. This will allow to identify the charging and discharging periods and how the power, voltage and current vary for different batteries. The batteries considered in this analysis have been charged under constant current typically for 3-4 hours and then discharged through a similar time period. From this analysis, insights can be obtained whether a specific battery is deeply/frequently discharged and if the battery can deliver power at a steady voltage/current. 2.2 Power: Fig. 2.2.1 shows how the power consumption pattern for different batteries varies over different hours in a day in 2016. The results for each hour are averaged for a period of 7 months to obtain the daily average characteristics. It can be seen that all batteries have approximately 3 cycles over 24 hour time period. LFP1 battery delivers power over a longer duration compared to the other batteries. LFP1 and NMC2 batteries can deliver power at a constant rate for 3 consecutive hours. On the other hand, NMC1 and LA1 battery delivers power at a smaller rate after 2 hours of discharge. It can be seen that LA1 and LC1 batteries are discharged at a much smaller rate (to DoD around 50%), which have been maintained to avoid deep discharge in these batteries. 3000
Power (W)
2000
Discharge
Discharge
Discharge NMC1
1000 0
LFP1 NMC2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
LC1
-1000
LFP2
-2000 -3000 Charge
LA1
Charge
Charge
Time (hours)
Charge
Fig. 2.2.1 Daily power consumption profiles for different batteries
Key takeaway: • • •
LFP1 batteries can deliver power over a longer duration and can be discharged deeply. LFP1, NMC1 and NMC2 batteries can discharge at a constant power, whereas for others, the charging power gradually decreases. LA1 and LC1 batteries are charged and discharged at a smaller rate to avoid deep discharge.
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2.3 Voltage: Fig. 2.3.1 illustrates how the battery terminal voltage varies over 24 hours in a day for different battery types. The nominal voltages of the NMC1, LFP1, LC1, NMC2, LFP2 and LA1 batteries are 51.8 V, 51.2 V, 48 V, 51.8 V, 51.2 V, and 48 V. It can be seen that NMC1 and NMC2 battery terminal voltages degrade during the discharge duration, whereas other battery terminal voltages remain mostly steady throughout the discharge. Contrarily, during charging period, the terminal voltages of NMC1, NMC2 and LFP2 batteries increase. NMC1 and NMC2 battery terminal voltages increase above the nominal value during charging, but during discharging, the voltage does not drop below the nominal value. LC1 battery terminal voltage is slightly lower than the nominal value during charging and discharging, which is caused due to some faulty cells, as reported by ITP. Note that the battery pack was replaced in January, 2018. LA1 battery voltage is lower than the nominal value during discharge. LFP1 battery has been charged and discharged at a slightly lower voltage compared to the nominal voltage. On the other hand, LFP2 battery is charged at a higher voltage compared to the nominal voltage. 70
Voltage (V)
60
Charge Charge Discharge
Discharge
Charge
Discharge
Charge
50
NMC1
40
LFP1
30
LC1
20
NMC2 LFP2
10 0
LA1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Time (hours)
Fig. 2.3.1 Daily voltage profiles for different batteries Key takeaway: • • •
LFP1 and LFP2 batteries have steady terminal voltages during discharge, so they are more suitable when constant voltage needs to be ensured. LA1 batteries discharge at a lower voltage than the nominal voltage, which needs to be carefully considered before connecting to a load NMC1 and NMC2 batteries cannot maintain a constant voltage during first half of the discharge cycle.
2.4 Current: Fig. 2.4.1 demonstrates the current drawn/delivered by the batteries over 24 hours in a day. The current profile is similar to the power consumption profile in Fig. 2.2.1. It can be seen that LFP1 battery can deliver a constant current for a larger duration during discharge compared to other batteries. On the other hand, LFP2 and LC1 batteries deliver current at a decreasing rate during discharge period. NMC1 and NMC2 batteries discharge at a slightly larger current towards the end of the discharge duration.
