REFERENCE
EV Battery Engineering Glossary
52 technical terms covering battery systems, BMS algorithms, cell chemistry, thermal management, charging infrastructure, and EV standards. Engineering-grade definitions you can cite.
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BMS
- Active Balancing
- A BMS technique that redistributes charge between cells using capacitors or inductors, transferring energy from higher-SOC cells to lower-SOC cells to maintain uniformity. More efficient than passive balancing but adds cost and complexity.
- Battery Management System (BMS)
- An electronic system that monitors and manages a rechargeable battery by measuring voltage, current, and temperature; estimating SOC and SOH; controlling cell balancing; and protecting against overcharge, over-discharge, overcurrent, and thermal events.
- CAN Bus
- Controller Area Network — a robust vehicle bus standard (ISO 11898) used for communication between ECUs in vehicles including BMS, inverter, and VCU. J1939 is the higher-layer protocol commonly used in commercial EVs.
- Cell Balancing
- A BMS function that equalizes the state of charge across cells in a series string to maximize usable capacity and prevent overcharge/over-discharge of individual cells. Can be passive (resistive bleed) or active (charge redistribution).
- Coulomb Counting
- A SOC estimation method that integrates current over time: SOC(t) = SOC(t0) + (1/C) ∫I dt. While simple, it accumulates measurement drift error over time and requires periodic correction via OCV-SOC lookup or Kalman filter updates.
- EKF (Extended Kalman Filter)
- A non-linear state estimator widely used in BMS for combined SOC and SOH estimation. Linearizes the battery model around the operating point using Jacobian matrices. Balances accuracy and computational efficiency for real-time embedded deployment.
- J1939
- A higher-layer protocol based on CAN bus (ISO 11898) widely used in commercial vehicles for ECU communication. Defines parameter groups (PGNs) for battery data, motor control, and diagnostic messages in EVs and HEVs.
- Kalman Filter
- An optimal state estimation algorithm that recursively estimates system states from noisy measurements. In BMS, used for combined SOC and SOH estimation by fusing voltage, current, and temperature measurements with a battery cell model.
- OCV-SOC Curve
- The relationship between open-circuit voltage and state of charge for a battery cell. Used as the primary reference for SOC estimation. LFP cells have a characteristically flat OCV curve (~3.2-3.4V for 20-80% SOC), making accurate estimation more difficult.
- Passive Balancing
- A simple cell balancing method that dissipates excess energy from higher-voltage cells through bleed resistors as heat. Lower cost than active balancing but wastes energy and generates heat. Typical balancing current: 50-200 mA.
- RLS (Recursive Least Squares)
- An adaptive parameter estimation algorithm used in BMS for online identification of battery model parameters (resistance, capacitance, OCV offset). Useful for tracking parameter changes due to aging and temperature.
- SOC (State of Charge)
- The ratio of remaining capacity to total usable capacity, expressed as a percentage. 100% SOC = fully charged; 0% SOC = empty at the lower voltage cutoff. The primary output of any BMS, estimated via coulomb counting, OCV lookup, or Kalman filtering.
- SOH (State of Health)
- A measure of battery degradation expressed as the ratio of current usable capacity to original rated capacity. An SOH of 80% indicates 20% permanent capacity loss. The threshold for battery replacement in most commercial EV applications.
Cell Chemistry
- Anode
- The negative electrode in a lithium-ion cell where oxidation occurs during discharge. Typically made of graphite or silicon-graphite composites. During charging, lithium ions intercalate into the anode structure.
- C-rate
- A measure of the rate at which a battery is charged or discharged relative to its capacity. A 1C rate means the current that will discharge the entire battery in 1 hour. A 2C rate discharges in 30 minutes, while C/2 takes 2 hours.
- Calendar Aging
- The irreversible capacity loss and impedance increase that occurs in a battery over time regardless of cycling, driven by side reactions between the electrolyte and electrodes. Accelerated by high temperature and high SOC during storage.
- Cathode
- The positive electrode in a lithium-ion cell where reduction occurs during discharge. Common cathode materials include LFP (LiFePO4), NMC (LiNiMnCoO2), and NCA (LiNiCoAlO2).
