Lithium-ion batteries for EV batteries| Understanding the Indian context

In this comprehensive article, Gurusharan Dhillon, Director of eMobility at Customised Energy Solutions, discusses the lithium-ion batteries used in electric vehicles, focusing on the Indian market.

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 the actual cost of a battery varies depending on composition and material, batteries continue to have a significant contribution to the total cost of the vehicle, providing us with a wonderful opportunity to dive deep and explore the exciting and wonderful world of EV batteries.

Major Components of an EV cell

Starting off with the major components of an EV cell:

The remaining cost contribution is towards Manufacturing and Depreciation [24%], Battery Housing and Current collectors.

Cell Characteristics

Cell chemistry plays a major role in effectiveness, energy density and thermal stability. The selection of cell chemistry requires deep consideration of the following:

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 to achieve 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 an individual cell.

High energy density helps increase driving range and reduces the number of cells required to deliver the same amount of power, thereby reducing the size of the battery pack.

The comparison of energy density for different cell chemistries is as follows:

Safety

Component materials like cathode, anode, electrolyte, and separator should have high thermal stability and intrinsic safety.

Cell Cycle

The life cycle of a battery is the number of charge and discharge cycles it can complete while 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, the battery slowly loses the capacity to retain full charge, resulting in a 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 inactive or stored.

Cycle Life [Lifespan]

The lifespan of a cell is normally measured in a number of cycles and refers to the useful life of a cell in EV application. The cycle life of a cell is defined as the number of charge-discharge cycles the cell undergoes at a particular (DoD) depth of discharge until the battery has degraded compared to its original capacity.

The actual operating life of the cell is affected by the rate and depth of cycles and other conditions such as temperature and humidity.

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

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 the Actual voltage of the cell varies around its nominal value.

Comparison of Nominal voltage for different cell chemistries is as below:

For eg. a nominal voltage of 3.6V indicates that the voltage will be between 2.8V and 4.2V.

(Recommended) Charge Current

The ideal current at which the cell is initially charged (to roughly 70 per cent SOC) under CC (constant charging) before CV (constant voltage) charging.

C rate (Battery Power or Charging Rate/Speed):

The charging rate (C rate) is the rate at which electrical current can be moved through a cell. A Cell with a Higher C rate can deliver its stored energy faster.

Capacity or Nominal Capacity (Ah for a specific C-rate):

The charge is delivered by a fully charged battery under a specific temperature and load.

Nominal capacity is usually measured in Ampere hours (Ah) for a certain C-rate.   

              Ampere Hour (Ah) = Current (I) x Discharge Time (T)

One Ampere Hour cell supplies 1A current for 1 Hour. The same cell can supply 0.5A (less current) for 2 hours same battery and 2A (more current) for 30 minutes.

To compare batteries, the the rate at which nominal capacity is determined must be known. Example of two batteries with 120 Ah nominal capacity:

If the battery is required to provide a 12A current for 10 hours (120 Ah), Battery 2 will not be suitable since the 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 the 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 the cell is discharged at a 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 the 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 a cell when stimulated by electric current. Increased impedance level indicates weakness in the cell, which can cause stored energy to convert to heat rather than useful current.

Discharge Current

Cell manufacturers usually define the Maximum Continuous Discharge Current to prevent excessive discharge rates that can damage the cell or reduce its capacity.

Cell Condition

The condition of a cell is normally measured by:

State of Charge (SoC):

The state of charge represents the 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.

It is calculated as the ratio of remaining/releasable charge in the battery divided by the maximum charge/rated capacity that can be delivered by the battery.

Depth of Discharge (DOD)

Depth of discharge describes the percentage of a battery’s capacity that has been discharged relative to its maximum capacity, which basically indicates energy taken out of the battery compared to its total storage capacity.

For e.g., If a battery has a total capacity of 100 Ampere hours (AH) and is discharged to 80 Ah, then the depth of discharge will be 80%, and if discharged to 50 Ah, then the depth of discharge will be 50% 

Depth of discharge is an important factor to consider in battery management, as it can impact the 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)

The state of health is an important indicator throughout the lifespan of the battery. EV owners need to know the 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 the battery’s state of health is essential to understand whether the battery is viable for reuse and repurposing or if it needs to be sent for recycling.

