Cylindrical lithium cells are named after their dimensions in millimetres. An 18650 is 18 mm across and 65 mm long; a 21700 is 21 mm across and 70 mm long. That extra 3 mm of diameter sounds trivial and is not — it means roughly 45% more internal volume, and it has reshaped what light EV packs look like.
The specifications that matter
| Property | 18650 | 21700 |
|---|---|---|
| Capacity | 2500–3600 mAh | 4000–5000 mAh |
| Weight | 45–48 g | 66–70 g |
| Energy density | 200–230 Wh/kg | 240–265 Wh/kg |
| Continuous current | 10–30 A | 15–45 A |
| Cells for 500 Wh at 13S | 52 (13S4P) | 39 (13S3P) |
| Cost per Wh | Slightly higher | Slightly lower |
The headline is energy density. A 21700 pack of the same capacity is typically 8–12% lighter and physically more compact, with fewer welds and fewer failure points. For a scooter, where every kilogram is carried and lifted onto trains, that matters.
Where 18650 still wins
- Shape flexibility. Thinner cells fit awkward deck cavities and slim frames that 21700s simply will not enter.
- Availability of high-drain options. The 18650 catalogue is enormous and mature, with well-characterised 20–30 A cells.
- Replacement and repair. Most existing scooter packs are 18650-based, so rebuilding an old pack usually means staying with the format.
- Price at the low end. Budget 18650s are cheap, though this is also where recycled and re-wrapped cells hide.
The cells people actually build with
| Cell | Format | Capacity | Continuous | Best for |
|---|---|---|---|---|
| Samsung INR18650-30Q | 18650 | 3000 mAh | 15 A | The sensible default |
| Samsung INR18650-25R | 18650 | 2500 mAh | 20 A | Higher current, less range |
| LG INR18650-MJ1 | 18650 | 3500 mAh | 10 A | Maximum range, gentle loads |
| Molicel INR21700-P42A | 21700 | 4200 mAh | 45 A | Performance builds |
| Samsung INR21700-50S | 21700 | 5000 mAh | 25 A | Long range with real current |
| LG INR21700-M50LT | 21700 | 5000 mAh | 14.6 A | Long range, cold tolerance |
Sizing the pack around your controller
Series count sets voltage; parallel count sets capacity and current. Pack continuous current is the per-cell rating times the parallel count, so a 4P pack of 30Q cells delivers 60 A — comfortably above what most 48 V controllers ask for. A 2P pack of the same cells gives 30 A, which a 1500 W controller will push against on every hill.
Design so that sustained draw sits below about 60% of the pack's rated maximum. Cells run near their limit get hot, and heat is the fastest route to capacity fade. Run your intended configuration through the pack calculator to see voltage, watt-hours, current headroom, weight and cost together before you order anything.
A worked comparison
Target: a 60 V scooter pack of around 1000 Wh, with at least 45 A continuous.
- 16S4P of 30Q (18650): 57.6 V nominal, 12 Ah, 691 Wh, 60 A continuous, 64 cells, about 3.1 kg of cells. Falls short on energy.
- 16S6P of 30Q (18650): 57.6 V, 18 Ah, 1037 Wh, 90 A, 96 cells, about 4.6 kg. Hits the target with a lot of welding.
- 16S4P of P42A (21700): 57.6 V, 16.8 Ah, 968 Wh, 180 A, 64 cells, about 4.5 kg. Same weight, a third fewer cells, three times the current headroom.
That is the modern case for 21700 in one table: fewer cells, fewer welds, more current headroom, same mass. It is why nearly every performance scooter released in the last few years uses them.
Buying without getting burned
- Buy from vendors who publish batch and IR data. Matched internal resistance matters more than brand loyalty.
- Reject unbranded and re-wrapped cells. "Fire" and "UltraFire" wrappers are not manufacturers; the cells inside are salvage.
- Check every cell before assembly. Voltage within 0.02 V across the batch, capacity within 2%, no dents or corrosion.
- Spot-weld, never solder. Soldering dumps heat into the cell can and damages the internals near the terminal.
- Fuse parallel groups where you can. A single shorted cell should not be able to take the pack with it.
A pack is only as good as its worst cell — which is why matched cells from a known batch beat a bargain every time.
One last thing that is easy to forget: size the wiring for the current the pack can now deliver, not the current the old pack delivered. A 180 A capable pack behind 12 AWG wire is a fire waiting for a hill. The wire gauge calculator takes thirty seconds and settles it.