The battery cell is the smallest functional unit of any battery system. Everything above it — the module, the pack, the BMS, the HV architecture — exists to house, protect, connect, and manage cells. Every performance target, every lifetime requirement, and every safety constraint in a battery system ultimately traces back to what an individual cell can and cannot do.
Understanding the cell is therefore not optional background knowledge for battery engineers. It is the foundation. The voltage window the BMS protects, the thermal management system that keeps cells within 2°C of each other, the busbar cross-sections sized to the continuous current — all of these are direct consequences of cell-level physics and cell-level specifications.
The cell datasheet capacity is measured at a specific C-rate and temperature. At 2C discharge and −10°C, the available capacity may be 70% of the datasheet figure. At the same rate and +45°C, it may be 95%. Real application duty cycles require characterising the cell across the full operating envelope, not just at the nominal condition.
This section covers the cell from first principles through to production data.
Where to Start
New to battery cells? This is a suggested reading order through the core material. For the complete list of every page in this section, see the Full Index below.
- Cathode, Anode & Electrolyte – the whole picture: the three core components every cell is built from.
- Formats – the physical shapes cells come in: cylindrical, pouch, prismatic and coin.
- Key Cell Metrics – the core numbers used to describe and compare cells.
- Battery Capacity – what capacity means and how it’s actually measured.
- Cell Design – how component and format choices come together into a finished cell.
- Cell Temperature – how temperature affects performance, ageing and safety.
- Cell Benchmarking – how to compare cells against each other on a level footing.
- Cell Roadmap – where cell technology is heading next.
Full Index
A complete, always up to date index of every page in the Battery Cell section, organised by hierarchy. Click any heading to expand it.
What is a Battery Cell?
A cell contains three core electrochemical components: an anode (negative electrode), a cathode (positive electrode), and an electrolyte that allows ions to move between them. A separator physically prevents the anode and cathode from touching while remaining permeable to ions.
During discharge, lithium ions move from the anode through the electrolyte to the cathode, while electrons flow through the external circuit — producing the electrical current that powers the load. During charge, this process reverses: an external voltage forces lithium ions back into the anode.
The voltage of the cell is determined by the electrochemical potential difference between the anode and cathode materials. The capacity is determined by how much lithium can be stored in the active materials. The power capability is determined by how fast that lithium can move — governed by ionic conductivity, electrode thickness, and particle morphology.
These three parameters — voltage, capacity, and power — cannot all be maximised simultaneously. They are the fundamental trade-off that drives cell design and chemistry selection.
→ Cell Design in depth → Cathode materials → Anode materials → Electrolyte
Key Cell Parameters
Every cell specification sheet expresses cell performance through the same core set of metrics. Understanding what each means — and critically, under what conditions it is measured — is essential before using any cell data.
Nominal voltage (V) — the representative working voltage of the cell, calculated from nominal energy divided by nominal capacity. Not a fixed value: cell voltage varies with state of charge, temperature, and current. An NMC cell has a nominal voltage of approximately 3.6–3.7V; LFP approximately 3.2V.
Capacity (Ah) — the charge a cell can deliver at a standard temperature (usually 25°C) and standard discharge rate (usually C/3 or C/5) down to the minimum voltage cutoff. A 100Ah cell can deliver 100A for one hour, or 10A for ten hours, to the cutoff voltage.
Energy (Wh) — the total electrical energy the cell delivers under standard conditions. Wh = Ah × V_nominal. This is the figure used to calculate pack energy, but it is discharge-rate and temperature dependent: the same cell may deliver 95Wh at C/5 and 80Wh at 2C.
C-rate — the charge or discharge current relative to cell capacity. 1C discharges the full capacity in one hour. 2C in 30 minutes. C/5 in five hours. C-rate normalises current across different cell sizes and allows direct comparison of rate capability.
Gravimetric energy density (Wh/kg) — cell energy per unit mass. The primary metric for applications where weight matters — EVs, aircraft, handheld devices.
Volumetric energy density (Wh/L) — cell energy per unit volume. Critical for space-constrained applications and for calculating pack-level volumetric density.
