Every rechargeable flashlight worth owning is built around a lithium-ion cell, and two formats own the market: 18650 and 21700. The numbers are just dimensions — 18mm × 65mm and 21mm × 70mm — but the choice ripples through everything: how the light carries, how long it runs, and how hard it can push. Here's how to pick, without the spec-sheet fog.
The short version
- 18650 — the veteran. Slimmer lights, huge cell selection, decades of proven designs. The right call when pocket comfort wins.
- 21700 — the successor. Roughly 30–50% more energy in a package only 3mm wider. The right call when runtime and output win.
Neither is "better." They're two answers to the same question: how much battery are you willing to carry?
What 3mm actually buys you
A quality 18650 like the Molicel P28A holds about 2800mAh and delivers serious current without breaking a sweat — it's the cell we recommend by default for lights like the Convoy S2+. Its bigger sibling, the Molicel P42A, packs 4200mAh with even higher drain capability. Same chemistry family, same brand, 50% more fuel.
That difference shows up two ways. In an efficient light it means hours of extra runtime. In a hot rod it means the driver can hold turbo longer before the cell sags. Lights built around 21700s — the Convoy H4 headlamp, the SPERAS E21, the L21B — simply have more headroom than their 18650 equivalents.
Picking cells that match the job
18650
- Molicel P28A — the enthusiast default: high drain, honest ratings, works in everything.
- Samsung 25R — the old reliable. 20A continuous, battle-tested for a decade.
- Nitecore 3600mAh USB-C — maximum capacity plus a charge port built into the cell. Ideal for moderate-drain lights and people who hate chargers.
21700
- Molicel P42A — high drain and high capacity at once; the do-everything 21700.
- Samsung 40T — 35A of continuous current for the hardest-driven lights.
- Vapcell P50 — a full 5000mAh when runtime is the whole point.
- Nitecore 5000mAh USB-C — big capacity with onboard USB-C charging.
One rule matters more than brand: match the cell's continuous discharge rating to the light. A high-output light with a low-drain cell gives you voltage sag, heat, and disappointment. When in doubt, buy the high-drain option — it's never wrong, just occasionally overkill.
The supporting cast
The 18650/21700 duopoly has smaller relatives worth knowing. 18350 cells like the Vapcell A11 power shorty conversions — the S2+ 18350 tube being the classic example. 14500 cells (AA-sized) like the Vapcell K10 turn AA lights such as the Convoy T3 into little monsters. And 26650 bricks like the Vapcell K62 exist for lights where size was never a concern. Browse by format: 18650, 21700, 18350, 14500, 26650.
Charging and housekeeping
A proper charger pays for itself in cell lifespan. The Vapcell Q2S-C covers two bays for a starter kit; step up to the Q4 or Q8 as the collection grows. On the road, the SPERAS PD21700 and Yonii PD2 pull double duty as power banks running off your spare cells.
Two cheap quality-of-life items: replacement wraps for any cell whose sleeve is nicked (a torn wrap is a short waiting to happen), and magnetic spacers to make flat tops behave like button tops in picky lights.
The honest recommendation
If you're buying your first serious light in 2026 and want maximum performance, buy the 21700 version and a P42A. If you value a slimmer light, an enormous ecosystem, and cells you can find anywhere, 18650 remains a superb choice — it's what half our own pockets carry. Or do what actually happens to everyone in this hobby: end up with both, and a drawer full of batteries you swear are all necessary.