Energy storage systems for electric vehicles

1. Energy
Storage

2. Parameters to be considered
• Costs per kWh
• Specific Energy density (Wh/kg)
• Vol. Energy density (Wh/litre)
• Life-cycles at certain C-rate and temperature at certain DoD
• Capacity and DoD usable
• C-rate usable
• Safety and safe-disposal

3. Slow Charging vs Fast Charging
• Depends on the C - rate of the Battery
• C- rate is the rate at which battery is charged/ Discharged
• Battery capacity and Charger ratings
• Charging at 1C and higher (1.5C, 2C, 3C, 4C) is fast Charging
• Charging at 0.5C and lesser (0.1C, 0.2C, 0.3C, 0.4C) is slow Charging
Ratings C - rate Time taken to
charge
10kWhr Battery charged with 10 kW charger 1C 1hr
10kWhr Battery charged with 5 kW charger 0.5C 2 hr
10kWhr Battery charged with 20 kW charger 2C 30 min
10kWhr Battery charged with 40 kW charger 4C 15 min

4. Battery Life..

5. Comparing..

6. Electrochemical Battery
Consists of: Cathode, Anode and Electrolyte
• Ions are atoms that have lost or gained electrons and thus electrically
charged: Ion flow made possible with an electrolyte
• A Separator which acts as insulator (electrically isolates the
electrodes) but allows the movement of ions
• Charging: Electrons (in electrolyte) move through Separator inside
the battery towards cathode - creates voltage potential between
cathode and anode
• Discharge: Current from positive cathode through external electric
circuit (load) to negative anode

7. Li Ion Battery

8. Li - Ion Battery Chemistry
Cathode Anode Characteristics
LCO (LiCoO2) Graphite Used in cell-phones; Cobalt-rich and expensive
NMC
(LiNixMnyCozO2)
Graphite
Most commonly used EV battery; NMC811 has minimal Cobalt,
Nickel rich version attempts even smaller Cobalt
NCA
(LiNiCoAlO2 )
Graphite
Similar to NMC; less expensive, lower number of cycles; used by
Tesla as its battery size is large
LFP
(LiFePO4)
Graphite
Safer than NMC; limited by specific energy; used to be dominant
in China, now on way-out
NMC
LTO
(Li4 Ti5O12)
LTO anode gives much longer life-cycles and temperature
tolerance, SAFE, but poor-specific-energy; high costs
LFP LTO Similar to NMC-LTO

9. Comparison of Li - Ion Battery
Li-Ion Cell
Chemistry
LCO/Graphite or
NCA/Graphite
NMC/Graphite LFP/Graphite NMC/LTO LFP/LTO (Nbdoped)
Spec. Energy density
(Wh/kg)
150 -300 160-325
90-120
(150 with Silicon
anode)
60 -100 50 -80
Charge/discharge
rates
0.5C/1C
1C/1C (2C with
Silicain anode)
1C/2C (4C with
Silicain anode)
4C/4C 5C/10C
Life-cycles 1000
2000 (8000 with
Silica)
3000 (4000 with
Silica)
10000 20000
safety Cell < 55°C Cell < 55°C safer safest safest
Cell costs / kWh $120 $145 $200 $500 High

10. Requirements of Battery
1. High specific energy : Long runtime in most appliances, build batteries with high ampere-hour (Ah)
2. High specific power : Batteries made for power tools and electric powertrains provide high load, but the
specific energy is low
3. Affordable price : Materials, refining processes, manufacturing, quality control and cell matching
add manufacturing cost; volume production helps a bit. Single cell use requires
no cell matching, lowers costs
4. Long life : High initial Investment is fine in countries with low interest rate: if battery life can
be 20 years, low cost per year. Depends on battery design as well as usage
temperature, charge-times and harsh discharge rates
5. Safety : High specific energy systems are often reactive and unstable. When used
correctly, brand-name Li-ion is very safe
6. Wide operating
range
: Cold temperatures slow the electrochemical reaction of all batteries. Li-ion
cannot be charged below freezing. High heat shortens battery life and
compromises safety

11. Requirements of Battery
7. Toxicity : Nickel and lithium-based batteries contain little toxic material, but they still pose a
hazard if disposed carelessly
8. Fast charging : Lithium batteries should be charged at 1C or slower. Fast charge possible only if
batteries built for it, be in good condition and at room temperature. Aged and
mismatched cells hurt during fast charging
9. Self-discharge : self-discharge: long storage and instant start-up. Self-discharge increases with
temperature and age. Long shelf life has a minimal performance degradation.