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60
Discharge
Current (A)
40
Discharge
Discharge NMC1
20
LFP1
0 -20
LC1 0 1 2 3 4 5 6789
10 11 12 13 14 15 16 17 18 19 20 21 22 23
NMC2 LFP2
-40 -60
Charge
Charge
Time (hours)
Charge
Charge
LA1
Fig. 2.4.1 Daily current profiles for different batteries
Key takeaway: • • •
LFP1 batteries can maintain a constant current during discharge NMC1 and NMC2 batteries initially discharge at a lower current which then increases at the last part of the discharge cycle LC1 and LFP2 batteries discharge at a gradually decreasing current, so may not be suitable when constant current needs to be maintained.
2.5 State of charge: Fig. 2.5.1 shows how the battery SoC varies over 24 hours in a day for different batteries. It can be seen that LA1 battery has not been discharged to less than 50% SoC, which conforms to accepted practice where the battery is not discharged deeply in a practical system to avoid the capacity fade. Similarly, LC1 battery has not been discharged to less than 30% SoC. On the other hand, NMC1 and NMC2 batteries have been discharged to less than 20% SoC, indicating that these batteries can be discharged deeply. The different DoD for different batteries is maintained according to manufacturers’ specifications on recommended DoD. Also, there are instances when a battery has been discharged to a level higher than the recommended DoD, which influences the average daily SoC profile. State of charge (%)
120 100 80
Charge Discharge
Charge
Discharge
Charge
Discharge
NMC1 LFP1
60
LC1
40
NMC2
20 0
Charge
LFP2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Time (hours) Fig. 2.5.1 Daily state of charge profiles for different batteries
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LA1
Key takeaway: • • •
LFP2, NMC1 and NMC2 batteries can be quickly charged to a higher level compared to other batteries LA1 batteries cannot be deeply discharged LA1 batteries cannot be charged to a level more than 70-80% due to low charge acceptance of these batteries with increasing SoC
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Chapter 3 Charging Characteristics: 3.1 Introduction to the analysis In this section, the voltage, current and power of different types of batteries during charging conditions have been illustrated. The batteries are charged first at constant current and then at constant voltage, though the constant voltage phase is short especially for LFP batteries. The charging conditions are controlled by the inverter to achieve a desired energy level. The characteristics are plotted with respect to time and SoC. The charging conditions are defined as when the SoC is increasing and the battery charging current/power is negative (i.e., the battery draws current/power). The results have been averaged over 2 years from June 2016 to May 2018. For each battery, the SoC values during charging conditions are identified and then for each SoC value, the power, voltage, and current measurements over the 2 year time period have been averaged. The results will be useful to model the relationship between SoC and charging power/voltage/current. This will enable state of charge estimation of a realworld battery storage system with different battery chemistries through measuring battery voltage, current and power. 3.2 Charging Power: Fig. 3.2.1 (a) demonstrates how the charging power of different batteries vary with respect to time during charging. It can be noted that the charging power remains constant during the majority of the charging time. Then the charging power gradually decreases to zero at the end of the charging time. From Fig. 3.2.1 (a), it can be seen that the battery LFP2 can switch to discharging state from charging state quite rapidly, as the slope of the power consumption during the last few minutes of discharge is steep compared to other batteries. Fig. 3.2.1 (b) illustrates how the charging power changes with different SoC for different batteries during charging conditions. It can be noted that the battery LA1 is consuming less power compared to other batteries because it is charged at a smaller current. The results show that the batteries LC1 and LFP1 start consuming lower power after they are charged to 6065%. On the other hand, the charging power of NMC1 battery gradually decreases after it has attained a state of charge more than 90%.
