Cell basics
Cell basics
The vocabulary you need before you can read a data sheet.
A cylindrical lithium-ion cell roughly 18 mm in diameter and 65 mm long, the most widely tooled format in the industry.
The name is dimensional: 18 mm diameter, 65 mm length, with the trailing zero denoting a cylindrical form. Because holders, nickel strip, welding jigs and pack tooling are standardised around 18 mm, the 18650 remains the default where a design does not need the extra volume of a 21700. Current models span about 2,000 to 3,500 mAh, and continuous discharge from 8 A on energy grades to 36 A on power grades.
Full entry: 18650 cell in pack design →
See also:
21700 cell, C-rate, Form factor
A cylindrical lithium-ion cell roughly 21 mm in diameter and 70 mm long, offering about 50% more volume than an 18650.
The extra volume over an 18650 is spent either on capacity at similar current or on current at similar capacity. In practice that means fewer parallel strings for a given pack capacity, fewer welds and a simpler BMS. It is also the only cylindrical format in which 5,000 mAh and 60 A continuous discharge currently coexist in one part. The trade-off is thermal: the high-current grades assume the pack can hold an 80 C surface cut.
Full entry: 21700 cell in pack design →
See also:
18650 cell, Temperature cut-off, Form factor
The mechanical envelope of a cell — cylindrical, prismatic or pouch — which sets the packaging efficiency and the interconnect method.
Cylindrical cells are steel cans with standardised dimensions and well-established welding methods. Prismatic cells are rectangular aluminium or steel cans that pack more efficiently into a box. Pouch cells are laminated film and give the highest volumetric efficiency but need external compression in the pack. The choice is usually driven by enclosure geometry and existing tooling rather than by electrical performance alone.
Full entry: Form factor in pack design →
See also:
18650 cell, 21700 cell
Charge or discharge current expressed as a multiple of the cell rated capacity, so that cells of different sizes can be compared on the same scale.
A 3,000 mAh cell discharged at 3 A is running at 1 C; at 30 A it is running at 10 C. The same current is a very different duty for a 5,000 mAh cell. This is why C-rate, not amperes, is the figure to compare across formats. It is also why a cycle life claim is meaningless without the C-rate it was measured at: a cycle at 0.5 C and a cycle at 3 C age a cell at very different rates.
Full entry: C-rate in pack design →
See also:
Capacity, Cycle life, Depth of discharge (DoD)
The charge a cell can deliver between its charge voltage limit and its discharge cut-off, stated in ampere-hours or milliampere-hours.
Data sheets normally give two figures: a typical capacity and a minimum capacity. Designing to the typical figure is designing to a value the manufacturer does not guarantee. Capacity is also conditional on rate and temperature: a cell rated at 3,000 mAh at 0.2 C will deliver measurably less at 10 A, and measurably less again at low temperature. The minimum figure at the rate you actually intend to run is the only safe design input.
Full entry: Capacity in pack design →
See also:
C-rate, Energy density
Energy stored per unit volume (Wh/L) or per unit mass (Wh/kg), the figures that decide how big and how heavy a pack has to be.
Volumetric density binds when the enclosure size is fixed; gravimetric density binds when the pack has to be carried or lifted. A cell can be strong on one and unremarkable on the other. Energy density also has to be read against the discharge rate: a figure quoted at 0.2 C does not describe what the cell delivers at 10 C, where voltage sag removes usable energy before the capacity is exhausted.
Full entry: Energy density in pack design →
See also:
Capacity, Internal resistance
The opposition to current flow inside the cell, measured as AC impedance at a stated frequency and state of charge, or as DC resistance across a current pulse.
Impedance decides voltage sag under load and how much of the capacity is usable before the pack hits its cut-off voltage. It is also the parameter that moves most with age: a cell that comfortably meets a current specification when new can fall out of specification partway through its life. AC and DC figures are not interchangeable, and a DC value is meaningless without the pulse duration and state of charge it was taken at.
Full entry: Internal resistance in pack design →
See also:
State of health (SoH), C-rate
The cell surface temperature at which a manufacturer discharge or charge rating ceases to be valid, commonly 80 C or 83 C on high-power cylindrical cells.
A continuous discharge figure is a conditional statement, not an absolute one. A rating of 60 A that holds only while the pack keeps the cell below an 80 C surface temperature cannot be used in a sealed housing with no thermal path. Where a manufacturer states a condition, it belongs next to the number; stripping it out to make a comparison table look tidy is how packs get built on ratings they cannot reach.
Full entry: Temperature cut-off in pack design →
See also:
C-rate, Thermal runaway
Chemistry
Chemistry
What the cell is made of, and what that costs or buys you.
A lithium-ion cathode chemistry using nickel, manganese and cobalt, dominant in cylindrical power cells and in most traction packs.