DC Internal Resistance — DCIR (Ω) — the cell’s resistance to a direct current pulse. Quoted at a specific condition: typically 25°C, 50% SoC, 10-second pulse. DCIR drives voltage drop under load (V = OCV − I×R), I²R heat generation, and available peak power. It increases with age, decreasing temperature, and at extreme states of charge.
Cycle life — the number of charge/discharge cycles the cell completes before capacity falls to 80% of its initial value, measured under defined conditions (temperature, C-rate, depth of discharge, voltage window). Cycle life is not a single number — it is a family of curves that vary strongly with operating conditions.
Calendar life — capacity and resistance degradation with time, independent of cycling. A cell stored at 40°C and 100% SoC degrades far faster than one stored at 20°C and 50% SoC.
→ Key Cell Metrics — all parameters explained → Cell benchmarking database
Cell Formats
The same electrochemistry can be packaged into several different physical formats. The format affects mechanical integration into the pack, thermal management approach, achievable energy density, and manufacturing cost.
Cylindrical
The electrode layers are wound into a tight spiral (jelly roll) and inserted into a cylindrical metal can. The can itself is typically the negative terminal; a cap assembly on the top is the positive terminal.
Cylindrical cells are the most mechanically robust format — the circular cross-section provides inherent structural strength and the metal can provides good protection. They are well-suited to automated high-volume manufacturing and have the lowest cost per Wh of any format for high-volume production.
The dominant sizes have been 18650 (18mm diameter, 65mm height) and 21700 (21mm × 70mm). The newer 46xx family — 46mm diameter cells including the 4680 (46mm × 80mm) — offers significantly higher energy per cell, lower manufacturing cost per kWh, and improved rate capability through tabless electrode design. The 4680 is used in the Tesla Model Y and Model 3 Highland; the 4695 in the Rivian R2.
The main limitation of cylindrical cells is packing efficiency — the circular cross-section leaves dead volume in any rectangular module or pack, capped at approximately 90.7% for hexagonal close packing.
→ Cylindrical cells → 46xx cylindrical cells
Prismatic
Prismatic cells use a rigid aluminium or steel case with a rectangular cross-section. The electrodes are either wound as a flat jelly roll or stacked as individual sheets. The terminals are located on the top face.
Prismatic cells pack efficiently into rectangular modules and packs — no dead volume from circular cross-sections. They are the dominant format for large-format automotive cells, particularly in China (CATL, BYD, CALB, Gotion). The trade-off is that prismatic cells require a controlled pressure applied to their main faces to maintain performance over lifetime — the cell expands during charging and permanently swells over thousands of cycles; the module structure must accommodate this while maintaining the required contact pressure.
Pouch
Pouch cells use a flexible aluminium-laminate pouch rather than a rigid case. Positive and negative electrode tabs protrude from one or both ends of the pouch. The absence of a rigid case gives maximum energy density at cell level and allows the cell to be shaped to fit a specific space — but requires external mechanical support and pressure management.
Pouch cells are widely used in consumer electronics, where the flexible form factor enables thin device profiles. In automotive applications, they are used by LG Energy Solution, SK On, and others, typically in a module structure that provides the required mechanical compression.
Blade
The blade cell is a high-aspect-ratio prismatic format developed by BYD, optimised specifically for cell-to-pack designs using LFP chemistry. The long, thin cell spans the full length of the pack and acts as a structural member, eliminating the module layer. The BYD Blade enabled LFP to become competitive with NMC in EV applications — the cell-to-pack efficiency gain compensates for LFP’s lower gravimetric energy density at cell level.
→ Blade cells → Cell to Pack (C2P)
Format Comparison
| Cylindrical | Prismatic | Pouch | Blade | |
|---|---|---|---|---|
| Mechanical robustness | High | High | Low | High |
| Packing efficiency | Moderate (~91%) | High | Very high | Very high |
| Thermal management | Side or base cooling | Base or face cooling | Face cooling | Base cooling |
| Cell expansion management | Via can | Module compression | Module compression | Pack-level structure |
| Manufacturing maturity | Very high | Very high | High | High |
| Typical application | Consumer, EV (cylindrical) | EV, BESS | EV, consumer | EV (LFP C2P) |
→ All formats — detailed comparison
Power vs Energy Cells
The same chemistry, in the same format, can be optimised for either energy density or power density — but not both simultaneously. This is one of the most fundamental trade-offs in cell design.
