Pocket Power: Which EDC Flashlight Battery You Should Actually Carry

Two mix-ups that ruin evenings:

• CR123A is not 16340 / RCR123A. Same neighborhood in size. Different voltage. Some lights take both. Some get weird. Read the spec sheet.
• A 14500 looks like an AA and runs at 3.7 V. A normal AA is 1.5 V. Wrong cell, unhappy light

Length is the tax.

• CR123 / 16340: Shortest common cylinder. Easy clip. Still a tube.
• 18350: Current shorty default. About half an 18650 long, same 18 mm diameter. High output without the cigar.
• AA: Slimmer than 18 mm cells, medium length. Looks like a normal pen light. Gas-station backup is the feature.
• 18650: The long one. A deep-carry clip and a slim head keep it livable. A fat head plus 65 mm of cell is what prints in khakis.
• Built-in USB-C / flat-form: This is why slim lights exist. The cell no longer forces a round tube. Best pocketability if you accept no hot-swap.

If it has to live in a jeans watch pocket or slim slacks, start with 18350, CR123, or built-in. If it lives in a jacket, bag, or pocket module, 18650 is the better daily driver.

FAA/TSA, not “what the agent guesses.”  Remember airlines themselves can have stricter rules so make sure to check with your specific airline. Pack for the written rule.

1. Spare lithium batteries are carry-on only. Loose 18650s, 18350s, CR123As, lithium AAs, and power banks do not go in checked bags. If the gate checks your backpack, pull the spares and keep them with you.
2. Lithium-ion limit: 100 Wh per battery with no airline approval. Every normal EDC cell is far under that. A 3,500 mAh 18650 is about 13 Wh. 101–160 Wh needs airline approval and is capped at two spares. Over 160 Wh is forbidden on passenger aircraft.
3. Lithium metal (CR123A, lithium AA) limit: 2 grams of lithium content per battery. Consumer flashlight cells are built to that limit. Still carry-on as spares, terminals covered.
4. Battery installed in the light: That is a portable electronic device. Carry-on is still the smart move. If it is checked, power it fully off and keep the switch from turning on in a bag.
5. Power banks count as spare batteries. The cable for a built-in light does not change that.
6. Cover the terminals. Hard case, original pack, or tape on the positive end. Bare cells plus keys is a short waiting to happen.

No common EDC flashlight cell is “too big” on watt-hours. The usual failure is loose cells in a checked bag, or flying with a swollen / damaged cell.

A kit that actually works:

1. Daily light on a rechargeable (18650, 18350, or built-in).
2. One matching spare cell or a power bank.
3. One primary-cell backup (CR123 or lithium AA) you do not touch until the rechargeable plan fails.

Pros

• About half the length of an 18650, same 18 mm platform.
• Enough voltage and current for modern high-output emitters.
• You can still hot-swap. The spare cell is small.
• Enthusiast output without the 18650 cigar.

Cons

• Capacity is closer to a CR123 than to an 18650.
• Turbo is a stunt unless you like swapping cells.
• Inflated mAh ratings are common. Treat ~1,200 mAh as the honest ceiling.
• Worse “sit in a drawer” story than a primary lithium.

Best for: Pocket-first EDC, short tactical lights, people who already own a 3.7 V charger.

Pros

• Long shelf life. Correct cell for a light that sits.
• Strong in the cold.
• Voltage stays usable instead of sagging like alkaline.
• One-cell lights stay short.
• Legal as a spare in carry-on when packed right.
• Dual-fuel 18650 lights can drink these when the charger stays home.

Cons

• Not rechargeable. Daily use gets expensive.
• Less total energy than a good 18650.
• 16340 rechargeables in the same size are a voltage trap if the light is not rated for them.
• Harder to find than AA at an airport kiosk.

Best for: Backup lights, glove box, nightstand, travel spare, cold weather, dual-fuel bodies.

Pros

• Available almost anywhere batteries are sold.
• Slimmer than 18 mm cells.
• NiMH is cheap and sane for low/medium output.
• Lithium AA is the grown-up disposable: better cold, better leak resistance, better high drain than alkaline.
• Easy to hand to someone who will not learn 18650 safety.

Cons

• 1.5 V (or 1.2 V NiMH) limits how hard a single-AA light can work.
• Alkaline sags and can leak if left loaded.
• 14500 lookalikes can destroy a 1.5 V-only light.
• Weakest runtime at modern EDC brightness.

Best for: Travel to places without hobby batteries, loaner lights, low-output pocket lights, households that already live on AA.

Pros

• No separate charger ritual. Use the cable you already carry.
• Enables flat / slim bodies round cells do not allow.
• No button-top vs. flat-top mistakes.
• Easier for someone who will not handle loose Li-ion cells.

Cons

• No hot-swap. Dead pack = dead light.
• You do not choose cell quality. When the pack fades, you buy a repair or a new light.
• The charge port is a lint and water point unless the cap is excellent.
• Spare strategy becomes a power bank — another lithium item for the airport.
• Poor “leave it in the glove box for five years” light unless it also takes CR123 or AA.

Best for: People who already charge a phone every night and want the light on that same pile. Slim 2026 EDC. Worst for a neglected backup unless there is a primary-cell fallback.

• Pocket carry, a few times a week: 18350 or built-in USB-C. Add CR123 or lithium AA if the light allows it.
• Every day, pocket or bag: 18650 plus one spare in a case.
• Flying: Light on your person, spares in carry-on with covered terminals. CR123 or lithium AA as the no-charger backup. Nothing loose in checked bags.
• Glove box / nightstand / kit that sits: CR123A or lithium AA. Rechargeables self-discharge. Primaries wait.
• One light for a non-hobby user: AA or built-in USB-C. Not unprotected 18650s.

The battery is not an accessory. It is why the light either stays in the pocket or moves to the junk drawer. Choose the cell for how you charge, how you fly, and how long the light can sit. Then buy the light that uses that cell well.