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(a)
(b) Fig. 3.2.1 Charging power vs (a) Charging time, (b) State of charge
16
Key Takeaway: • • •
LA1 has lower charging power due to a lower charging current LC1 is charged to 80% SoC (in 2016 and 2017 data), so its charging power is 0 after 80% SoC NMC1 can be charged to more than 90% SoC, after which the charging power becomes 0
3.3 Charging voltage: Fig. 3.3.1 (a) shows how the battery terminal voltage changes with respect to time during charging conditions. It can be identified that for LFP1 and LFP2 batteries, the battery voltage remains relatively steady compared to other batteries during charging. Note that the charging current and power also remain steady for these batteries. On the other hand, NMC1 and LC1 batteries have increasing voltage over time during charging. The charging current for these batteries decrease slightly over time during this period. Fig. 3.3.1 (b) demonstrates the battery terminal voltage at different SoC during charging conditions. The battery voltages show similar characteristic as identified in the previous discussion. The voltage across NMC1 increases linearly for the SoC 60%-90%. The inverter maintains a constant current to deliver the desired energy and so, the voltage varies by around 17% over the entire charging period to satisfy the condition. This characteristic can be used to estimate SoC from terminal voltage. The batteries LFP1, LC1 and LA1 have their terminal voltages steady after a SoC of 70%. So, charging voltage would not give a true estimate of SoC beyond this range for LFP1, LC1 and LA1 batteries.
(a)
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(b) Fig. 3.3.1 Charging voltage vs (a) Charging time, (b) State of charge
Key Takeaway: • • •
LFP1 and LFP2 battery terminal voltages do not increase significantly during charging NMC1 and LC1 battery terminal voltages increase above the nominal value during charging NMC1 battery voltage increases linearly with SoC and this correlation can be used to determine SoC from voltage measurements
3.4 Charging current: Fig. 3.4.1 illustrates the variation in charging current for different batteries with respect to charging time. The charging current follows a similar pattern as the charging power. That is, the current is constant up to a certain time and then gradually it becomes zero during the end of charging period. The charging current with respect to state of charge will be similar to that as charging power vs state of charge characteristics.
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Fig. 3.4.1 Charging current vs charging time
Key Takeaway: • • •
LC1 charging current drops to zero earlier because the battery is charged only to 80% SoC (in 2016 and 2017 data) LFP1 and LFP2 batteries draw constant charging current during the charging period NMC1 charging current slightly decreases over the charging duration
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Chapter 4 Discharging Characteristics 4.1 Introduction to the analysis In this section, the voltage, current and power of different types of batteries during discharging conditions have been evaluated. The discharging conditions are defined as when the state of charge is decreasing and the battery discharging current/power is positive (i.e., the battery delivers current/power). The results have been averaged over 2 years from June 2016 to May 2018. For each battery, the state of charge values during discharging conditions are separated and then the power/voltage/current for each sample of a certain state of charge have been averaged. The results will enable state of charge estimation with different battery chemistries through measuring battery voltage, current and power during discharging conditions. 4.2 Discharging voltage: Fig. 4.2.1 shows the battery terminal voltage during discharging conditions over the discharging time period for different batteries. It can be seen that LFP1 and LFP2 batteries can discharge at a steady level of voltage for certain time duration and then gradually drops. However, NMC1, LA1 and LC1 battery voltages decrease over time in a linear fashion, though LC1 batteries exhibit a smaller degree of decrease for terminal voltage during discharging conditions.
Fig. 4.2.1 Discharging voltage vs discharging time Figs. 4.2.2 (a) and 4.2.2 (b) show how the terminal voltage changes with respect to state of charge for batteries installed in phase 1 and 2, respectively during discharging. It can be noted that the terminal voltage drops as the battery state of charge decreases. As expected from the previous discussion, the LFP1 and LFP2 battery terminal voltages decrease at a lower rate compared to the other batteries in phase 1. LC1 battery exhibits a steady terminal voltage till 70% state of charge and then gradually drops as LA1 battery. It can be seen that
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LFP3, LFP4, and LFP5 batteries have a steady terminal voltage for a wide range of SoC. Thus, these batteries are useful for constant voltage applications, which can be satisfied even when the batteries experience deep discharge. Note that the terminal voltage for LPF4 battery decreases to 0 when SoC drops below 18%. This is because BMS shuts down the battery operation beyond this point.