The maximum 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:

The swelling of lithium-ion batteries is caused by heat and the build-up of gases. The swell rate is the amount that anode material expands when charged. The 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 the rate at which they can be cooled or heated, mainly due to the 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 the smallest ratio of surface area to volume.

Cell format directly affects several factors like pack design, 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 formats: 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 are 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.

The prismatic cell format makes it possible to manufacture larger cells, which reduces the 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 a minimalistic approach to packaging and do not have a rigid enclosure, using sealed flexible foil as a cell container, helping reduce weight and allowing for flexible and easy fitment in the 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.

Cell Form Factors used in the Indian context

Factors impacting the life of EV Cell

Temperature & Climate Conditions:

One of the most important factors affecting battery performance is temperature. Cell performance is impacted when the battery faces extreme temperatures.

Charge and Discharge Cycles:

EV cells are normally rated for charge/discharge cycles (cell discharges from 100% to 0% and then recharges from 0% to 100%). Charging frequency affects battery life. Every discharge/recharge cycle strains the battery.

Cell Chemistries in use in EV’s

There are 4 critical minerals mainly used in EV Cells:

Based on detailed consideration of various factors, NMC and LFP cell chemistries are most prominently utilised in EVs, with alternate chemistries like NCA and LTO selectively in use.

Lithium Nickel Manganese Cobalt Oxide (NMC):

Several NMC combinations in application in India are as follows:

Lithium Iron Phosphate (LFP):

LFP cells are comparatively cheaper to make as compared to nickel-based variants because of the higher use of iron and phosphate.

Lithium Nickel Cobalt Aluminium Oxide (NCA):

NCA batteries enjoy nickel-based advantages with NMC, including high energy density and specific power. NCA utilise aluminium (instead of manganese) to increase stability.

Nickel-rich variants (Nickel more than 80%) are low in cobalt and enjoy a cost advantage since cobalt is several times more expensive as compared to nickel.

Lithium Titanate (LTO):

Unlike other chemistries above, where the cathode composition makes the difference, LTO batteries use a unique anode surface made of lithium and titanium oxides. These batteries exhibit excellent safety and performance under extreme temperatures but have low capacity and are relatively expensive, limiting their use at scale.

Cell Chemistries used in the Indian context

Fun fact: Cylindrical cells with NMC (nickel manganese cobalt) power Formula E

Important consideration factors when finalising Cell chemistry for a particular application are:

While the majority of cells being utilised in Electric vehicles are imported, several companies are investing in Gigafactories to help locally source EV cells in India.

In addition to Giga factories, companies are also actively engaged in creating local ecosystem for Anode and Cathode Active Materials, Electrolyte and Separators

Module:

When a number of cells are assembled into a rigid frame to protect them better from external shocks such as heat or vibration, it is called a module.

Individual cells, when assembled together in a module, are exposed to road shocks during driving conditions and need high reliability & stability to withstand high and low temperatures.

A module is basically a collection of cells connected in a series, parallel or series-parallel configuration.

Pack:

A pack refers to a series of individual modules and protection systems organised in a shape that can be installed on the electric vehicle.

EV battery modules are connected in series, in parallel or in series-parallel to form an EV battery pack with modules assembled in specific configurations to achieve output levels to meet the power requirements of various applications in an EV.

Important components of a Battery Pack

Cell Contacting Systems:

The cell contacting system helps monitor batteries for better performance and safety on a real-time basis.

Cell contacting systems (CCS) are the first level of electrical power transmission between the cell and the power source and help connect individual cells to one unit.

Additionally, functions like sensors for voltage and temperature are built directly into cell contacting systems for enhanced efficiency.

Cell contacting systems also include the interconnection of different modules, managing thermal consideration and ensuring overall structural integrity in a pack.

Components of cell contacting systems include:

Busbars:

Busbars are solid metal bars used to carry current. Bus bars carry electrical current between cells within a module or between modules in a pack.

Typically made from copper or aluminium, busbars are rigid, flat, and wider than cables. Busbars can carry more current than cables with the same cross-sectional area.

