Two battery packs can share a voltage, a capacity, a chemistry and a price, and still be built from cells of completely different physical construction. That choice shapes how the pack behaves in service more than most buyers realise, and it is almost never mentioned on a quotation.
There are three formats in commercial use. This page explains what each one is, how it fails, and which applications suit it — because format is one of the few remaining places where a supplier can make a genuinely different engineering decision.
Cylindrical cells
The oldest and most recognisable format: electrode layers wound into a spiral and sealed in a rigid metal can. The numbering describes the dimensions in millimetres, so an 18650 is 18 mm across and 65 mm long, a 21700 is 21 mm by 70 mm, and a 32700 is 32 mm by 70 mm.
| Format | Typical chemistry | Typical capacity | Commonly found in |
|---|---|---|---|
| 18650 | NMC | 2.0–3.5 Ah | Laptops, power tools, older EV packs |
| 21700 | NMC | 4.0–5.0 Ah | Modern EV packs, premium two-wheelers |
| 26650 | LFP | 3.0–3.4 Ah | Light EV packs, storage |
| 32700 | LFP | 5.5–6.5 Ah | E-rickshaw packs, solar storage |
The rigid can is the defining feature. It contains the pressure that builds inside a cell as it ages, so cylindrical cells do not swell — they hold their shape from the day they are built to the day they fail. They are also made in enormous volume on highly automated lines, which makes them cheap, consistent and easy to source.
The cost is packing efficiency. Stack cylinders and you leave gaps; roughly 9% of the volume is unavoidably wasted no matter how you arrange them. In a space-constrained pack that matters.
The bigger consequence is the number of connections. Building a 100 Ah 19S pack from 32700 cells at 6 Ah each needs about 17 cells in parallel per series group — 19 × 17 = 323 cells, each requiring two welded joints. That is well over 600 connections, and every one of them is a place where resistance can develop.
A joint with a few extra milliohms of resistance dissipates heat every time current passes, which degrades the joint further, which raises resistance again. Most packs that fail early fail at a connection rather than in a cell. More cells means more chances, which is why cylindrical construction demands genuinely good welding rather than adequate welding.
Prismatic cells
Electrodes either stacked flat or wound and pressed into a rectangular shape, sealed in a rigid aluminium or steel case. Individual cells are large — 50 to 300 Ah is typical — with threaded terminals for bolted busbars.
The consequence of that size is a dramatic simplification. That same 100 Ah 19S pack needs nineteen prismatic cells, not 323. Nineteen cells, 38 bolted connections, and every single cell individually monitored by the BMS.
| 32700 cylindrical (6 Ah) | Prismatic (100 Ah) | |
|---|---|---|
| Cells required | ~323 | 19 |
| Connections | 600+ welded | 38 bolted |
| BMS monitoring | 19 parallel groups | 19 individual cells |
| A single weak cell is | Hidden inside a group of 17 | Directly visible |
| Repairable? | Difficult — welded groups | Yes — unbolt and replace |
| Packing efficiency | Lower | Higher |
Rows four and five are why we use prismatic LFP cells for traction packs. In a parallel group of seventeen cylindrical cells, the BMS sees the group as one unit — a single failing cell inside it is invisible until the whole group starts to sag, by which point the neighbours have been carrying it for months and have aged accordingly. With one cell per series position, a drifting cell shows up in the cell voltage spread long before it affects range, which is exactly the early warning our BMS guide tells you to watch for.
Repairability follows from the same thing. A pack with one bad cell out of nineteen is an afternoon’s work: unbolt, replace, rebalance. The same fault in a welded cylindrical pack means cutting into a nickel-strip assembly, and most suppliers will quote you a new pack instead.
Prismatic cells do swell. Aluminium cases flex under internal pressure as cells age, which is why a properly built prismatic pack compresses the stack between end plates. A prismatic pack assembled without compression is a pack whose cells will gradually push each other apart.
Pouch cells
No rigid case at all — electrodes sealed in a laminated aluminium foil envelope, with flat metal tabs for connection. Lightest of the three, best packing efficiency, and cheapest to manufacture at scale.
They are also the least forgiving. With no rigid container, a pouch cell must be supported by the pack structure. Every pouch cell needs a frame, controlled compression, and protection from anything sharp. Get any of that wrong and the failure modes are unpleasant: the foil punctures easily, and the cells swell visibly as they age.
Swelling is not a defect in a pouch cell; it is expected behaviour that the pack must accommodate by design. A pouch pack built without room to swell will deform its own enclosure.
The format dominates consumer electronics, where the pack is designed around the cells by the same company. It appears in vehicles where weight is critical and the manufacturer controls the whole assembly. It is a poor fit for the replacement and custom-build market, which is why we do not use it.
How cells get joined, and why it decides pack life
Format determines how many connections a pack has. How those connections are made determines whether the pack survives.
The physics is unforgiving. A joint with even a few milliohms of excess resistance dissipates heat every time current passes through it. That heat oxidises and degrades the joint, which raises resistance, which produces more heat. The process accelerates, and it ends at a joint hot enough to melt insulation. Most packs that fail early fail here rather than in a cell.
| Method | Used with | Strength | Weakness |
|---|---|---|---|
| Spot welding, nickel strip | Cylindrical | Fast, cheap, no heat into the cell | Weld quality varies; strip limits current capacity |
| Laser welding | Cylindrical, prismatic tabs | Consistent, low resistance | Capital equipment; needs process control |
| Bolted busbar | Prismatic | Serviceable, high current capacity | Loosens with vibration if not torqued and locked |
| Soldering | Sometimes used on cylindrical | Requires no special equipment | Heats the cell — genuinely damaging; avoid |
| Ultrasonic welding | Pouch tabs | No thermal damage to the cell | Specialised equipment |
Two rows deserve comment. Soldering onto a cylindrical cell puts sustained heat into the cell body, which damages the internal separator near the terminal. It is done because a soldering iron is cheap and a spot welder is not, and it is a reliable indicator of a pack you should not buy. If someone offers you a hand-assembled pack, ask how the cells were joined.
