18650 vs 21700 vs AA: Flashlight Batteries Explained

Quick answer: An 18650 is a rechargeable 3.6V lithium-ion cell holding about 2,500 to 3,500 mAh, and a 21700 is a larger 3.6V cell holding about 4,000 to 5,000 mAh, which is why the brightest pocket lights use one. AA and AAA are the 1.5V (or 1.2V rechargeable) sizes you can buy anywhere. For a daily flashlight, a 21700 or 18650 gives the most runtime; for an emergency light that sits in a drawer, lithium AA or CR123A cells last a decade on the shelf and do not leak.

Flashlight spec sheets assume you already know what a 14500 is. Most people do not, and the wrong guess can range from disappointing runtime to a dead light. The number names are simpler than they look: the first two digits are the diameter in millimeters, the next two are the length, and a trailing zero means a round cell. An 18650 is 18 mm by 65 mm. A 21700 is 21 mm by 70 mm. A 14500 is 14 mm by 50 mm, which is the size of an AA.

If you are shopping for a light and not just batteries, our best EDC flashlights of 2026 lists the battery each pick uses, and how many lumens you need explains why battery size and sustained brightness are tied together.

Flashlight battery sizes compared
Battery	Size (mm)	Chemistry	Nominal voltage	Typical capacity	Rechargeable?
21700	21 x 70	Lithium-ion	3.6 to 3.7V	4,000 to 5,000 mAh	Yes
18650	18 x 65	Lithium-ion	3.6 to 3.7V	2,500 to 3,500 mAh	Yes
16340 (RCR123A)	16 x 34	Lithium-ion	3.6 to 3.7V	About 550 to 800 mAh	Yes
CR123A	17 x 34.5	Lithium primary	3V	About 1,500 mAh	No
14500	14 x 50	Lithium-ion	3.6 to 3.7V	About 800 to 1,000 mAh	Yes
AA	14 x 50	Alkaline, NiMH or lithium primary	1.5V (NiMH 1.2V)	About 2,000 to 3,000 mAh	NiMH only
AAA	10 x 44	Alkaline, NiMH or lithium primary	1.5V (NiMH 1.2V)	About 800 to 1,200 mAh	NiMH only

Why mAh alone is misleading

Capacity in milliamp-hours only compares batteries at the same voltage. To compare across sizes, multiply by voltage to get watt-hours, the actual stored energy. A 3,000 mAh 18650 at 3.6V holds about 10.8 Wh. A 5,000 mAh 21700 holds about 18 Wh. A 2,000 mAh NiMH AA at 1.2V holds about 2.4 Wh. So one good 21700 carries roughly as much energy as seven rechargeable AAs, in a package about the size of two.

That gap is why the table in our EDC flashlight roundup splits so cleanly: lights that sustain 1,000 lumens or more run on 18650 or 21700 cells, while AA and AAA lights top out at a few hundred sustained lumens. High output needs both energy and current, and lithium-ion cells deliver both.

Be skeptical of capacity claims. No 18650 on the market holds 9,900 mAh, whatever an online listing says. The real ceiling for 18650s is around 3,500 mAh, and for 21700s it is around 5,000 mAh. Cells from Molicel, Samsung, Sony (Murata) and LG, or brand-wrapped cells from flashlight makers such as Nitecore, Fenix and Olight, are the safe bet.

18650 vs 21700: which is better for a flashlight?

The 21700 wins on energy, about 40 to 50 percent more than an 18650, and on sustained current, which lets a light hold high output longer before stepping down. The 18650 wins on size. A 21700 light is a few millimeters fatter and longer, which is noticeable in a pants pocket. For a jacket or bag light, go 21700. For a light that rides clipped in a jeans pocket all day, an 18650 or smaller is easier to live with.

The two are not interchangeable. Some 21700 lights ship with a sleeve adapter so an 18650 can fit in a pinch, but never assume a light takes both unless the manual says so.

What about 14500, 16340 and CR123A?

These are the small-light sizes. A 14500 is a lithium-ion cell shaped exactly like an AA, but at 3.6V instead of 1.5V. In a light designed for both, such as many dual-fuel EDC lights, it roughly doubles or triples brightness. In a light designed only for AA, it can burn out the LED or driver instantly. Check the manual before you try it.

A CR123A is a 3V lithium primary cell, the long-time standard for tactical lights. It is not rechargeable, it keeps most of its charge for about 10 years, and it works in deep cold. A 16340 (also sold as RCR123A) is the rechargeable lithium-ion version of the same size, but it runs at 3.6V to 4.2V instead of 3V, and it holds far less energy. Many lights accept both; some older incandescent and low-voltage lights do not.

Alkaline, NiMH or lithium AA?

For AA and AAA lights, chemistry matters more than brand.

