21700 battery reference
The 21700 is a 21 mm × 70 mm lithium-ion rechargeable cell. It is physically larger than the 18650 and offers 15–50% more capacity per cell. Not interchangeable with 18650 without an adapter sleeve.
Specifications
| Property | Value |
|---|---|
| Diameter | 21.0 mm |
| Length | 70.0 mm (unprotected); up to 74 mm (protected) |
| Nominal voltage | 3.6–3.7 V |
| Fully charged voltage | 4.2 V |
| Minimum discharge voltage | 2.5–3.0 V |
| Typical capacity | 3000–5000 mAh |
| Chemistry | Lithium-ion (Li-NMC, Li-NCA, LiFePO4) |
| Cycle life | 300–500+ cycles to 80% capacity |
21700 vs 18650 at a glance
| Spec | 21700 | 18650 |
|---|---|---|
| Diameter | 21 mm | 18.6 mm |
| Length | 70 mm | 65 mm |
| Capacity | 3000–5000 mAh | 1800–3500 mAh |
| Energy per cell | ~11–18.5 Wh | ~6.5–13 Wh |
| Device support | Newer high-performance devices | Very wide — majority of devices |
Common devices using 21700
- High-performance flashlights (Olight Warrior 3, Fenix E35R, others)
- Tesla Model 3 and Model Y battery packs (2170 cell)
- High-end vaping mods
- Some cordless power tools
Interchangeability with 18650
21700 cells do not fit in 18650 device slots without an adapter. Some devices (notably Olight and Nitecore flashlights) ship with adapter sleeves specifically to allow using 18650 cells in a 21700 device for backward compatibility. The reverse — 21700 in an 18650 slot — is not possible without modification.
Safety notes
Same as 18650: charge only with Li-ion charger, do not short, do not charge damaged cells, recycle at battery drop-off. Never exceed the maximum continuous discharge rating (CDR) of the cell.