Every rechargeable flashlight battery has a four or five digit number on it, and almost nobody explains what it means. It is not a model number. It is the size, in millimetres.
The numbers are just measurements
Take an 18650. The first two digits are the diameter: 18mm. The next two are the length: 65mm. The final zero means it is cylindrical. That is the whole code.
Once you know that, the rest fall into place:
- 14500 — 14mm x 50mm (roughly AA sized)
- 16340 — 16mm x 34mm (roughly CR123A sized)
- 18650 — 18mm x 65mm
- 21700 — 21mm x 70mm
- 26650 — 26mm x 65mm
A bigger cell holds more energy. That is the trade you are making every time: runtime against how big the light is in your pocket.
18650 — the default
The 18650 is the most common lithium cell in flashlights, and it is the one to buy if you are not sure. Decades of production mean the cells are cheap, well understood, and available everywhere. Most mid-size tactical and EDC lights are built around one.
Our TrustFire 18650 is 3000mAh, which is a sensible real-world capacity. Be sceptical of any 18650 claiming much beyond 3500mAh — the chemistry does not currently support it, and inflated numbers on cheap cells are the oldest trick in this category.
21700 — more runtime, slightly bigger light
The 21700 is 3mm wider and 5mm longer than an 18650. That sounds trivial. It is not: the extra volume buys a large jump in capacity. Our 21700 is 6000mAh against 3000mAh for the 18650 — double the runtime from a cell most people cannot tell apart at a glance.
This is why most new high-output lights are designed around 21700. If you are buying a light today and runtime matters, it is the format worth choosing.
26650 — for high-drain lights
At 26mm across, the 26650 is a noticeably fat cell, and the lights built for it are chunky. In exchange you get high capacity and the ability to deliver serious current without the cell struggling. Our 26650 is 5000mAh.
These are for long-range throwers and high-output work lights, not pocket carry.
The small ones: 14500, 16340, 10440
These matter mostly because of what they resemble. A 14500 is about the size of an AA. A 16340 is about the size of a CR123A. A 10440 is about the size of an AAA.
Here is the part that catches people out: they are not interchangeable with the batteries they resemble. A 14500 is a 3.7V lithium cell. An AA alkaline is 1.5V. Putting a 14500 into a light designed only for AA can destroy it. Check what your light accepts before you swap.
How to find out what your light takes
In order of reliability:
- The manual or the product spec. It will name the cell size directly.
- The cell already in it. Take it out and read the number printed on the wrapper.
- Measure the battery tube. An 18650 needs about 18.5mm of internal diameter, a 26650 about 26.5mm.
If the light has a sealed, built-in battery and charges over USB-C, none of this applies — there is nothing to swap.
Flat top or button top
Look at the positive end of the cell. A button top has a small raised nub. A flat top is level.
Some lights need the nub to complete the circuit and simply will not work with a flat top. Others take either. If a brand new cell seems dead in your light, this is the first thing to check — it is far more often the cause than a faulty battery.
Protected cells
A protected cell has a small circuit at one end that cuts power on over-discharge, overcharge or short circuit. It adds a couple of millimetres of length and a little cost.
For flashlight use, protected cells are the sensible default. Lithium cells that are run flat or shorted can fail badly, and the protection circuit is cheap insurance. The cells we stock are protected.
Charging them
Lithium cells need a charger built for lithium chemistry and set to the right voltage. Most flashlight cells are 3.6/3.7V and charge to 4.2V. LiFePO4 cells are 3.2V and charge to 3.6V. Charging one at the other's voltage is how cells get damaged.
Our TrustFire TR-006 charger handles 16340, 18650, 25550, 26650 and 26700 and has a switch for lithium-ion or LiFePO4. It does not charge AA, AAA or NiMH — a lithium charger is a different device.
Short version
- The number is the size in millimetres, nothing more
- 18650 if you want the standard, well-supported option
- 21700 if you want roughly double the runtime for a slightly larger light
- 26650 for high-drain, high-output lights
- Never assume a lithium cell can replace the alkaline it resembles
- Check flat top versus button top before deciding a cell is faulty
Browse all sizes in our rechargeable batteries collection, or see the chargers that go with them. Not sure what your light takes? Email Customersupport@brightlightled.shop with the model and we will tell you.
