Decarbonization of the transportation sector has an important role to play in helping reduce Greenhouse Gas (GHG) emissions and meeting net zero emission targets.
New energy vehicles play a vital role in this transition and Electric vehicles are leading the change to sustainable mobility. Key components of an Electric vehicle include Battery Pack, Electric Motor, Motor Controller and Inverter all of which significantly impact total vehicle cost.
While actual cost of a battery varies depending on the composition and material, batteries continue to have a significant contribution on the total cost of the vehicle providing us with a wonderful opportunity to deep dive and explore the exciting and wonderful world of EV batteries.
Major Components
Starting off with major components of a EV cell:
Cell Characteristics
Cell chemistry plays a major role in effectiveness, energy density and thermal stability. Selection of cell chemistry requires deep consideration of:
Performance:
Each cell should have consistent performance for parameters like capacity, internal resistance, open circuit voltage and quality.
Power Density (Specific Power):
Power Density is the amount of power that can be delivered per unit mass (Watts per kilogram) or maximum available power per unit volume (Watts per L).
Power Density is a characteristic of battery chemistry and packaging and determines the battery size required to achieve a given performance target. Power Density is the battery’s ability to deliver electrical power quickly, for achieving high acceleration and vehicle responsiveness.
Energy Density (Specific Energy):
Energy Density: Energy is the total electrical energy obtainable from a cell in one discharge cycle divided by the mass of individual cell.
Safety
Component materials like cathode, anode, electrolyte and separator should have high thermal stability and intrinsic safety.
Cell Cycle
A life cycle of a battery is the number of charge and discharge cycles it can complete while still maintaining most of its performance.
A full cycle would mean one complete discharge and one full recharge. In normal usage, a battery goes through multiple recharge and discharge cycles. With every charge cycle, battery slowly loses capacity of retaining full charge resulting in decrease in performance over time (reduced range and faster discharge).
Calendar Life: Calendar life is the degradation amount that occurs over time (not cycles) while the battery is either inactive or stored.
Cycle Life [Lifespan]
Lifespan of a cell is normally measured in number of cycles and refers to the useful life of a cell in EV application. Cycle life of a cell is defined as number of charge-discharge cycles the cell undergoes at a particular depth of discharge (DoD) until battery has degraded compared to its original capacity.
Actual operating life of a cell is affected by rate and depth of cycles and by other conditions such as temperature and humidity.
Higher DOD à Lower Cycle Life
(Higher the DOD, lower the cycle life)
Normally when energy-generating capability falls below 80%, batteries may be deployed in stationary applications, such as renewable energy storage and other stationary applications.
Cell Specifications/Characteristics
For cell selection, cell specifications/characteristics need to be closely studied:
Nominal Voltage (V): Nominal voltage is the amount of voltage output a cell gives out when charged and is the standard voltage by which cells are referred to.
Nominal voltage is a function of anode and cathode materials, as well as impedance and Actual voltage of cell varies around its nominal value.
The ideal current at which cell is initially charged (to roughly 70% SOC) under CC (constant charging) before CV (constant voltage) charging.
C rate (Battery Power or Charging Rate/Speed):
Charging rate (C rate) is the rate at which electrical current can be moved through cell Cell with Higher C rate can deliver its stored energy faster.
Capacity or Nominal Capacity (Ah for a specific C-rate)
Charge delivered by a fully charged battery under specific temperature and load.
Nominal capacity is usually measured in Ampere hours (Ah) for certain C-rate.
Ampere Hour (Ah) = Current (I) x Discharge Time (T)
One Ampere Hour cell supplies 1A current for 1 Hour. Same cell can supply 0.5A (less current) for 2 hour same battery and 2A (more current) for 30 minutes.
To compare batteries, rate at which nominal capacity is determined needs to be known. Example of two batteries with 120 Ah nominal capacity:
If battery is required to provide 12A current for 10 hours (120 Ah), Battery 2 will not be suitable since C/10 rate is 11.7 A and only Battery 1 will be suitable.
Battery capacities can be compared by Amp-hour only, if they have same voltage. If two batteries have different voltages, using Watt-hour will be more accurate.
Energy or Nominal Energy Wh (for a specific C-rate)
“Energy capacity” is the total Watt-hours available when cell is discharged at specific discharge current (C-rate) from 100% SOC to cut-off voltage.
Nominal Energy (Wh) = Discharge Power (W) x Discharge Time (h)
Internal Resistance (IR)
Internal resistance is different for charging and discharging, and is dependent on state of charge. As internal resistance increases Battery Efficiency decreases and thermal stability is reduced since more charging energy is converted into heat.