The ratios vary: nickel-rich formulations push energy density up, manganese contributes thermal stability, and cobalt supports rate capability and cycle life. Most high-power cylindrical cells, including the Samsung INR, Molicel INR and LG INR families, are nickel-rich NMC. Some variants add silicon or silicon oxide to the graphite anode to gain capacity, which is why a cell may be described as NMC-Si.
Full entry: NMC (lithium nickel manganese cobalt oxide) in pack design →
See also:
NCA (lithium nickel cobalt aluminium oxide), LFP (lithium iron phosphate)
A nickel-rich cathode chemistry using aluminium as the stabilising element, associated with high energy density in cylindrical cells.
NCA is the chemistry behind the Panasonic NCR and NCA cylindrical families. It delivers high specific energy, which is why it appears in cells optimised for capacity rather than extreme discharge current. As with any nickel-rich chemistry, the cell depends on the pack managing voltage and temperature windows properly rather than on the chemistry being forgiving.
Full entry: NCA (lithium nickel cobalt aluminium oxide) in pack design →
See also:
NMC (lithium nickel manganese cobalt oxide), LFP (lithium iron phosphate)
A lithium-ion cathode chemistry using iron phosphate, trading energy density for thermal stability and long cycle life.
LFP cells have a nominal voltage near 3.2 V against roughly 3.6 V for NMC and NCA, and lower energy density by both volume and mass. What they buy is a much flatter and safer thermal behaviour and cycle counts that suit stationary storage. This is why LFP dominates residential and commercial energy storage while nickel-rich chemistries dominate traction and portable power, where volume and weight bind.
Full entry: LFP (lithium iron phosphate) in pack design →
See also:
NMC (lithium nickel manganese cobalt oxide), NCA (lithium nickel cobalt aluminium oxide)
Pack design
Pack design
From single cell to assembled pack.
The electronics that monitor cell voltages, current and temperature and disconnect the pack when any of them leave the manufacturer window.
A BMS is not optional for unprotected industrial cells. It enforces the charge voltage limit, the discharge cut-off, the current limits and the temperature window, and it decides when to stop charging because one cell group is out of balance with the rest. The important specification is not the feature list but whether the thresholds match the cell actually fitted: a BMS configured for a different chemistry or voltage window will either cut early or fail to protect.
Full entry: BMS (battery management system) in pack design →
See also:
Cell balancing, Depth of discharge (DoD)
Equalising the state of charge of cells connected in series, either by dissipating excess charge from the highest cells or by moving charge between them.
Cells in series see the same current but drift apart because they are not identical. Without balancing, the pack is limited by its weakest group: the BMS must stop charging when the highest cell is full and stop discharging when the lowest is empty, so usable capacity shrinks to the narrowest cell rather than the average. Passive balancing bleeds energy from high cells; active balancing moves it, at the cost of more hardware.
Full entry: Cell balancing in pack design →
See also:
BMS (battery management system), State of charge (SoC)
How cells are connected to reach the required voltage (series, written S) and capacity or current capability (parallel, written P).
A 4S2P pack has four groups in series with two cells in each group: nominal voltage is four times the cell voltage, capacity is twice the cell capacity, and current capability is roughly doubled. Series strings need balancing; parallel groups need cells matched in capacity, impedance and state of charge. The configuration is usually settled by the load voltage and the required runtime, and it determines the BMS channel count.
Full entry: Series and parallel configuration in pack design →
See also:
Cell balancing, BMS (battery management system)
Joining cells to busbars by resistance welding or laser welding rather than soldering, which is the accepted method for cylindrical cell assembly.
Soldering directly to a cell can puts heat into a sealed component that was not designed to receive it, and the joint is difficult to make repeatable at production volume. Resistance and laser welding make the joint in milliseconds with a controlled heat-affected zone. The weld is also an electrical component: a poor joint adds resistance that produces local heating, and it is one of the more common causes of a pack that tests well and fails in the field.
Full entry: Interconnect welding in pack design →
See also:
Series and parallel configuration, Internal resistance
Life and ageing
Life and ageing
How cells age, and which numbers describe it honestly.
The number of charge and discharge cycles a cell completes before its capacity falls below a stated fraction of its original value, usually 80%.
A cycle life figure is only comparable with another when charge rate, discharge rate, depth of discharge and temperature all match. This is the most commonly misused number in battery procurement: a claim measured at 0.5 C and 20% depth of discharge is not comparable with one measured at 1 C and 90%. Ask for the test condition before comparing quotes, and treat any figure quoted without one as marketing.
Full entry: Cycle life in pack design →
See also:
Depth of discharge (DoD), C-rate
The fraction of a cell capacity removed during a discharge, expressed as a percentage.