Power cells have thinner electrode coatings — more current collector area relative to active material, shorter ion diffusion paths, and lower DCIR. They deliver and accept current faster, making them suitable for high-rate applications: HEV/PHEV hybrid systems, power tools, fast-charging stationary storage. The penalty is lower Wh/kg.
Energy cells have thicker electrode coatings, which stores more active material per unit volume and increases Wh/kg and Wh/L. The thicker electrode also increases the diffusion path length for lithium ions, raising internal resistance and limiting the maximum safe charge/discharge rate. Energy cells are designed for applications that need maximum range or storage capacity — long-range EVs, grid-scale storage.
The Ragone plot — power density versus energy density — is the standard visualisation of this trade-off across cell types and formats.
Cell Lifetime and Degradation
Battery cells degrade with use and with time. Understanding the mechanisms — and which operating conditions accelerate them — is essential for designing packs that meet their lifetime targets.
Degradation manifests in two ways: capacity fade (the cell stores less energy) and resistance growth (the cell delivers less power and generates more heat at a given current). In most applications, 80% of initial capacity is defined as end of life.
The five principal degradation mechanisms in lithium-ion cells are:
1. SEI growth — the solid electrolyte interphase that forms on the graphite anode during the first charge is essential for cell operation, but continues to grow slowly throughout the cell’s life. As it thickens, it consumes lithium irreversibly and increases cell impedance.
2. Lithium plating — at high charge rates, low temperatures, or high states of charge, lithium can deposit as metallic lithium on the anode surface rather than intercalating into the graphite. Plated lithium can form dendrites — conductive filaments that can penetrate the separator and cause an internal short circuit.
3. Particle cracking — repeated expansion and contraction of cathode particles during cycling generates mechanical stress. Particles crack, exposing fresh surfaces to electrolyte, consuming active material, and increasing impedance.
4. Electrolyte decomposition — continuous slow oxidation of the electrolyte at the cathode surface and reduction at the anode surface consumes electrolyte and creates resistive interphase layers.
5. Mechanical loss of active material — electrode particles lose contact with the current collector or with each other, becoming electrically isolated and unavailable for charge storage.
These mechanisms are temperature-dependent, voltage-dependent, and rate-dependent. The BMS usable SoC window, charge rate limits, and temperature management targets are all set to slow these mechanisms to the rate required to achieve the pack lifetime target.
→ Cell lifetime and degradation in depth → Calendar ageing → Cycle ageing
Cell Manufacturing
Manufacturing a battery cell is one of the most demanding industrial processes in any sector. The tolerances on electrode thickness are measured in microns; the cleanliness requirements rival pharmaceutical manufacturing; the formation and ageing process ties up capital in slow electrochemical cycling for days or weeks.
The cell manufacturing process has three main stages:
Electrode manufacturing — active materials (cathode powder or graphite), binder (typically PVDF), solvent (NMP), and conductive additives (carbon black) are mixed into a slurry and coated onto a metal foil current collector (aluminium for cathode, copper for anode). The coated foil is dried to remove solvent, then calendered (compressed between rollers) to achieve target thickness and porosity.
Cell assembly — the dried electrodes are slit to width, then wound (cylindrical) or stacked (pouch, prismatic) with a separator between each anode-cathode pair. Tabs are welded to the current collectors, and the cell stack is inserted into the case or pouch and sealed.
Cell finishing — the sealed cell is filled with electrolyte under vacuum, then charged for the first time in a controlled process called formation. Formation creates the SEI layer and establishes the electrochemical baseline of the cell. After formation, cells are aged (rested) and subjected to final electrical testing, grading, and quality inspection.
Manufacturing scrap rates, energy consumption (50–180 kWh per kWh of cell produced), and formation/ageing capacity are the dominant cost drivers in cell production.