- Cell Chemistry
- The electrochemical system composition of a battery cell defined by its cathode, anode, and electrolyte materials. Common chemistries include LFP, NMC, NCA, and emerging Na-ion and solid-state.
- Cycle Aging
- Capacity fade and impedance rise caused by repeated charge-discharge cycling. Driven by SEI layer growth, lithium plating, particle cracking, and electrolyte decomposition. Accelerated by high C-rates, extreme temperatures, and deep DOD.
- Cylindrical Cell
- A battery cell format with a cylindrical form factor (e.g., 18650, 21700, 4680). Offers good mechanical stability and high production throughput but lower packing density compared to prismatic or pouch cells.
- DCIR
- Direct Current Internal Resistance — the internal resistance of a battery cell measured under DC load pulses. DCIR increases with aging and varies with temperature and SOC. A key diagnostic parameter for SOH estimation.
- Depth of Discharge (DOD)
- The percentage of a battery's capacity that has been discharged relative to its total capacity. A DOD of 80% means 20% SOC remains. Deep cycling (high DOD) accelerates cycle aging compared to shallow cycling.
- Electrolyte
- The ionically conductive medium in a battery that enables lithium ion transport between anode and cathode. Typically lithium hexafluorophosphate (LiPF6) salt dissolved in organic carbonate solvents.
- Energy Density
- The amount of energy stored per unit volume (Wh/L) or mass (Wh/kg). Cell-level gravimetric energy density ranges from ~150 Wh/kg (LFP) to ~300 Wh/kg (NMC 811). Pack-level density is typically 30-50% lower due to packaging overhead.
- LFP (LiFePO4)
- Lithium Iron Phosphate — a cathode chemistry known for excellent thermal stability, long cycle life (2000-8000 cycles), and low cost. Lower energy density (~150 Wh/kg) than NMC but superior safety and calendar life. Dominant in commercial EVs and ESS.
- Lithium Plating
- A degradation mechanism where lithium ions deposit as metallic lithium on the graphite anode surface instead of intercalating. Occurs during high-rate charging at low temperatures or high SOC. Causes capacity loss and creates dendrite safety risk.
- Na-ion (Sodium-Ion)
- An emerging battery chemistry using sodium ions instead of lithium. Offers lower cost and abundant raw materials but lower energy density (~100-150 Wh/kg). CATL and other manufacturers have begun commercial production for ESS and entry-level EVs.
- NMC (LiNiMnCoO2)
- Lithium Nickel Manganese Cobalt Oxide — a cathode chemistry offering high energy density (~200-300 Wh/kg) with tunable Ni:Mn:Co ratios. Common variants: NMC 111, NMC 532, NMC 622, NMC 811. Higher energy but shorter cycle life than LFP.
- Prismatic Cell
- A rectangular battery cell format with rigid casing, commonly used in EV battery packs for higher packing density. Offers good thermal management via flat surfaces but may swell under aging.
- SEI Layer
- Solid Electrolyte Interphase — a thin passivation layer that forms on the graphite anode surface during initial cycling. Permits lithium ion transport while preventing further electrolyte decomposition. SEI growth is the primary driver of calendar aging.
Charging
- CC-CV Charging
- Constant Current — Constant Voltage charging protocol. The battery is first charged at a constant current until a voltage threshold is reached, then held at constant voltage while current tapers down. Standard protocol for Li-ion batteries.
- CCS
- Combined Charging System — a fast-charging standard that supports both AC and DC charging through a single connector, capable of up to 350 kW. Uses Power Line Communication (PLC) per ISO 15118 for vehicle-charger handshake.
- CHAdeMO
- A DC fast-charging protocol developed by Japanese automakers (TEPCO, Nissan, Mitsubishi) supporting bidirectional charging (V2H, V2G). Delivers up to 400 kW with CHAdeMO 3.0. Uses CAN-based communication.
- DC Fast Charging
- High-power DC charging typically above 50 kW that bypasses the vehicle's onboard charger and delivers DC power directly to the battery. Standards include CCS (up to 350 kW), CHAdeMO (up to 400 kW), and GB/T (up to 250 kW).