Impedance
Amount of resistance within cell when stimulated by electric current. Increased impedance level indicates weakness in cell which can cause stored energy to convert to heat rather than useful current.
Cell manufacturers usually define Maximum Continuous Discharge Current to prevent excessive discharge rates that can damage cell or reduce its capacity.
State of Charge (SoC)
State of charge represents percentage of energy stored in a cell relative to its full capacity. SOC is an important metric for evaluating energy availability and overall system performance.
100% SOC Fully Charged
0% SOC Fully Discharged
It is calculated as ratio of remaining/relesable charge in the battery, divided by maximum charge/rated capacity that can be delivered by the battery.
Depth of Discharge (DOD)
Depth of discharge describes percentage of a battery’s capacity that has been discharged relative to its maximum capacity, which basically indicates energy taken out of battery compared to its total storage capacity.
For example.: If a battery has total capacity of 100 Ampere hours (AH) and is discharged to 80 Ah then depth of discharge will be 80% and if discharged to 50 Ah then depth of discharge will be 50%.
Depth of discharge is an important factor to consider in battery management, as it can impact overall lifespan and performance of the battery. Deeper discharge cycles generally lead to more wear and tear on the battery, potentially reducing its longevity.
State of Health (SOH)
State of health is an important indicator throughout the lifespan of the battery. EV owners need to know reliability of their vehicles and during resale, battery state of health information helps in accurate valuation due to increased confidence in the EV’s worth, longevity, and range.
When the EV is retired, knowing battery’s state of health is essential to understand whether battery is viable for reuse and repurposing, or if it needs to be sent for recycling.
The max releasable capacity represents the upper limit of what the battery can theoretically release which might not be achievable in real-world applications.
Max releasable capacity is higher than rated capacity because rated capacity is set considering safety, longevity, and reliable performance.
Swell Rate
Swelling of lithium-ion batteries is caused due to heat and build-up of gases. Swell rate is the amount that anode material expands when charged. Anode tends to swell when charged and contract when discharged.
Form Factor
The form factor of an EV battery refers to its physical shape and size. The form factor of cells has an important effect on rate at which they can be cooled or heated, mainly due to ratio of surface area to volume.
The three most common form factors for EV batteries are cylindrical, prismatic, and pouch. Of the three main form factors used, cylindrical cells have smallest ratio of surface area to volume.
Cell format directly affects several factors like pack design, cell cooling, pack safety, cell-to-module integration, and mechanical stability.
Cylindrical
Sheet-like anodes, separators, and cathodes are sandwiched, rolled, and packed into a cylinder-shaped can.
Cylindrical cells are the least expensive cell format to manufacture because they are self-contained in a casing offering good mechanical resistance.
Cylindrical cells are largely used in two format 18650 and 21700.
Prismatic
Prismatic cells consist of large sheets of anodes, cathodes, and separators sandwiched, rolled up, pressed and fitted into a metallic or hard-plastic housing in cubic form.
Prismatic battery cells have a rectangular shape and normally used in EVs that require a high power-to-weight ratio. Prismatic cells have a higher energy density than cylindrical cells, making them more space-efficient and lighter.
Prismatic cell format makes it possible to manufacture larger cells, which reduces number of electrical connections that need to be cleaned and welded. However, prismatic cells have a lower lifespan than cylindrical cells, making them less durable and more prone to degradation over time.
Pouch Cells
Pouch cells display minimalistic approach to packaging and do not have a rigid enclosure, using sealed flexible foil as cell container helping reduce weight and allows for flexible and easy fitment in pack.
Pouch cells are an excellent candidate for use in EVs since pouch cells have the lightest form factor amongst all cell formats. However, pouch cells have lower energy density than cylindrical and prismatic cells, making them less suitable for high-power EVs requiring higher energy density cells. Also, pouch cells are prone to punctures and leaks and are less durable as compared to cylindrical cells.
Factors impacting life of EV Cell
Temperature & Climate Conditions:
One of the most important factors affecting battery performance is temperature. Cell performance is impacted when battery faces extreme temperatures.
To be continued…
Gurusharan Dhillon is the Director – e Mobility, Customised Energy Solutions. He is a Mechanical Engineering graduate with MBA in Sales & Marketing. He has 30+ years of experience in areas of Strategy, Business Planning, Operations, Sales, Marketing and Product Planning in Asia and Middle East with leading Japanese and Korean automotive OEMs like Toyota, Nissan, Honda and Hyundai.