→ Cell manufacturing process — full detail → Manufacturing CAPEX breakdown → Cell to Cell Variation → Energy used to manufacture a cell
Cell Testing
Cell testing underpins the entire design process — from initial sizing through to control system parameterisation and final pack sign-off. The cell supplier’s datasheet gives nominal values at standard conditions; the engineer’s job is to characterise the cell across the full operating envelope that the application will impose.
Test categories:
Performance testing — capacity vs temperature, capacity vs C-rate, DCIR vs SoC and temperature, OCV curve, peak power vs temperature and SoC. This data feeds directly into BMS parameterisation and pack sizing models.
Ageing tests — calendar ageing (cells stored at fixed temperature and SoC) and cycle ageing (cells cycled at fixed conditions). These tests establish the lifetime model used to predict capacity fade and resistance growth over the pack service life.
Abuse testing — overcharge, overdischarge, external short circuit, nail penetration, crush, thermal exposure. Establishes the boundaries of safe operation and the failure modes when those boundaries are exceeded.
Legislative testing — UN 38.3 (transport), IEC 62133 (portable applications), IEC 62619 (industrial), and application-specific standards. Required for shipping and for market access in most regions.
Cell sample maturity — cells move from A-sample (first physical prototypes, hand-built, variable) through B, C, and D samples as the design matures and the production process stabilises. The test programme at each stage is calibrated to the maturity of the cell.
→ Cell testing in depth → Abuse testing → UN 38.3 transport test → Cell sample maturity
Cell Benchmarking
The cell benchmarking section assembles measured performance data from real production cells across the market — energy density, capacity, DCIR, voltage, dimensions, mass, and more. It is one of the most widely used resources on the site.
The cell database is available as a downloadable Excel file — updated regularly, currently covering over 1,550 cells from 156 manufacturers. The benchmarking pages group cells by chemistry and format for direct comparison.
→ Cell Benchmarking — online → Cell Database — Excel download → A to Z Cell Manufacturers
Cell Roadmap
The cell technology roadmap traces the trajectory of energy density, cost, and chemistry from current production to the next generation of cells. The key developments to track are:
High-nickel cathodes (NCMA, NMC90+) — pushing gravimetric energy density above 300 Wh/kg at cell level through higher nickel content with aluminium stabilisation.
Silicon anodes — silicon can store 10× more lithium per unit mass than graphite, but expands ~300% during charge. Partial silicon addition (5–10% blended with graphite) is in production today; higher silicon content cells are in development.
46xx cylindrical — the 4680 and 4695 formats are driving cell-level cost reduction and energy density improvement simultaneously through tabless electrode design and increased cell size.
Solid state batteries — replacing the liquid electrolyte with a solid ionic conductor eliminates the flammability risk and could enable lithium metal anodes, potentially reaching 500+ Wh/kg at cell level. Commercial production at automotive scale remains several years away.
Sodium-ion — a commercially available alternative to lithium-ion for applications where cost and supply chain security are priorities over energy density. The best current Na-ion cells now approach the energy density of mainstream LFP.
→ Cell Technology Roadmap → Chemistry Roadmap
Related Sections
The cell sits at the foundation of the entire battery design stack. Every section of this site is connected to cell-level properties:
- Chemistry — the electrochemical basis of cell voltage, capacity, and safety behaviour
- Module — how cells are grouped and mechanically integrated
- Battery Pack — how cell specifications determine pack architecture
- BMS — how cell parameters are measured, estimated, and protected
- Thermal — how cell heat generation and temperature sensitivity drive thermal management design
- Safety — thermal runaway, propagation, and the cell-level origins of pack-level safety requirements
- Lifetime — degradation mechanisms and their relationship to operating conditions
- Manufacturing — the production process and its influence on cell variability and quality
- Testing — the test programme that characterises and validates cell performance
- Costs — cell cost as the dominant driver of pack economics
See also: Cell Capacity Testing: Standards, Diagnostics and Safety, Cell Design, Cylindrical Cells, Cell Energy Density Roadmap to 2035, Large-Format BESS Cells, Cell to Pack Mass Ratio by Chemistry and Format, Self Discharge of Cells