Depth of discharge and cycle life are inversely related: shallow cycles cost far less ageing per cycle than deep ones. A pack cycled to 20% DoD will complete many more cycles than the same pack cycled to 90%, which is why storage warranty terms are written against a stated DoD. It also means that cycles alone is not a comparable unit of battery life; energy throughput over life is closer to it.
Full entry: Depth of discharge (DoD) in pack design →
See also:
Cycle life, State of charge (SoC)
The remaining charge in a cell at a given moment, expressed as a percentage of its current usable capacity.
SoC is a relative measure, so it depends on the capacity figure it is measured against, and that figure falls as the cell ages. It is also not directly measurable; it is estimated from voltage, current integration and temperature, with the estimate drifting between full-charge references. Peak power capability varies strongly with SoC, which is why power figures for high-drain cells are quoted at a stated SoC.
Full entry: State of charge (SoC) in pack design →
See also:
Depth of discharge (DoD), State of health (SoH)
The current condition of a cell expressed as a percentage of its original capability, usually capacity, sometimes resistance.
SoH is the number that decides whether a pack is still fit for its application. Capacity-based SoH captures how much energy the cell holds; resistance-based SoH captures how well it can still deliver current. A cell can pass a capacity test and still fail a power application because its impedance has grown. This is why impedance growth over cycling matters as much as the cycle count itself.
Full entry: State of health (SoH) in pack design →
See also:
Internal resistance, Cycle life
Ageing that occurs with time rather than with use, driven mainly by temperature and state of charge during storage.
A cell that is never cycled still ages. Storage at high temperature and high state of charge accelerates it; cool storage at moderate SoC slows it. For stationary storage and backup applications, where the pack may sit near full charge for most of its life, calendar ageing can dominate over cycle ageing. This is why storage warranty terms specify both a cycle count and a calendar period.
Full entry: Calendar life in pack design →
See also:
State of charge (SoC), Cycle life
Safety and compliance
Safety and compliance
Transport, safety and what the documents actually cover.
The transport test regime lithium cells and batteries must pass before they can be moved by air or sea, covering eight tests including altitude, thermal, vibration, shock and external short circuit.
UN38.3 is about transport, not about product safety in use. What a buyer normally receives is the test summary rather than the full laboratory report, and that summary is the document a carrier or forwarder will ask for. It is model-specific, so a summary covering one model does not cover a sibling model that was added to the range later.
Full entry: UN38.3 in pack design →
See also:
IEC 62133, MSDS / SDS
The international safety standard for sealed portable secondary lithium cells and batteries, with part 2 covering lithium systems.
IEC 62133 is the reference for portable applications: devices, tools, instruments. Where the end use is industrial or stationary rather than portable, IEC 62619 is normally the applicable standard instead. Both are model-scoped: the certificate lists what it covers, and the check is whether the model being purchased appears in that list.
Full entry: IEC 62133 in pack design →
See also:
IEC 62619, UN38.3
The international safety standard for secondary lithium cells and batteries used in industrial applications, including stationary energy storage.
If the pack is going into an energy storage system, a UPS or fixed industrial equipment, IEC 62619 is usually the standard that applies rather than IEC 62133. It addresses the failure modes relevant to large stationary installations, including thermal propagation between cells. As with any certificate, the coverage list is the part that matters, not the existence of the certificate.
Full entry: IEC 62619 in pack design →
See also:
IEC 62133, Thermal runaway
The North American standard covering lithium cells, with UL 2054 covering battery packs assembled from them.
Recognition is issued under a file number that lists the models covered. The useful question is therefore not whether a supplier has a UL certificate but whether the specific model being purchased appears in that file. A supplier holding a UL file is not the same claim as a given cell being listed in it.
Full entry: UL 1642 in pack design →
See also:
IEC 62133, UN38.3
A self-sustaining internal heating process in which a cell generates heat faster than it can dissipate it, leading to venting, fire or propagation to adjacent cells.
Thermal runaway is triggered by internal short, overcharge, mechanical damage or external heat, and once started it cannot be stopped by cutting the current. The design response is layered: prevent the triggers with a correctly configured BMS, prevent propagation with thermal barriers or spacing between cells, and manage the consequences with venting paths and enclosure design. Chemistry choice matters here, which is part of why LFP is preferred for stationary storage.
Full entry: Thermal runaway in pack design →
See also:
BMS (battery management system), IEC 62619
The safety data sheet describing the chemical hazards of a cell, its handling requirements and the response to fire, leak or exposure.
The SDS travels with the shipment and is what a warehouse, a carrier and a fire service will ask for. It has to match the actual shipping configuration: cells, batteries and batteries installed in equipment are classified differently and the documentation differs accordingly. An SDS that describes a different configuration is worse than none, because it sends the responder to the wrong procedure.
Full entry: MSDS / SDS in pack design →
See also:
UN38.3, Thermal runaway
