Flashlight Battery Guide: 18650 vs 21700 vs CR123A
Comparing the most common flashlight batteries — capacity, size, availability, and which lights use them.
The battery inside a flashlight matters as much as the LED. It determines runtime, output stability, size, and long-term cost. Here is a practical comparison of the three most common flashlight battery formats.
18650
The most popular rechargeable flashlight battery. The name describes its dimensions: 18mm diameter × 65mm length (roughly the size of a AA battery, but fatter and longer).
- Capacity: 2,600–3,600 mAh typical
- Voltage: 3.6V nominal (4.2V charged)
- Best for: EDC, tactical, and general-purpose lights
- Pros: Widely available, good energy density, fits pocket-sized lights. Thousands of charge cycles.
- Cons: Not available at gas stations. Requires a charger (or USB-C on the light).
The 18650 is the default recommendation. Most lights in the $30–$150 range use this cell, and quality cells from Samsung, Sony/Murata, and LG cost $5–8 each and last for years.
21700
The newer, larger alternative: 21mm × 70mm. Originally developed for electric vehicles (Tesla Model 3), now increasingly common in flashlights.
- Capacity: 4,000–5,000 mAh typical
- Voltage: 3.6V nominal
- Best for: High-output lights, throwers, and lights where extended runtime matters
- Pros: 40–50% more capacity than 18650. Supports higher sustained current for brighter sustained output.
- Cons: Slightly larger and heavier. Fewer options at retail. Light bodies are a bit wider (~28mm vs ~25mm tube).
If size is not your top priority, 21700 is the better cell. The extra runtime and current capacity are meaningful, and the size penalty is small — most 21700 lights still fit comfortably in a pocket.
CR123A
A 3V lithium primary (non-rechargeable) cell. Shorter and fatter than an 18650. Common in SureFire and other legacy tactical platforms.
- Capacity: ~1,500 mAh
- Voltage: 3.0V nominal
- Best for: Backup and long-term storage lights
- Pros: 10-year shelf life. Works in extreme cold. Compact. Available at many hardware stores.
- Cons: Expensive per use ($2–5 each, not rechargeable). Low capacity means short runtime at high output. Two cells often required.
CR123A lights are best for “stash it and forget it” scenarios — a glove box light, emergency kit, or weapon-mounted light that needs to work after sitting unused for years. For daily use, the cost-per-hour is dramatically higher than rechargeable cells.
Dual-Fuel Lights
Many modern tactical lights accept both an 18650 and two CR123A cells. This gives you the best of both worlds: rechargeable for daily use, disposable lithium for backup. If this flexibility matters, look for “dual fuel” in the listing specs.
Quick Comparison
| Spec | 18650 | 21700 | CR123A |
|---|---|---|---|
| Diameter | 18 mm | 21 mm | 17 mm |
| Length | 65 mm | 70 mm | 34 mm |
| Capacity | 2,600–3,600 mAh | 4,000–5,000 mAh | ~1,500 mAh |
| Rechargeable | Yes | Yes | No |
| Shelf Life | ~1 year (charged) | ~1 year (charged) | 10 years |
| Cost per Cell | $5–8 | $6–10 | $2–5 (single use) |
Our Recommendation
For most people: buy a light with USB-C charging that uses an 18650 or 21700 cell (included). You will never need to buy batteries separately. Plug it in like your phone. If you need a backup light for emergencies, get a small CR123A-powered option and leave it in your kit.