High-power EV battery packs utilise a combination of cells either in series, parallel or series-parallel configurations to achieve desired voltage ratings. Connecting individual cells requires a material good at both conducting and insulating as compared to traditional insulated cables, making Busbars, which are electric conductors and with ground planes separated by insulators, the best choice.

The main advantages of using Busbars over traditional insulated cables are as follows:

Busbars help support thermal management and assist in distributing power more efficiently due to the provision of mounting of active components for power conversion on the Bus bar. Higher power density is achieved through the mounting of busbar active components like IGBT (Insulated Gate Bipolar Transistor) semiconductors and passive circuit elements, such as capacitors and EMI noise reduction filters.

Connectors:

The connector’s function is to connect individual cells and modules inside the battery pack (e.g. EV busbars, wires, and other distribution connectors). Connectors connect and protect the battery pack while managing the flow of power in, out, and around the pack.

The two main components of a connector are contacts and housing, also called as plug or receptacle. The housing holds the terminals, isolates the terminals from other electronic components, and prevents short-circuiting, thereby ensuring connection stability.

Connector terminals made from electrically conductive materials like brass, phosphor bronze, beryllium copper, and high copper alloy provide a continuous path for the electrical current to flow between circuits. 

Cell Interconnects:

Structures that physically link adjacent cells, aiding in both electrical and mechanical connections.

Terminal Blocks:

Components used at the ends of a module or pack for external electrical connections.

Other key EV battery components that form Battery pack are:

Battery Management System (BMS):

 BMS monitors vital parameters like voltage, current and temperature to ensure the safe operation of the battery pack. BMS is also equipped with a failsafe mechanism that shuts off the battery pack when necessary.

The BMS communicates with the onboard charger to monitor and control battery pack charging. It also helps maximize the vehicle range by optimally using stored energy.

BMS manages the cooling system of Battery pack and helps lower pack temperature in case of cell overheating.

Battery Thermal Management System (BTMS):

Maintains the thermal energy and temperature in an EV battery, heating or cooling it down as needed. Types of cooling that are deployed are as follows:

Passive Cooling:

Passive cooling utilises natural convection along with conduction to transfer heat generated inside the pack to the external environment. Passive cooling is low-cost and “energy-efficient,” as it requires no energy from a vehicle.

Active Air Cooling:

Active air-cooling uses a fan to force air over batteries to remove heat. This helps extend the lifetime of the battery pack by maintaining batteries at consistent operating temperatures.

It is cheaper and lighter than liquid cooling because it does not interface with other cooling networks in the vehicle.

Liquid Cooling:

Liquid cooling allows the battery pack to be operated with higher peak power loads because it dissipates more heat than other cooling methods.

There are three main approaches to liquid cooling: cooling tubes, cooling plates with cooling channels inside them, and direct/immersive cooling. 

Thermal interface materials (TIMs):

Thermal interface materials (TIMs) facilitate the transfer of heat between components in EV Battery assemblies. Thermal Interface Materials (TIM) remove excess heat from battery pack cells to regulate battery temperature, improve battery functionality and prolong battery life. 

Thermal Interface Materials are placed at the bottom plate of the battery or between an array of cells and a cooling plate to help conduct heat and provide a thermal path for heat to flow away from the battery.

Gap fillers (also commonly referred to as thermal pads) are a specific type of TIM ideal for conforming to curved or uneven surfaces typically seen in a pack. Thermal pads help maximize surface area contact between the battery and heat sink, minimizing potential thermal impedance and providing the shortest pathway to conduct heat.

Contactor System:

It is very important for pack safety since it helps isolate the high-voltage battery from the high-voltage bus, which delivers current to the traction motor and other voltage components of the vehicle in the event of danger.

There are two main types of contactors:

Normally Open (NO) and Normally Closed (NC).

Contactors are the only moving part in a battery pack and are critical to the safe operation of the pack. Contactor faults like permanently closed, permanently open and overheating can stop the operation of the battery pack.

Possible reasons for contactor failure include incorrect design, manufacturing quality, Incorrect sizing—mechanical vibration and selection.

