18650 vs 21700 vs AA: Which Flashlight Battery

The numbers are just dimensions in millimetres. An 18650 cell is 18mm across and 65mm long; a 21700 is 21mm by 70mm. That extra few millimetres is worth a great deal: a good 18650 holds around 3,500mAh while a good 21700 holds 5,000mAh, and the larger cell can also deliver more current without sagging, which means higher sustained output rather than just longer runtime.

21700 has become the default for full-size lights because it buys both at once. The cost is bulk — a 21700 light does not disappear into a pocket the way a slim 18650 light does. 18650 remains the most versatile format simply because two decades of lights were built around it and the cell market is enormous.

Two details trip people up. Protected cells have a small circuit at the positive end that cuts power on over-discharge, which makes them a few millimetres longer and sometimes too long for a given light. And button-top versus flat-top matters: some lights need the physical bump of a button top to make contact, and a flat-top cell in one of those simply will not work.

An AA light runs about 1.5V from an alkaline or NiMH cell, against 3.6V nominal from lithium-ion, and holds far less energy. The practical consequence is a much dimmer light with a shorter runtime. What you buy in exchange is genuine: you can find AA cells in any petrol station on earth, they do not care about being stored flat, and they are not a fire risk.

Many AA lights also accept a 14500, which is a lithium-ion cell in the AA form factor at 3.7V. This typically doubles or triples output — but only in lights explicitly rated for it. Putting a 14500 in a light designed only for AA can destroy the driver immediately, and the light's manual is the only authority on this.

AAA lights are a different proposition again. Nobody chooses one for performance; they are chosen because they weigh nothing on a keyring. As an emergency light that is always with you, a good AAA torch beats an excellent 21700 light sitting at home.

CR123A is worth knowing about mainly for cold and storage. It is a 3V lithium primary — not rechargeable — with an exceptionally long shelf life and good performance well below freezing, which is why it persists in duty and emergency lights. The rechargeable 16340 is the same size but not always a safe substitute, for the same voltage reasons as the 14500.

Built-in batteries have taken over the small-EDC end of the market, and for good reason: the light can be slimmer, sealing is easier, and charging is a cable away. The tradeoff is a hard lifespan. Lithium-ion loses meaningful capacity after a few hundred cycles, and when a sealed light's cell is worn out the whole light is usually finished.

A useful middle ground is a light with a removable cell and built-in USB-C charging. You get the convenience of charging in place, and when the cell degrades in three years you spend a few pounds instead of replacing the light. Where a light offers this, it is generally the better buy.

One thing to check on any USB-C light is whether the port does anything useful besides charge. Several lights now also act as a power bank for a phone, which is genuinely handy on a long trip, and a few support pass-through use while charging.

Lithium-ion cells store a lot of energy in a small space and fail badly when abused. The rules that matter are few. Buy cells from a reputable vendor, because counterfeit high-capacity cells are widespread and are often low-capacity cells in a printed wrapper. Never carry a bare cell loose in a pocket with keys or coins, where a short across the terminals can start a fire — use a plastic case.

Inspect the wrapper. A nick that exposes bare metal near the positive end can short against the body tube, and re-wrapping a cell is a cheap and standard fix. Don't mix cells of different ages or brands in a multi-cell light, because the weakest cell gets driven into over-discharge by the others. And charge on a hard, non-flammable surface rather than a bed or sofa.

Most enthusiast lights have low-voltage protection built into the driver, which handles the most common risk. Treated sensibly, these cells are unremarkable — but they are the one part of the hobby where cutting corners has consequences beyond a disappointing beam.