(a)
(b) Fig. 4.2.2 Discharging voltage vs state of charge for batteries installed in (a) phase 1, (b) phase 2.
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Key takeaway: • • •
NMC1 and LA1 battery voltages decrease linearly with decreasing SoC and this correlation can be used to estimate SoC from voltage during discharge. LC1 battery voltage decreases linearly with decreasing SoC when SoC is below 75%. LFP3, LFP4 and LFP5 battery voltages do not decrease linearly with decreasing SoC, indicating their suitability to maintain constant voltage during discharge.
4.3 Discharging current: Fig. 4.3.1 illustrates the changes in discharging current over time for a number of batteries. It can be seen that the discharging current is constant for most part of the discharge duration, and then gradually drops to zero at the end of the discharge cycle. The NMC1 battery discharging current initially increases linearly with discharge time to the nominal value and then delivers the nominal current throughout the discharge interval. The results demonstrate that the LFP1 and LC1 batteries are being discharged over a longer period of time compared to other batteries and these batteries can exhibit a steady discharge characteristics over this period.
Fig. 4.3.1 Discharging current vs discharging time Fig. 4.3.2 (a) and Fig. 4.3.2 (b) show how the battery discharging current changes with state of charge for batteries installed in phase 1 and phase 2. The results illustrate that the LFP2 battery can exhibit a steady discharge current over almost the entire discharge cycle, which can be useful for applications when deep discharge is required. LFP1 battery can also deliver steady discharge current at low levels of state of charge, however it is initially discharged (at higher state of charge) at a smaller current. The NMC1 battery is discharged at a lower discharge current after its state of charge drops to 50%. This is due to increase in battery temperature and because the BMS lowers the discharge current to maintain the temperature.
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On the other hand, the LA1 battery current is mostly stable up to a SoC level of 40%. The data points below 50% SoC level for LA1 battery can be ignored since these data points result from sulfation when SoC falls below the recommended range. The results also show that LC1 battery can be discharged at a higher current compared to LA1 battery. The LFP3, and LFP5 batteries can deliver a steady level of current even if they are discharged to a state of charge 5%-10% (90%-95% depth of discharge). LFP4 batteries demonstrate this characteristic to a state of charge of 30% (70% depth of discharge).
(a)
(b) Fig. 4.3.2 Discharging current vs state of charge for batteries installed in (a) phase 1, (b) phase 2.
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Key takeaway: • • •
NMC1 battery discharges at a larger current when SoC is more than 50% and then at a lower discharge current which gradually decreases with decreasing SoC to avoid over-temperature. LFP2, LFP3, LFP4 and LFP5 can deliver a constant discharge current and can be useful to maintain constant current during discharge. LA1 discharge current is stable until SoC level more than 40%
4.4 Battery Temperature Fig. 4.4.1 (a) shows the battery temperature during discharging conditions. The battery temperature does not change significantly over the duration. This is because the batteries are in a regulated temperature condition, which does not vary significantly over time. It can be observed from the figure that NMC1 battery has a slightly higher temperature, whereas LC1 battery has a slightly lower temperature, compared to other batteries. Fig. 4.4.1 (b) demonstrates the battery temperature at different state of charge during discharging. The characteristics are similar to the one in Fig. 4.4.1(a). From this figure, it can be identified that the battery temperature data from this dataset cannot be used for battery capacity estimation, since the temperature does not change with state of charge. Key takeaway: • • •
Battery temperature did not vary with charge-discharge, so this parameter cannot be correlated with SoC. The batteries can perform in standard conditions, if the temperature does not change with SoC. LA1 battery discharges at a lower current, which may cause battery temperature to be lower compared to other batteries.