Alkaline is cheap and everywhere, but its capacity collapses under high drain, and it leaks potassium hydroxide as it ages. A leaked alkaline cell is the single most common way a good flashlight gets ruined. Do not leave alkalines in a light you store for months.
NiMH rechargeable cells run at 1.2V, handle high drain well, and do not leak like alkalines. Low self-discharge types such as Panasonic eneloop pro (2,550 mAh, up to 500 cycles, about 85 percent charge after a year on the shelf) are the best everyday AA.
Lithium primary AA cells, such as Energizer Ultimate Lithium, are 1.5V, lighter than alkalines, rated to store for up to 20 years, and work in extreme cold. They are expensive, which makes them ideal for emergency kits, glove boxes, and lights you rarely use.

Protected vs unprotected cells

A protected lithium-ion cell has a small circuit board at the negative end that cuts power if the cell is overcharged, over-discharged, or shorted. It adds a few millimeters of length, so a protected 18650 may be about 67 to 70 mm long instead of 65. An unprotected cell has no board and relies on the flashlight and charger to keep it within safe limits.

Which to buy depends on the light. Most modern lights from reputable brands have low-voltage protection built in and specify a cell type in the manual. Some very high-output lights call for unprotected high-drain cells because the protection circuit trips under their current draw. Some compact lights have no room for a protected cell. Also check the tip: button-top cells have a raised positive terminal, flat-top cells do not, and some lights make contact with only one type.

Are built-in USB-C batteries worth it?

Many flashlight brands now sell 18650 and 21700 cells with a USB-C port built into the cell itself. You pull the cell out and plug it into any phone charger. It is the easiest way to live with a lithium-ion light if you do not want a separate charger. The trade-offs: they cost more, they usually hold a little less than a standard cell of the same size because the charging circuit takes up space, and some do not fit lights with tight tubes. Plenty of lights also charge through a port in the body, which makes the question moot.

For loose cells, a smart charger such as the Nitecore UM2 handles 18650, 21700, 14500, 16340 and NiMH AA and AAA in the same two bays, and it shows voltage and charge status on a screen.

Lithium-ion battery safety

Never carry loose lithium-ion cells in a pocket with keys or coins. A short circuit can make a cell vent hot gas and catch fire. Use a plastic battery case.
Replace any cell with a torn or nicked wrapper. The wrapper is the insulation between the positive and negative ends.
Do not mix old and new cells, or different brands, in a multi-cell light.
Charge on a hard surface, with a charger meant for the chemistry, and do not leave damaged or swollen cells charging.
For flights, spare lithium-ion cells go in carry-on bags only, never checked luggage, under FAA rules. Cells installed in a flashlight can travel in either.

The Bottom Line

For a daily light, pick a 21700 if you want maximum runtime and do not mind the size, or an 18650 for a slimmer pocket light. Buy real cells from a known brand, match protected or unprotected to what your manual asks for, and store them in a case. For AA and AAA lights, use NiMH for daily use and lithium primaries for emergency kits, and keep alkalines out of anything you care about. Our EDC flashlight picks list the battery each light takes.

Keep reading: How Many Lumens Do You Need? | The Best EDC Flashlights | USB-C Charger Wattage Explained | The Best Everyday Carry

Frequently Asked Questions

Is a 21700 better than an 18650?

For runtime, yes. A 21700 holds about 4,000 to 5,000 mAh versus 2,500 to 3,500 mAh for an 18650, at the same 3.6V, and it sustains higher current. The 18650 is smaller and lighter, so it makes a slimmer pocket light. The two sizes are not interchangeable unless the light includes an adapter sleeve.

Can I use a 14500 battery instead of an AA?

Only if the flashlight maker says so. A 14500 is the same size as an AA but runs at about 3.6V instead of 1.5V. In a light built for both, it makes the light much brighter. In a light built only for AA, it can destroy the LED or driver the moment you switch it on.

Should I buy protected or unprotected 18650 batteries?

Follow the flashlight manual. Protected cells add a circuit that stops overcharge, over-discharge and short circuits, and are the safer default for lights without built-in protection. Some high-output lights require unprotected high-drain cells, and some compact lights cannot fit the extra length of a protected cell.

What is the difference between CR123A and 16340?

They are the same physical size, but a CR123A is a 3V disposable lithium cell with about 1,500 mAh and a roughly 10-year shelf life, while a 16340 is a rechargeable 3.6V lithium-ion cell with far less capacity. Many lights take both, but check first, because the higher voltage can damage some lights.

Why do alkaline batteries leak in flashlights?

As alkaline cells discharge and age, internal gas pressure can push potassium hydroxide electrolyte out through the seal. It corrodes springs and contacts and can ruin a light. NiMH rechargeables and lithium primary AA cells are much less prone to leaking, so use them in lights you store for long periods.

This post contains affiliate links. If you buy through them, we may earn a small commission at no extra cost to you.