- GB/T
- China's national standard for EV charging (GB/T 20234 and GB/T 27930). Supports both AC and DC charging up to 250 kW. Uses CAN-based communication protocol. Mandatory for all EVs sold in China.
- ISO 15118
- International standard for vehicle-to-grid communication interface. Enables Plug & Charge (automatic authentication and billing), smart charging, and bidirectional power transfer. Uses TCP/IP over Power Line Communication.
- V2G (Vehicle-to-Grid)
- Technology enabling bidirectional power flow between an EV battery and the electricity grid. Allows the vehicle to discharge stored energy back to the grid during peak demand. Requires compatible hardware (bidirectional charger) and communication protocol (ISO 15118).
Standards
- AIS-156
- Automotive Industry Standard issued by MoRTH (India) mandating safety and performance requirements for traction battery packs. Covers vibration, thermal runaway propagation, IP rating, short circuit protection, and overcharge behavior. Required for EV homologation in India.
- ISO 26262
- Functional safety standard for automotive E/E systems. Defines Automotive Safety Integrity Levels (ASIL A to D) for risk classification. BMS functions like overvoltage protection typically require ASIL C or D compliance.
- UL 2580
- Underwriters Laboratories standard for EV battery safety. Defines abuse test matrix including overcharge, short circuit, crush, immersion, thermal shock, and fire exposure. Widely referenced in North American EV certification.
- UN ECE R100
- United Nations Economic Commission for Europe regulation for electric vehicle battery safety. Covers electrical safety, thermal runaway, mechanical integrity, and chemical hazard protection. Required for EV type approval in UN ECE member countries including India.
Thermal
- Heat Pump
- An efficient cabin heating system for EVs that uses a refrigeration cycle to move heat from ambient air or the battery to the cabin. Can reduce winter range loss by 30-50% compared to resistive PTC heaters.
- Immersion Cooling
- A thermal management technique where battery cells are directly submerged in a dielectric coolant fluid. Offers superior heat transfer compared to indirect cooling, enabling higher C-rates and more uniform cell temperatures.
- Lumped Thermal Model
- A simplified thermal model that treats the battery cell as a single mass with uniform temperature. Uses the heat capacity and thermal resistance to estimate temperature rise: ΔT = Q / (m × c_p). Suitable for system-level studies.
- Nusselt Number (Nu)
- A dimensionless number in heat transfer representing the ratio of convective to conductive heat transfer across a boundary. Used in cooling plate design: Nu = h × D_h / k, where h is heat transfer coefficient and D_h is hydraulic diameter.
- PTC Heater
- Positive Temperature Coefficient heater — a resistive heating element used for cabin heating in EVs. Simpler and cheaper than heat pumps but less efficient, drawing 5-7 kW continuously and significantly reducing winter driving range.
- Reynolds Number (Re)
- A dimensionless number characterizing flow regime: Re = ρ × v × D_h / μ. Re < 2300 indicates laminar flow; Re > 4000 indicates turbulent flow. Used in cooling plate channel design to determine heat transfer correlations.
- Thermal Runaway
- A self-accelerating exothermic reaction within a battery cell that causes rapid temperature rise, venting, fire, or explosion. Triggered by internal short circuit, overcharge, or external heating. UN ECE R100 requires demonstrated thermal runaway propagation resistance.
Vehicle
- HVAC Load
- The power consumed by heating, ventilation, and air conditioning systems in an EV. Can account for 3-7 kW during extreme temperatures, significantly reducing driving range especially in cold climates.
- MIDC
- Modified Indian Drive Cycle — the standard drive cycle used for EV range certification in India. A less aggressive cycle compared to WLTP, with lower average speeds and acceleration rates, resulting in higher claimed range figures.
- Regenerative Braking
- An EV technology that recovers kinetic energy during deceleration by operating the electric motor as a generator, converting motion back into electrical energy to recharge the battery. Typical efficiency: 60-80% depending on speed and battery state.
- WLTC
- Worldwide Harmonized Light Vehicles Test Cycle — a global drive cycle used for EV range and energy consumption certification. More dynamic than MIDC with four phases (low, medium, high, extra-high speed) and a total duration of 30 minutes.