The 18650 and 21700 are the two most common cylindrical lithium cell formats, with the 21650 sitting between them. Choosing the right format early saves cost and rework at the pack level. Here is how they compare and when to use each.
The formats at a glance
- 18650 — Ø18 × 65 mm. The industry standard: a huge variant range (~1500–4200 mAh in NCM), mature supply and the lowest unit price.
- 21650 — Ø21 × 65 mm. Same height as the 18650 but a wider can. High-rate NCM (Mn) — a niche for power-dense, height-limited designs.
- 21700 — Ø21 × 70 mm. The higher-energy successor: ~2400–6000 mAh across NCM, LMFP and LiFePO4.
Energy and capacity
A 21700 holds roughly 30–50% more energy than an 18650. Fewer cells for the same pack energy means fewer welds, simpler BMS wiring and lower assembly cost — often offsetting the higher per-cell price.
Power and internal resistance
Bigger cans have lower internal resistance and shed heat better, so the 21700 (and the high-rate 21650) generally handle higher continuous current with less temperature rise. For peak-current designs, check the specific model’s discharge rate rather than the format alone.
Pack cost and design
- Fewer cells (21700) → lower assembly labor, fewer interconnects, simpler thermal design.
- More mature ecosystem (18650) → widest choice, easiest second-sourcing, lowest cell price.
Which to choose
- Choose 18650 when cost per cell matters most, you need a specific niche variant, or you are matching an existing design — common in power tools and entry-level e-bikes.
- Choose 21700 when you want higher energy density and lower pack cost at scale — e-bikes, premium tools, drones and storage.
- Choose 21650 when you need high discharge current but pack height is fixed around 65 mm.
Don’t forget chemistry
Format is only half the decision — you also pick the chemistry (NCM for energy/power, LiFePO4 for cycle life and safety). See LiFePO4 vs lead-acid for the storage angle.