21700 Batteries: Selection Guide, Specs, and Safety

21700 batteries are cylindrical lithium-ion cells sized 21 mm x 70 mm. Compared with 18650 cells, they offer higher capacity and stronger current handling, making them a solid choice for high-output flashlights, tools, and managed battery packs.

At a Glance

• Size: 21 mm diameter, 70 mm length (protected/button-top variants can reach ~73–76 mm)
• Capacity: ≈3000–5200 mAh (trade-off: higher capacity often means lower max current)
• Continuous discharge: ≈10–40 A (model-specific; check datasheet)
• Nominal voltage: 3.6–3.7 V (Li-ion). LiFePO4 variants are ~3.2 V and require compatible devices/chargers.
• Top style: flat-top or button-top
• Protection: protected (with protection circuit) or unprotected (bare cell)

How to Choose the Right 21700

1. Confirm physical fit: Many lights and chargers accept 21 mm diameter, but length varies. Protected/button-top cells can be 73–76 mm long. Devices made strictly for 18650 (18 mm) typically do not fit 21700.
2. Match chemistry and voltage: Most 21700 cells are 3.6/3.7 V Li-ion (INR/NMC/NCA). Only choose LiFePO4 (3.2 V) if your device explicitly supports it, and use an LFP-capable charger.
3. Pick capacity vs. current:
   • High-drain tools/performance lights: 30–40 A continuous, ~3000–4000 mAh.
   • General-purpose and long runtime: 15–25 A continuous, ~4500–5200 mAh.
4. Choose protection level:
   • Protected: safer for single-cell consumer devices; adds length and cost.
   • Unprotected: for packs with a dedicated BMS or devices that manage protection internally.
5. Select top style:
   • Button-top: required by many flashlights and devices with recessed contacts.
   • Flat-top: common for pack building and some chargers.
6. Verify charger compatibility: Ensure the charger supports 21700 size and chemistry. Extra-long protected/button-top cells may need slots that accommodate up to ~76 mm.

Quick Recommendations by Use Case

Use case	Capacity	Continuous discharge	Top / Protection
High-output flashlight (turbo modes)	3000–4000 mAh	25–40 A	Button-top, protected if device lacks safeguards
Everyday/long-runtime flashlight	4500–5200 mAh	10–20 A	Button-top, protected recommended
Power tools / high-drain devices	3000–4000 mAh	30–40 A	Flat-top, typically unprotected (within a BMS-managed pack)
DIY battery packs (e-bike, storage)	Varies by design	Match BMS & motor/controller needs	Flat-top, unprotected; spot-welded in managed packs

Chemistries and What They Mean

• INR/NMC/NCA (Li-ion, 3.6–3.7 V nominal): Most common; good balance of capacity, current, and cycle life. Charge to 4.2 V.
• IMR (LiMn-based, Li-ion): Typically higher current capability, sometimes at the expense of capacity.
• LiFePO4 (LFP, ~3.2 V nominal): Lower voltage, excellent cycle life and thermal stability; requires LFP-compatible device and charger (charge to ~3.6 V).

Protected vs. Unprotected, Button vs. Flat

• Protected cells: Add overcharge, over-discharge, and short-circuit protection; increase length and sometimes diameter slightly.
• Unprotected cells: For packs/devices with a BMS; never use bare cells without appropriate protection in multi-cell configurations.
• Button-top: Raised positive terminal for devices with deep contact wells.
• Flat-top: Preferred for pack building and some chargers; may not make contact in devices that expect a button-top.

21700 vs. 18650 vs. 26650

Format	Size (mm)	Typical capacity	Notes
18650	18 x 65	2500–3600 mAh	Good availability; fits older/compact lights; lower capacity than 21700.
21700	21 x 70	3000–5200 mAh	Strong balance of size, capacity, and current; increasingly common.
26650	26 x 65	4000–5500 mAh	Larger diameter; may be heavier; compatibility varies.

Estimate Runtime

A quick estimate for a single-cell device: runtime (hours) ≈ (mAh × 0.85) ÷ device draw (mA). The 0.85 factor accounts for typical efficiency and voltage sag.

Example: A 5000 mAh cell powering a light that draws 1000 mA ≈ (5000 × 0.85) ÷ 1000 ≈ 4.25 hours.

Charging and Care

• Use a quality charger that supports 21700 size and your chemistry (Li-ion 4.2 V, or LFP 3.6 V).
• Recommended charge rate: ~0.5C for longevity (e.g., 2 A for a 4000 mAh cell). Some cells allow up to 1C—verify datasheet.
• Temperature: charge roughly 0–45°C (32–113°F); discharge roughly -20–60°C (-4–140°F). Follow cell-specific specs.
• Storage: cool, dry place at ~40–60% charge; check and top up every 3–6 months.
• Avoid over-discharge (generally below ~2.5–2.8 V for Li-ion). Many devices cut off around 2.8–3.0 V.

Safety Essentials

• Do not carry loose cells in pockets or bags—use a protective case.
• Inspect wraps regularly; rewrap or recycle if the wrap is torn, if the cell is dented, or shows swelling/corrosion.
• Do not short, puncture, crush, or expose to water/heat. Use correct polarity.
• Do not mix old and new cells, different brands, or different capacities in multi-cell devices.
• Do not replace individual cells inside managed packs (e-bike/tool packs); service or replace the entire pack.
• Do not leave charging batteries unattended. Charge on a nonflammable surface.

FAQ

Will a 21700 fit in a device made for 18650?

Usually no—the 21700 is 3 mm wider. Some newer lights accept both formats with adapters; check your device manual and maximum supported dimensions.

Do I need a protected or unprotected cell?

For single-cell consumer devices without built-in protection, choose protected. For battery packs or devices with a robust BMS, unprotected (bare) cells are typical.

What charger should I use for 21700?

Use a charger that supports 21700 length (up to ~76 mm for protected/button-top) and the correct chemistry. Many multi-bay Li-ion chargers list supported sizes—verify before charging.

How long do 21700 batteries last?

Expect roughly 300–800 full cycles for Li-ion depending on usage, depth of discharge, charge rate, and temperature—often 2–5 years in typical use.

Are USB-rechargeable 21700 cells a good idea?

They are convenient and include protection but are longer/heavier and may not fit tight battery tubes. Ensure your device can accommodate the added length.