(a)
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(b) Fig. 4.4.1 Battery temperature vs (a) discharge duration, (b) state of charge 4.5 Voltage Regulation: Fig. 4.5 shows voltage regulation for different batteries. This will help determine internal impedance of batteries, which can distinguish the no load condition from full load condition. The figure has been plotted for a current range of 30-60 A and the change in terminal voltage has been identified. It can be seen that LA1 and NMC1 batteries have higher internal impedance compared to that of LFP1 and NMC2 batteries.
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Fig. 4.5 Voltage regulation for different batteries Key takeaway: • • •
LFP1 and NMC2 batteries have very good voltage regulation LA1 battery voltage degrades significantly with increasing discharge current NMC1 battery has a higher internal impedance compared to LFP1 and NMC2, but it is much lower than LA1
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Chapter 5 Impact of Weather Conditions 5.1 Introduction to the analysis: In this section, the difference in power, voltage and current characteristics of different batteries in different weather conditions from the average characteristics outlined in Section 3 and 4, has been investigated. For charging characteristics, the power and voltage deviations have been considered. On the other hand, the voltage and current deviations have been illustrated for discharging conditions. Note that, the average charging and discharging conditions have been obtained by averaging the characteristics over two years. The winter, spring, summer and autumn characteristics have been obtained by averaging the data over June-August, SeptemberNovember, December-February and March-May, respectively for 2 years. In this section, the difference between the characteristics in winter, spring, summer and autumn from the average characteristics have been calculated and then the mean deviation has been plotted for different batteries. 5.2 Deviation from average charging Characteristics 5.2.1
Mean deviation in Power Fig. 5.2.1 shows how the charging power in different weather conditions differs from the average charging power characteristics. It can be seen that NMC1 battery shows smaller deviation compared to the LC1, LFP1 and LA1 batteries. This deviation is more than 10% for LC1 and LFP1 batteries during winter and spring and LA1 batteries during winter and autumn. The maximum deviation is observed for LC1 battery during spring, which is around 20%. This can be attributed to the unavailability of the LC1 battery charging data during most part of the spring 2017.
Fig. 5.2.1 Mean deviation of charging power in different weather conditions from the average charging power over 2 years
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Key Takeaway: • •
LC1 and LA1 battery shows significant variation across different seasons in terms of charging power NMC1 and LFP1 battery shows less variation across different seasons in terms of charging power
5.2.2
Mean Deviation in Voltage Fig. 5.2.2 shows how the charging voltage in different weather conditions differ from the average charging voltage characteristics for different batteries. Note that this variation is observed at the system level and may not represent the chemical characteristics of the batteries. The deviation is within 2-3% for all the batteries for different weather conditions. The LFP1 battery shows smaller variation compared to the other batteries. The maximum deviation is observed for the LC1 battery during spring for the reasons mentioned in the previous discussion.
Fig. 5.2.2 Mean deviation of charging voltage in different weather conditions from the average charging voltage over 2 years
Key takeaway: • LC1 and LA1 battery shows some variation across different seasons in terms of charging voltage • NMC1 and LFP1 battery shows less variation across different seasons in terms of charging voltage • Overall, the voltage variation is not more than 1.5 V across different seasons 5.3 Deviation from average discharging characteristics 5.3.1
Mean Deviation in Voltage Fig. 5.3.1 shows how the discharging voltage changes in different weather conditions when compared with the average discharging voltage characteristics, computed over 2 years for each battery. It can be seen that NMC1 and LA1 batteries have significant fluctuations in 28
discharging voltage. For LFP1 and LC1 batteries, the maximum deviation is between 8-10%. On the other hand, the maximum deviation for NMC1 and LA1 batteries are around 18% in Summer. This deviation results due to the voltage levels set by BMS to achieve a constant power across the loads and could be caused by variations in the load.
