Fenix Batteries Guide: Choosing the Right Battery for Your Flashlight
Flashlight batteries are named by size, and the names look like code. A 18650, a 21700, a 14500 and a 16340 are all rechargeable lithium-ion cells, but they are not interchangeable, and some of them look almost identical to batteries you already know, like AA and CR123A. This guide explains what each size is, how they compare, what the mAh number on the label means, and which Fenix battery fits which kind of light.
What the Battery Numbers Mean
Most cylindrical lithium-ion cells are named for their dimensions. The first two digits are the diameter in millimeters, and the last three are the length in tenths of a millimeter. So an 18650 is about 18 mm wide and 65 mm long, and a 14500 is about 14 mm wide and 50 mm long.
That is the nominal size. Cells with a built-in protection circuit are a little bigger, which is why Fenix's own batteries measure slightly larger than the name suggests.
| Battery | Nominal size | Fenix cell size | Rechargeable? |
|---|---|---|---|
| 14500 | 14 × 50 mm | 14.5 × 50 mm (ARB-L14-1600U) | Yes |
| 16340 | 16 × 34 mm | See each product page | Yes |
| 18650 | 18 × 65 mm | 18.6 × 69 mm, or 18.6 × 70 mm for the USB "U" series | Yes |
| 21700 | 21 × 70 mm | 21.5 × 76 mm | Yes |
| CR123A | About 17 × 34.5 mm | Not a Fenix cell | No (primary lithium) |
The Fenix sizes come from Fenix's own battery specifications. Because lights are built around the protected-cell length, always buy the battery your light's product page lists rather than going by the nominal size alone.
18650 vs 21700
These are the two cells that power most full-size rechargeable flashlights. The 21700 is wider and longer, so it holds more energy, but a 21700 will not fit in a light built for an 18650.
- 18650: the most common flashlight battery. Fenix makes it in capacities from 2,600 mAh up to the 4,000 mAh ARB-L18-4000U. A popular pick is the ARB-L18-3500U, which ships with the Fenix UC35 V2.0.
- 21700: more capacity in a larger cell. The ARB-L21-5000U is rated at 5,000 mAh, and the ARB-L21-6000 goes up to 6,000 mAh.
If you own a light built for a 21700 and only have 18650 cells, the Fenix ALF-18 battery adapter is a sleeve that makes an 18650 the size of a 21700. It does not work the other way around, because a 21700 is physically too big for an 18650 light. For a deeper comparison, including adapter advice, read Are 21700 Batteries Better Than 18650?
14500 vs AA
A 14500 is the same size as a standard AA battery, but it is a different battery. Many 14500 cells are lithium-ion at about 3.6 V, such as the ARB-L14-1050, while an alkaline or NiMH AA is 1.5 V or 1.2 V. A light designed for 1.5 V can be damaged by a 3.6 V cell.
The safe rule is to check your light's product page. For example, the Fenix E12 V3 runs on one AA and specifically does not allow a rechargeable 14500 Li-ion battery. Other Fenix lights are built to accept a 14500 and say so on their listing.
Fenix also makes a 1.5 V AA-size rechargeable designed as an alternative to a standard AA: the ARB-L14-1600U, which has a built-in USB charging port. Whether a standard AA charger can charge a 14500 depends on the charger. The ARB-L14-1600U is meant to be charged through its own port, not in a separate charger.
16340 vs CR123A
A 16340 and a CR123A are close in size and shape, but they are not the same battery. The CR123A is a non-rechargeable lithium cell at 3 V. The 16340 is a rechargeable lithium-ion cell at about 3.6 V, such as the ARB-L16-800UP with USB-C charging.
Some Fenix lights accept both, some accept only one, and a 16340 does not make a CR123A light brighter by default. In some lights the higher voltage can work fine, run poorly, or damage the electronics. Check the light's listing or manual before you substitute. If you want a non-rechargeable spare, the Tenergy Propel CR123A 4-pack is a CR123A option.
What mAh Means
mAh stands for milliamp-hours. It is a measure of how much charge a battery holds, so a higher number generally means a longer runtime on the same light. Compare mAh between batteries of the same size and voltage, like two 18650 cells.
The gain is not one-to-one. A 3,500 mAh 18650 holds up to about 35 percent more than a 2,600 mAh one (3,500 divided by 2,600), but your actual runtime gain depends on the light and the mode you use.
mAh also does not tell you the total energy across different cell sizes, because voltage matters too. Multiply volts by amp-hours to get watt-hours. A 5,000 mAh 21700 at 3.6 V is 18 Wh. Watt-hours are the number airlines use for batteries, which is covered in our guide to taking a flashlight on a plane.
Button Top vs Flat Top
The positive end of a cell is either flat or has a raised button. Older Fenix lights used flat-top cells, but many current models accept only button-top cells. If you are buying a battery for a current Fenix light, choose a button top unless the light's listing says otherwise.
Other Battery Types
- AA: easy to find anywhere. The Fenix LD22 V2.0 runs on two AA batteries as a backup to its included rechargeable battery; its 800-lumen Turbo mode works only with the rechargeable battery. Browse AA flashlights.
- AAA: common in keychain lights and small penlights, like the Fenix LD02 V2.0. Browse AAA flashlights.
Frequently Asked Questions
Can I use a 21700 battery in a light made for an 18650?
No. A 21700 is wider and longer than an 18650 and will not fit. You can go the other way with the Fenix ALF-18 adapter, which lets an 18650 work in a 21700 light.
Can I use a 14500 battery instead of AA?
Only if your light says it accepts a 14500. A 14500 is the same size as an AA but usually runs at a higher voltage, and some lights, like the Fenix E12 V3, do not allow it.
Can I use a 16340 in place of a CR123A?
It depends on the light. They are similar in size and shape, but they are different batteries, so check your light's listing or manual first.
Is a higher mAh battery always better?
A higher mAh gives a longer runtime in the same light, as long as the battery is the size and type the light accepts. For most lights, choose the highest capacity in the correct size.
For charging, storage and care, read our lithium-ion battery safety guide. To see every option, browse the flashlight batteries collection, or contact our customer support team if you are not sure which battery your light takes.