Temperature Sensors:

Electric vehicle battery packs are impacted by temperature fluctuations, which can affect their performance, safety and lifespan.

Temperature sensors placed directly inside the cells provide precise temperature readings, ensuring each cell operates within a safe/optimum range by controlling heating and cooling mechanisms.

Continuous monitoring allows for assessment of battery health and, in case of overhearing, will trigger safety measures like disconnecting the battery or reducing charging rates.

Gas Sensors

During the early onset of a thermal runaway, gradual changes in temperature, discharge voltage, and discharge current are not easily detected.

Gas sensors can enable real-time gas measurement, which is needed by Battery Management Systems (BMS) to detect cell failure when specific gas concentrations exceed certain thresholds and help in early warning of thermal runaway of lithium-ion batteries.

Series and Parallel Connection for Battery Pack

Parallel Cell Module

Connecting cells in parallel causes voltage to remain the same and current to increase due to a decrease in internal resistance.

Each cell supplies energy through a set number of electrons/second. When two batteries are connected in parallel, the number of electrons they push out each second is added, and the total energy supplied and total current increases.

Connecting two cells with different amp-hour ratings in parallel creates an unbalanced flow of current and voltage between cells, causing cells with higher capacity to discharge at a slower rate, resulting in lower performance, shortened battery life, and potential safety risk.

Series Cell Module

Connecting cells in series causes the current to remain the same while voltage gets summed up, due to which voltage across cells is increased (Kirchoff’s Law). However, if the internal resistance of a cell increases, it will affect the maximum discharge rate and overall efficiency.

Series and Parallel Module

A series-parallel configuration helps provide the desired voltage and capacity in the smallest possible size. The total power of the above is 50.4 Wh (Voltage x Current). This configuration is called 2SP2 (2 in Series and 2 in Parallel).

If the configuration consists of eight cells with the configuration of 4SP2 (4 in Series and 2 in Parallel), then the total power produced by the pack will be 100.8 Wh (double of 2SP2).

Terminology of a Battery Pack    XX S YY P

XX: No. of Cells connected in Series        YY: No. of cells connected in Parallel

Cell Balancing

BMS helps maximise battery capacity and increase cell longevity by helping maintain an equivalent state of charge for every cell.

Choosing the Right Cell Balancing Method for Electric Vehicle Battery Packs

Choosing the appropriate cell balancing method depends on various factors, including vehicle performance, cost, and performance.

Battery Pack makers and EV OEMs need to carefully evaluate the advantages and limitations of passive and active cell balancing while considering voltage differential, cell capacity, power efficiency, and overall system considerations. The choice of cell balancing method needs to ensure optimal performance, safety, and cost-effectiveness for the working life span of the battery pack.

Battery Testing

Battery testing is critical in helping ensure safe and reliable performance for EV applications. Battery performance, safety and reliability need to be thoroughly evaluated, and comprehensive testing is essential to ensure the same.

Battery testing involves detailed evaluations to assess the battery’s overall health, capacity, energy density, efficiency, cycle life, self-discharge rate, thermal stability, and safety. 

As a leading e Mobility services provider, Customized Energy Solutions works closely with industry partners to support them in the areas of Cell selection, Testing, Pack Design and other areas to help address business challenges with cost-effective and sustainable solutions.

Customized Energy Solutions manages the India Energy Storage Alliance (IESA), a member-driven alliance which has active engagement from stakeholders across the value chain from Mining to Recycling, including Cell components (Anode, Cathode, Electrolyte, Separator), Cell Manufacturing, BMS, Electronics, Components, Assembly, OEM’s and Recycling. CES is also working closely with companies setting up Giga factories in India to help fast-track local start of production.

About the author:

Gurusharan Dhillon is an automotive professional with 30+ years of expertise in strategy, operations, sales and marketing. He currently serves as a Director of eMobility at Customised Energy Solutions. With a focus on the electric mobility sector, Dhillon specializes in Powertrain, Battery Technology, Charging Infrastructure, and emerging technologies. He has worked with leading automotive OEMs like Toyota, Nissan, Honda and Hyundai.

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