Fig. 5.3.1 Mean deviation of discharging voltage in different weather conditions from the average discharging voltage over 2 years
Key takeaway: • Discharge voltage varies significantly for NMC1 and LA1 batteries during summer • LFP1 battery demonstrates smaller variation in voltage across different seasons • A variation of at least 2 V in discharging voltage across different seasons 5.3.2
Mean Deviation in Current Fig. 5.3.2 demonstrates the mean deviation of the discharging current in different weather conditions from the average discharging characteristics obtained over 2 years for different battery types. It can be seen that LC1 and LA1 batteries have more fluctuations in discharge current compared to the NMC1 and LFP1 batteries. The NMC1 battery does not deviate more than 2% from the average discharging characteristics. The LFP1 battery follows similar pattern except in summer. This can be attributed to the fact that LFP1 battery has not been discharged during December 2016 and February 2017 which caused fluctuation in the average characteristics. The maximum deviation of 10% is observed for LA1 and LC1 batteries during autumn.
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Fig. 5.3.2 Mean deviation of discharging current in different weather conditions from the average discharging current over 2 years
Key takeaway: • LC1 and LA1 battery discharge currents show more variation across different seasons • NMC1 battery shows less variation in discharge current across different seasons • LFP1 shows a variation of 3A in discharge current in summer 5.3.3
Energy throughput Fig. 5.3.3 illustrates how the energy throughput changes in different weather conditions for different batteries. It can be seen that LA1 battery has higher energy throughputs most of the time. This is because LA1 battery has a larger installed capacity, compared to others. The LA1 battery has smaller throughput in winter, as it has not been discharged for most of the time in that time period. The energy throughput of other batteries do not vary for more than 15% across different weather conditions.
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Fig. 5.3.3 Average energy throughput for different batteries in different weather conditions Key takeaway: • LC1 battery has a lower energy throughput which can be attributed to the faulty cells • LA1 battery has a lower energy throughput in Winter since it has not been discharged many times during that time period.
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Chapter 6 Battery Performance with number of Cycles 6.1 Introduction to the analysis: In this section, the residual capacity, discharge voltage and round trip efficiency with respect to the number of cycles have been investigated for the different batteries. The analysis considers how many times the battery state of charge starts increasing (charge) and then decreasing (discharge) and records that as number of cycles. Sometimes the batteries have undergone beyond the minimum SoC level and started charging from that point. However, these short cycles have been ignored, as these do not represent a nominal cycle. To determine the discharge capacity, the discharge duration is calculated for each discharge cycle and compared with the nominal discharge duration. For NMC1, LFP1 and LFP2 battery, the discharge duration is computed when the state of charge falls below 20%. On the other hand, for LC1 and LA1 batteries, the analysis considers a state of charge around 30% and 55%, since these batteries cannot be discharged deeply. 6.2 Discharge Capacity: Fig. 6.2.1 shows the capacity fade with number of cycles for a number of batteries. It can be seen that all batteries are demonstrating some form of capacity degradation with increasing number of cycles. This indicates that the batteries will not be able to deliver the same amount of energy after a number of cycles. LC1 and LA1 batteries demonstrate a larger degree of capacity fade compared to the LFP1 and NMC1 batteries. NMC2 batteries have data for only 7 months in 2016, so further analysis could not be conducted.
(a)
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(b) Fig. 6.2.1 Discharge Capacity vs number of cycles for different batteries (a) raw data (b) linear fit Key takeaway: • NMC1 and NMC2 batteries show smaller fade in capacity compared to other batteries • LA1 and LC1 batteries show larger fade in capacity compared to other batteries • LFP1 batteries show larger fade in capacity compared to LFP2 batteries 6.3 Discharge voltage: Fig. 6.3.1 shows how the battery terminal voltage starts degrading after a number of cycles. It can be observed that LFP1 and LFP3 batteries can deliver power at a steady terminal voltage across the number of cycles considered in the analysis. Note that LFP3 batteries have data available for only 5 months, and thus the fade in discharge voltage may not be visible at this short duration. NMC1 and NMC2 batteries demonstrate the same rate of degradation of discharge voltage with number of cycles. LA1 battery terminal voltage degrades at a larger rate compared to the other batteries. LC1 battery terminal voltage degrades at a slightly slower rate compared to LA1 battery.
