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 EVs
There are 4 critical minerals mainly used in EV Cells:
- LITHIUM (Li): Lithium is reactive. Cells using Lithium can hold high voltage and exceptional charge, making for an efficient, dense form of energy.
- NICKEL (Ni): Nickel, when refined and alloyed suitably, enhances the properties of the battery components by increasing their energy density. This superior energy density directly translates into improved performance parameters such as extended driving range and longer battery life for electric vehicles.
- COBALT (Co): Cobalt plays a very important role in enhancing energy density and ensuring stability in lithium-ion batteries.
- MANGANESE (Mn): Manganese plays an important role as a stabilizer in NMC structure.
Based on detailed consideration of various factors, NMC and LFP cell chemistries are most prominently utilised in EV’s, with alternate chemistries like NCA and LTO selectively in use.
Lithium Nickel Manganese Cobalt Oxide (NMC)
NMC cathodes contain large proportion of nickel. Nickel helps increases energy density. Additionally, manganese and cobalt are used to improve thermal stability and safety.
Several NMC combinations in application in India are as below:
Lithium Iron Phosphate (LFP)
LFP cells are comparatively cheaper to make as compared to nickel based variants because of 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 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.
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 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 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 condition and need high reliability & stability to withstand high and low temperatures.
A module is basically a collection of cells connected in 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 EV battery pack with modules assembled in specific configurations to achieve output levels to meet power requirements of various applications in a EV.
Cell Contacting Systems
Cell Contacting system help in monitoring batteries for better performance and safety on real time basis.
Cell Contacting System (CCS) are the first level of electrical power transmission between cell and 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 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 and flat and wider than cables. Busbars can carry more current than cables with same cross-sectional area.
High-power EV battery packs utilise 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 conductor and with ground plane separated by insulator the best choice.
Main advantages of using Busbars over traditional insulated cables are as below:
Busbars help support thermal management and assist in distributing power more efficiently due to provision of mounting of active components for power conversion on the Bus bar. Higher power density is achieved through mounting of busbar active components like IGBT (Insulated Gate Bipolar Transistor) semiconductors, and passive circuit elements, such as capacitors and EMI noise reduction filters.
Gate Collector Emitter
Connectors
Connectors function is to connect individual cells and modules inside the battery pack (eg. EV busbars, wires, and other distribution connectors).
Connectors connect and protect battery pack while managing flow of power in, out, and around the pack.
The two main components of a connector are contacts and housing, which are also called plug or receptacle. The housing holds the terminals and 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 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 as below:
Battery Management System (BMS)
BMS monitors vital parameters like voltage, current and temperature to ensure safe operation of battery pack. BMS is also equipped with a failsafe mechanism that shuts off battery pak when necessary.
The BMS communicates with onboard charger to monitor and control battery pack charging. It also helps maximize the vehicle range by optimally using stored energy.
BMS manages cooling system of Battery pack and helps lower pack temperature in case of cell overheating.
Battery Thermal Management System (BTMS)
It 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 below:
Passive Cooling:
Passive cooling utilises natural convection along with conduction to transfer heat generated inside pack to external environment. Passive cooling is low-cost and ‘energy-efficient,’ as it requires no energy from vehicle.
Active Air Cooling:
Active air-cooling uses a fan to force air over batteries to remove heat. This helps extends lifetime of battery pack by maintaining batteries at consistent operating temperature.
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 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 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 array of cells and cooling plate, to help conduct heat and provide a thermal path for heat to flow away from 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 battery and heat sink, minimizing potential thermal impedance and providing shortest pathway to conduct heat.
Contactor System
It is very important for pack safety since it helps isolates high-voltage battery from the high-voltage bus, which delivers current to traction motor and other high 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 safe operation on 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 that 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 change in temperature, discharge voltage, and discharge current, are not easily detected.
Gas sensors can enable real-time gas measurement, needed by Battery Management System (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 decrease in internal resistance.
Each cell supplies energy through set number of electrons/second. When two batteries are connected in parallel number of electrons they pushed out each second is added and total energy supplied and total current increases.
Connecting two cells with different amp-hour ratings in parallel, creates 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 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
Series-parallel configuration helps provide 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 configuration of 4SP2 (4 in Series and 2 in Parallel) then total power produced by 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 equivalent state of charge of 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 advantages and limitations of passive and active cell balancing, while considering voltage differential, cell capacity, power efficiency, and overall system considerations. 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 application. 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 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.
Concluded
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.
