What Is an 18650 Battery? The Number, Explained
Our customer asked:
What is an 18650 battery, and why is it inside almost every rechargeable flashlight?
TL;DR
An 18650 is a rechargeable lithium-ion battery shaped like an oversized AA. The name is its size: 18 mm in diameter, 65 mm long, and the 0 means it is cylindrical. It runs at a nominal 3.6 to 3.7 volts, about two and a half times an AA, and typically holds 2,000 to 3,500 mAh. It is the most common battery in rechargeable flashlights because it packs a lot of energy into a size that still fits in your hand.
I run product innovation at LUXPRO, and the 18650 has shaped more of our flashlight designs than any other single part. Here is what makes it special, and the details that decide whether a given 18650 will work in your light.
What makes the 18650 special
- Energy for its size. One 18650 stores roughly what four to six alkaline AAs do, in a cell that weighs less than two ounces. That is what makes a 1,000-lumen light possible in a body you can clip to a belt.
- It can deliver current. Bright LEDs pull hard. A good 18650 handles high current without its voltage sagging the way alkalines do, so the light stays bright.
- It is standardized and mass-produced. The 18650 went into laptop battery packs for years, and later into early electric car packs. That scale drove the price down and the quality up, and it means a flashlight designer can count on a cell that behaves predictably.
- It is replaceable. In a light with a removable 18650, a worn-out battery does not mean a worn-out flashlight. You swap in a new cell and keep the light.
Not all 18650s are the same
I see this catch people out all the time: two cells can both say 18650 and still not be interchangeable. These are the differences that matter in a flashlight:
Protected vs. unprotected. A protected cell has a small circuit board added to the bottom that cuts power if the cell is over-charged, over-discharged or shorted. The board makes the cell a few millimeters longer. Some lights need protected cells, and some are too short to fit them.
Button top vs. flat top. Some lights need a raised button on the positive end to make contact. A flat-top cell may simply not turn the light on.
Built-in USB-C charging. Some 18650s have a USB-C port in the cell itself, so you can charge it without a separate charger. The charging circuit takes up room inside the can, which makes these cells longer than a standard 65 mm cell.
Our own spare cells are an example. The LUXPRO 18650 two-pack has a built-in USB-C port and measures 2.69 inches tall, about 68 mm. We list it as compatible only with the XP920, LP309, XP918 and XP918v2, because those lights were designed around that length and contact layout. I would rather tell you that up front than have you find out after it arrives. Charging cables are not included in that pack.
18650 vs. other lithium-ion sizes
| Cell | Size | Typical capacity | Where you see it |
|---|---|---|---|
| 14500 | AA size | About 800 mAh | Small EDC lights. 3.7 V, so never put it in a light designed only for AA. |
| 16340 / RCR123A | CR123A size | About 700 mAh | Compact tactical lights |
| 18650 | 18 × 65 mm | 2,000 to 3,500 mAh | Most rechargeable flashlights, including the XP920 and XP713 |
| 21700 | 21 × 70 mm | 4,000 to 5,000 mAh | Newer high-output lights, power tools |
| 26650 | 26 × 65 mm | About 5,000 mAh | Large search lights like our XP715, which uses two |
The way I remember it, the naming system works the same for every size: the first two digits are the diameter in millimeters and the next two or three are the length.
Why we design some lights with 18650s and others with built-in batteries
We use a removable 18650 when we want to give you a replaceable battery, the option to carry a charged spare, and in some designs the choice of a backup power source. The XP920 also runs on two CR123A lithium primaries, which you can find in most sporting goods stores.
A built-in battery lets us seal the light better, shape the body more freely, and keep the price down. Our XP913 takes that route. I do not think either approach is better in every case. If you want a light you can keep running for many years, a removable standard cell is the one to choose.
Handling 18650s safely
- Never carry loose cells in a pocket with keys or coins. Metal across the terminals can short the cell and make it dangerously hot. Use a plastic battery case.
- Do not use a cell with a torn wrapper. The wrapper is the insulation. Tears near the positive end can cause a short.
- Insert it the right way round. Most lights mark the direction. Reversing a lithium-ion cell can damage the driver.
- Flying: spare lithium-ion cells go in carry-on bags, not checked luggage, with their terminals protected. Cells installed in a flashlight can generally travel in the device. Check your airline's current rules.
Frequently Asked Questions
Q: What does 18650 stand for?
Its size: 18 mm diameter, 65 mm length, and 0 for cylindrical.
Q: What batteries are 18650?
Any cylindrical lithium-ion cell of that size. They are sold under many brand names, and older laptop battery packs are full of them. Capacity, protection and terminal style vary, so match the cell to what your light's manual specifies.
Q: Can I use AA batteries instead of an 18650?
Only if the light is designed for both. An AA is a different size and 1.5 volts instead of 3.7. A few lights include an adapter or carrier. Our LP309 takes either 4 AA or one or two 18650s. More: What batteries do flashlights use?
Q: How long does an 18650 last?
From under an hour to several days per charge depending on brightness, and usually three to five years of regular use. Full breakdown with the math: How long does an 18650 battery last?
Q: What is the best rechargeable flashlight that uses an 18650?
I compare ours and competitors' side by side in 10 Best Rechargeable Flashlights.