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(a)
(b) Fig. 6.3.1: Discharge voltage vs number of cycles for different batteries (a) raw data (b) linear fit Key takeway: • • •
LA1 battery terminal voltage degrades at a larger rate compared to the other batteries. LC1 battery terminal voltage degrades at a slightly slower rate compared to LA1 battery LFP2 battery terminal voltage degrades at a slightly higher rate compared to LFP1 battery
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6.4 Efficiency: Fig. 6.4.1 shows how the battery efficiency degrades over the number of cycles. It can be seen that LFP1 battery has better efficiency compared to the other batteries. NMC1 battery has high efficiency initially, which then gradually degrades across its lifetime. LC1 battery is less efficient but the efficiency does not degrade significantly with number of cycles.
(a)
(b)
Fig. 6.4.1: Efficiency vs number of cycles for different batteries (a) raw data (b) linear fit Key takeaway: • •
LC1 battery has a lower round trip efficiency (can be attributed to the faulty cells) NMC1 and LFP1 batteries have more than 90% round trip efficiency, though it decreases with the number of cycles
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Chapter 7 Conclusions and recommendations Throughout this analysis, important characteristics for a range of batteries in diverse conditions have been investigated. The analysis shows that LFP batteries are able to provide a comparatively stable voltage during discharge. This is an important attribute when constant voltage needs to be maintained across load. LA batteries cannot be discharged deeply which makes them unsuitable for applications where deep discharging is required. LC batteries can deliver a steady voltage compared to the LA batteries, though the variation in voltage is more compared to LFP batteries. NMC batteries demonstrate a lower rate of capacity fade with respect to number of cycles, though they are not able to maintain a steady voltage during discharge. The discharge voltage linearly decreases with the battery state of charge. The characteristics obtained from this analysis have been summarised in Appendix A. The results from this analysis will be very useful in the ABPS project. The analysis can help differentiate different battery chemistries in terms of their charging-discharging characteristics and lifecycle performance. From the findings, practical battery models can be developed for different battery chemistries. The analysis of battery characteristics can enable the capacity estimation by exploiting the correlation between measurable parameters like voltage, current and state of charge. Overall, the analysis will allow the battery performance to be measured in a shorter and longer time and benchmark performance to be established.
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Chapter 8 Appendix A The following table summarizes the battery characteristics. Note that NMC2 is not included as it was no longer fit for purpose after 4 months of operation. Table 8.1: Summary of battery characteristics derived from the analysis
Battery Characteristics Maximum deviation in charging power Maximum deviation in charging voltage Maximum deviation in discharging voltage Maximum deviation in discharging current Maximum deviation in energy throughput Voltage regulation Slope and intercept of capacity degradation with number of cycles Slope and intercept of terminal voltage degradation with number of cycles Slope and intercept of efficiency degradation with number of cycles
NMC1
LFP1
LC1
LA1
11.1%
15.5%
22.3%
36.67% 9.12%
----
1.7%
1.5%
2.69%
2.22%
----
17.1%
7.17%
10.6%
16.35% 0.5%
3.9%
0.7%
0.7%
10.8%
14.3%
1.8%
6.4%
11.5%
2.3%
20%
35%
20%
----
23.32% 5%
---
53.3%
10.9%
----
-1.04% and 100.32 at 1000 cycles -0.09% and 52.2 at 1000 cycles
-1.75% and 96.66 at 1000 cycles
-3.7% and 100.92 at 750 cycles -0.13% and 47.94 at 750 cycles
-0.1% and 91.07 at 1000 cycles -0.1% and 50.93 at 1000 cycles
----
-0.002% and 51.23 at 1000 cycles
-2.59% and 100.06 at 1000 cycles -0.24% and 48.65 at 1000 cycles
0.006% and 96.8 at 1000 cycles
-0.003% and 96.04 at 1000 cycles
---0.002% and 87.77 at 750 cycles
----
----
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LFP2
0.81%
LFP3
0.007% and 57.5 at 300 cycles