Bolted busbars are the reason a prismatic pack is serviceable, but they carry an obligation: they must be torqued to specification and secured against loosening. Vibration on a vehicle works fasteners loose over months, and a loosening busbar follows exactly the heating spiral described above. This is a real maintenance item on a traction pack, and one worth asking about.
What actually changed in industrial packs
Twenty years ago, industrial equipment that needed portable power used lead-acid, and the few lithium applications were built from small cylindrical cells borrowed from the laptop industry — because those were the only cells made in volume.
Three things shifted that, and none of them was a chemistry breakthrough.
Large-format prismatic cells became available. Once cells were manufactured at 100 Ah and above, an industrial pack stopped being an assembly of hundreds of small parts and became an assembly of a dozen large ones. That single change made per-cell monitoring practical, made packs repairable, and removed most of the connection count that had been the dominant failure mode.
LFP moved from niche to default. Better thermal tolerance and two to three times the cycle life mattered more to industrial buyers than energy density ever did, because industrial equipment is rarely weight-constrained and always life-constrained.
BMS capability caught up. Early packs had protection boards. Modern ones have controllers that count coulombs, log history and talk to the machine over CAN or Modbus — which is what allows a machine to slow down as its pack depletes rather than simply stopping. We cover specifying that in our post on industrial battery pack selection.
How each format fails
This is the comparison that matters most, and it rarely appears in a specification.
| Cylindrical | Prismatic | Pouch | |
|---|---|---|---|
| Ageing | Capacity fade, no shape change | Capacity fade, some swelling | Capacity fade with visible swelling |
| Pressure relief | Engineered vent in the can | Engineered vent in the case | Seam splits, unpredictably |
| Mechanical damage | Very resistant | Resistant | Vulnerable |
| Thermal propagation | Small cells limit energy per event; many neighbours | Larger energy per cell; fewer neighbours, easier to space | Hardest to contain |
| Early warning | Poor — masked within parallel groups | Good — per-cell visibility | Visual, once swelling starts |
The thermal propagation row connects directly to certification. India’s traction battery testing includes forcing one cell into runaway and requiring that it not cascade through the pack — and format changes how you engineer for that. Fewer, larger cells with deliberate spacing and defined venting paths is a more tractable design problem than several hundred small cells packed tightly together. Our guide to Indian battery safety standards covers what that testing involves.
Choosing a format
| Application | Format | Reason |
|---|---|---|
| E-rickshaw, e-loader traction | Prismatic LFP | Per-cell monitoring, repairable, fewer connections, easier thermal design |
| Solar and stationary storage | Prismatic LFP | Weight irrelevant; cycle life and serviceability decisive |
| Two-wheeler | Prismatic or cylindrical LFP | Depends on the shape of the space available |
| Awkwardly shaped enclosures | Cylindrical | Small cells fill irregular volumes prismatic cannot |
| High power density, small pack | Cylindrical NMC | Best available power-to-size in a rigid can |
| Weight-critical, integrated design | Pouch | Only where the pack is engineered around the cells |
Note that this decision sits alongside, not instead of, the chemistry decision. Format is construction; chemistry is what is inside. You can have prismatic LFP or prismatic NMC, cylindrical LFP or cylindrical NMC. Our LFP vs NMC comparison covers the other axis.
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A note on reclaimed cells
Format has one more consequence worth knowing, because it interacts with the cheapest packs on the market.
Cylindrical cells are the easiest to recover from scrapped batteries and resell, which makes them the format most commonly encountered as reclaimed stock. A pack built from salvaged 18650s can test acceptably on day one and cost 30–40% less than one built from new cells.
What you cannot see is the spread. Reclaimed cells have unknown and differing histories, so their capacities vary far more than new cells from a single batch — and capacity spread is precisely the condition that defeats cell balancing. The pack drifts out of balance faster than the BMS can correct, and loses usable capacity within eighteen months while no individual cell has actually failed.
This is not a format problem as such. But if you are quoted a cylindrical pack at a price that seems too good, cell provenance is the first thing to ask about.
Questions worth asking a supplier
What format and what cell, by model number?
“Lithium” is not an answer. A model number identifies format, chemistry, capacity and manufacturer in one go, and a supplier who will not give you one is telling you something.
How many cells in parallel per series group?
This determines whether your BMS can see individual cells or only groups. One is far better than seventeen for early fault detection.
Welded or bolted connections?
Both work when done properly. The follow-up question is what quality control applies to whichever they use.
Is the stack compressed?
Prismatic and pouch cells need it. If the answer is confusion, the pack was assembled rather than designed.
If one cell fails in year two, what happens?
The most revealing question on this page. A prismatic pack from a manufacturer is a repair. Almost anything else is a replacement.
Format is not the most important decision in a battery pack — chemistry and BMS quality both matter more. But it is the decision that quietly determines whether your pack can be diagnosed and repaired three years from now, or whether it is a sealed box that gets thrown away.