12. 6 Important Parameters
LCO LMO NMC
LFP NCA LTA

14. Li Ion Battery Construction
Every Cell has
• Container
• Cathode
• Anode
• Separator between cathode and Anode
• Electrolyte in between Cathode and Anode
(LiPF6 used as Electrolyte
• Terminals
- Cylindrical / Pouch / Prismatic

15. Cylindrical Cell
• Cylindrical solid body without terminals
• Higher energy-density than pouch and prismatic
• Lesser number of cycles
• Spot-welding to make a pack
• Capacity - 2.2Ah - 3.8 Ah @ 3.7 V
• Cell (18 mm diameter and 65 mm length)
• volume of 16cm3 with capacity of around 3Ah (11 Wh)
• 21700 Cell (21mm dia & 70 mm length) - used by Tesla (3 – 4.8 Ah)
• Larger battery 26650 with solid body with large threaded terminals now available

16. Pouch Cell
• soft, flat body, thinner sizes
• Improves space utilization and allows flexible design
• More expensive to manufacture
• Less efficient in thermal management.
• Allow for swelling
• 90 to 95% packing efficiency
• Laser / Ultrasonic welded
• Cells available at 24 Ah, 44Ah, 50 Ah and higher
• Used in larger vehicles

17. Prismatic Cells
• Semi-hard plastic case with large threaded terminals
• Cells from 7 Ah, 15 Ah, 24 Ah, 30 Ah, 40 Ah, 44, 50 Ah
• Used in two-wheelers and three-wheelers
• Can be spot-welded, Laser-welded and Ultrasonic welded

18. SoC Estimation
Percentage of total charge available at any particular time in the battery
• 70% SoC implies that battery is 30% empty (70% full)
In Lead Acid battery, Open circuit Battery Voltage directly
proportional to its SoC
• 12V battery varies from 11.7V to 12.85V
• 48V battery varies from 46.5V to 51.5V
In Li Ion battery, SoC is nor proportional to O.C voltage (Both charging and discharging)

19. Li-Ion Battery: Voltage vs SOC

20. SOC Curve - Implications
• Constant Current (CC) Charging at High rate (say 2C)
• Only partially charge battery: possible only up to some low SoC (say 57%)
• Beyond that it will be a Constant Voltage (CV) Charging, which is very low-
current charging
• High-rate charging only meaningful for large Battery
• High-rate Charging also impacts life badly High-rate discharge also hurts
battery life
• Energy pumped into Cell between 3.5V and 4.2V when slow-charged
• For fast charge, it is between 3.9V and 4.2V
• Slow-discharge energy is between 3.4V and 4.1V SoC not a linear function of
voltage

21. Measuring SoC
Voltage Method
• Obtain the Open Circuit Cell Voltage (OCV) Vs SoC accurately in lab at very low charging rate (C/25
going to C/100) for different temperatures
• Does OCV Vs SoC curve depend on SoH
• SOC is a non-linear function of open-circuit voltage, only when Battery is fully at rest (very slow
charge or discharge is ok)

22. Coulomb Counting Method
• Very Accurate but dependent on accurate SoH and precision of current measurement
• Measuring the current (total Coulombs) flowing in and out of battery
• Coulomb Counting requires correct starting point (initial SoC)
• A reset to 100% is done after full charge cycle
• Coulomb count between two instant will indeed be a good measure of energy added or removed
from a battery
• But will represent SoC only to the extent that initial SoC was good!
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑆𝑜𝐶 ∆𝑆𝑜𝐶 =
𝐶ℎ𝑎𝑟𝑔𝑒 𝑃𝑢𝑚𝑝𝑒𝑑 𝑖𝑛 𝑜𝑟 𝑜𝑢𝑡 𝑜𝑓 𝑡ℎ𝑒 𝐵𝑎𝑡𝑡𝑒𝑟𝑦
𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 ∗ 𝑆𝑜𝐻
(converted to percentage)
• Where Charge pumped in and out is Coulomb Count * electron charge or integration of current over
time.
• If computed charge is IN the ΔSoC is positive, else it is negative.

23. Coulomb Counting Method
𝑆𝑜𝐶𝑁𝑒𝑤 = 𝑆𝑜𝐶𝑜𝑙𝑑 + ∆𝑆𝑜𝐶
• Requires SoH to be correctly known as ΔSoC is dependent on SoH
• A repeated partial charge and discharge (without a 100% reset cycle) builds up the
accumulation errors in SoC
• The extent of error directly depends on the errors in current measurement device and the
degradation of battery (SoH)

24. Estimating Battery Life
• Not very accurately, but can be estimated
• Referred to as age/health of a battery - State of Health (SOH)”
• Represents the amount by which battery has deteriorated due to irreversible physical and chemical
changes
Method 1
• Completely discharge (3 V) and then charge the battery slowly
• Track the open-circuit voltage and carry out the coulomb count
• Give several hours rest after full charge
• Indicates maximum charge that the battery can hold currently
• Compare it with past data: Gives an estimate of SoH
Alternate method: (Internal resistance)
• As battery electrodes deteriorate, its capacity to deliver current also reduces
• Internal resistance of a cell indicates the capability to deliver current
• Difference between internal resistance of fresh and used cell, helps in estimating SOH

25. Self Discharge of Battery
• Self discharge defines the rate at which the battery looses its energy while on shelf

26. A fully charged battery of 500 Ah capacity is supplying a
load of 0.6Ω. Find the state of charge of the battery after
3 hrs. Nominal voltage of the battery is 12V. SoH of the
battery is 80%.

27. A li-ion battery when fully charged(to 4.2V) has an
internal resistance of 20mΩ at BoL. At the EoL, the cell
suffers a 150% rise in internal resistance when the cell is
fully charged. Find the maximum instantaneous current
the cell can supply (with 4.2V) at BoL and EoL.

