18500 Batteries

What is an 18500 battery?

An 18500 is a cylindrical, rechargeable lithium-ion cell measuring approximately 18 mm in diameter and 50 mm in length. Most have a nominal voltage of 3.6–3.7 V (4.2 V full). Typical capacities range from 1100–1800 mAh, with continuous discharge ratings that vary by model. They are popular in compact flashlights, headlamps, instruments, and custom electronics where an 18650 is too long but more runtime is needed than an 18350.

When to choose an 18500

• Use if your device tube or battery bay is limited to ~50 mm length but you want longer runtime than an 18350.
• Use for moderate current draw (often up to 5–10 A depending on cell) in compact lights or devices.
• Avoid if you need maximum runtime or very high current; an 18650 or 21700 will generally perform better if they fit.
• Avoid if your device requires primary cells (non-rechargeable) or a different nominal voltage.

How to choose the right 18500

1) Capacity vs. discharge rating (CDR)

There is a trade-off between mAh and current capability. Higher capacity models usually support lower max current; high-drain cells trade some capacity for safer current delivery.

• Light/medium draw (≤2 A): prioritize higher capacity (e.g., 1500–1800 mAh) for longer runtime.
• Medium/high draw (3–8 A): choose cells with published CDR in that range; expect ~1200–1600 mAh.
• Very high draw (>8 A): only select cells rated for it; verify datasheet and device requirements.

2) Button top vs. flat top

• Button top improves contact in devices with recessed positive terminals.
• Flat top is slightly shorter and may be required in tight battery tubes or battery packs.

3) Protected vs. unprotected

• Protected cells add a protection circuit (over-charge, over-discharge, short-circuit). They are longer (often 50.5–52.5+ mm) and may not fit all devices.
• Unprotected cells are standard length (~50 mm) and common in high-drain lights designed with their own protection. Only use if your device and your handling practices are appropriate.

4) Chemistry and voltage

• Li-ion (NMC/NCA/IMR/INR): nominal 3.6–3.7 V, charge to 4.2 V. Most 18500 fall here.
• LiFePO4: nominal ~3.2 V, charge to 3.6–3.65 V. Only choose if your device specifically supports LiFePO4.

5) Fit and tolerances

• Diameter can vary by brand (about 18.0–18.6 mm). Tight battery tubes may require slimmer cells.
• Length: protected and button-top versions can exceed 52 mm; verify clearance in your device.

Runtime estimates (quick reference)

Approximate continuous runtimes for a 1500 mAh (1.5 Ah) 18500. Real-world results are typically 10–25% lower due to voltage sag, driver efficiency, and cutoff settings.

Device draw	Theoretical runtime	Typical real runtime
0.5 A (e.g., low/med flashlight)	3.0 h	2.3–2.7 h
1.0 A (e.g., bright headlamp)	1.5 h	1.2–1.4 h
3.0 A (e.g., turbo burst)	0.5 h	0.3–0.45 h

Alternatives compared

Size	Dimensions	Typical capacity	Best for
18350	18 x 35 mm	700–1200 mAh	Ultra-compact lights; short bursts
18500	18 x 50 mm	1100–1800 mAh	Compact devices needing mid-level runtime
18650	18 x 65 mm	2400–3600 mAh	Longest runtime in single-cell handhelds

Safety and care

• Use a charger designed for lithium-ion 4.2 V cells. Only charge LiFePO4 with a compatible 3.6–3.65 V charger.
• Do not over-discharge; recharge before dropping below ~2.8–3.0 V.
• Store at 30–60% charge in a cool, dry place. Avoid heat, crushing, and punctures.
• Never carry loose cells in pockets; use a protective case to prevent short circuits.
• Do not mix old and new cells or different brands/chemistries in the same device. Use cells only as your device manual specifies.

Quick buying tips

• If your device draws over 5 A, choose a cell with a published CDR ≥ the device’s draw; expect lower mAh.
• If runtime matters more than peak output and your device draws ≤3 A, prioritize higher mAh models.
• Check whether your device needs button-top or flat-top, and whether protected cells will fit.
• Confirm chemistry and maximum charge voltage in your device manual before purchasing.

FAQ

Will an 18500 fit where an 18650 is specified?

Usually no. An 18500 is shorter and will not make reliable contact unless the device supports spacers or spring travel designed for it. Use only the size the manufacturer recommends.

Can I replace two CR123A cells with one 18500?

No. Two CR123A in series are about 6 V (or 6.4–6.6 V fresh), which is not equivalent to a single 3.6–3.7 V Li-ion cell. Only use the battery configuration your device supports.

What’s the difference between protected and unprotected 18500 cells?

Protected cells include a small circuit that guards against over-charge, over-discharge, and short circuits, adding length and sometimes limiting peak current. Unprotected cells are standard length and common in high-drain devices that manage protection themselves.

Do I need a button-top or a flat-top?

It depends on the device’s positive contact design. Recessed contacts often require button-tops; tight tubes may need flat-tops. Check the device manual or measure available space.

How long do 18500 batteries last?

Cycle life typically ranges from about 300–800 full cycles depending on the cell, charge voltage, discharge rate, and storage temperature. Charging to 4.1 V and avoiding deep discharges can extend lifespan.