28. Battery Pack
Number of cells assembled to form a battery-pack for required voltage and capacity
▪ Safety Issues
▪ Cell Balancing
▪ Careful electrical design including BMS (every cells get equally charged / discharged)
▪ Thermal Design is very critical
▪ Mechanical design considerations
Cell Chemistry evolves continuously bringing down the cost
▪ Materials used depends on chemistry
▪ Quantity used depends on Wh/kg
▪ Cost of material and Availability

29. Design considerations for Battery Pack
Thermal design must remove the heat generated from the pack immediately
• Cells temperature need control
Mechanical design should include safety considerations
• Right Pressure needs to be applied to cells, else they will bulge
Battery Management System (BMS)
• Only balanced cells used in a pack: requires voltage/current/temperature monitoring of each
cell and balancing cells during charging as well as discharging
• Pack should get cut off if the temperature increases: key to safety
• Communicates with charger to decide charging strategy

30. Battery Pack Design - Electrical
Battery-pack required certain Voltage and Capacity (in Ah and Wh terms)
• Voltage chosen based on requirement of drive-train components and total battery Capacity
• High Currents implies large ohmic losses (thick cables) - normally limited to < 200A
• Therefore depending upon energy (kWh) of storage, certain voltage preferred
✓ 48V or 72V for small batteries for 2W /3W and small 4W
✓ 350V for medium sized batteries for larger cars and pick-ups
✓ 750V for motors for buses and trucks

31. Battery Pack from Cells
• Cell voltage typically 3.7V (usage voltage varying from 3.1V to 4.1V)
• Cell Capacity is 3.4 Ah (cylindrical) to 50 Ah (prismatic / pouch)
• Requires cells connected in Series to get higher voltage: 14 cells in Series is 51.8V
• Required cells in Parallel for higher Capacity: 8 cells (50 Ah) in parallel gives 400 Ah
Generally, cells must be connected in series and parallel to make a pack
• mSnP implies n cells in series to form strings and then connecting m strings in parallel
• nPmS implies m cells in parallel to form modules and then connecting n modules in series

32. mSnP Battery Pack
• Cells can be connected in series to form a STRING
Eg. 14 cells connected in series to form 48V battery
20 cells in series for 72V battery
100 cells in series for 365V battery
200 cells in series for 730V battery
• Strings can be connected in parallel to increase capacity
Eg. 14S2P strings with 15 Ah cells would be of 48V * 2 * 15 Ah capacity
• Any capacity can be built
• Drawback - if strings do not have exactly same voltage, current will flow from one string to another for balancing
• Happen continuously while charging or discharging and even when IDLE
• Not good for battery

33. nPmS Battery Pack
• Cells can be connected in parallel to form MODULES
Eg. Four 15 Ah cells give a module of 60 Ah
Eight 15 Ah cell give a module of 120 Ah
Sixteen 15 Ah cell give a module of 240 Ah
• Modules can now be connected in series to make a battery of higher voltage
Eg Battery Pack of 8P14S with 15Ah (3.7 V) cell will have a capacity is 51.8V x 120 Ah or 6.21 kWh
• No major balancing issue in mPnS pack
• Auto-balancing between cells in a module all the time

34. Modules in Battery Pack
• Multiple Cells packed in parallel to form a Module
✓ Cells selected are of same voltage (balanced)
✓ Cells connected with a metal bar that conducts electricity
• Multiple Modules in series to form a battery Pack
• Battery Management System (BMS) a must to get optimal performance
• Cell equalization during Charging
✓ Monitor voltages and temperature of each module and total pack current
• If a module is over-charged (impacts life), equalize by
✓ Passive balancing: bleed module with higher voltage through a resistor, so that it charges slower, or just drain it
✓ Active balancing: stop charging module with higher voltage; instead, use its output to charge the rest of pack
(using a DC-DC converter)
• BMS could limit temperature of each module if active cooling is done

35. Failures in Battery Pack
In a nPmS pack
• If one of n parallel cells may become open circuit, (n-1) cells in module continue to function
• Module capacity goes down by (1/n) Ah - Battery will function with reduced capacity (n-1)PmS
• One of n parallel cells become short circuit – the full module has zero voltage
• Battery voltage will be (m-a)S: the pack is now nP(m-1)S
• Can continue to function only if the drive train functions at lower voltage - poor performance
• Repair will involve replacement of whole module – difficult (depends upon pack)
• Replacement of a cell is difficult
If a serial connection between module fails - battery fails
• Generally easier to repair – may require bus-bar replacement

36. Failures in Battery Pack
• Failure in BMS - replace BMS
• MOSFET in BMS heated - design issue
• Temperature sensor failure: detect and replace
Failure to detect rapid heating and cutting-off (BMS function) may take cells to meltdown
• Cell capacity deterioration - Battery Pack Capacity deterioration
Unbalanced cells in modules may lead to Battery Pack Capacity deterioration
• Incorrect SoH or SoC estimation
BMS may cut-off battery (and thereby vehicle cut-off) even when charge is not low
Wrong display of charge remaining