What Do Battery Numbers Mean? A Guide to Efest 18650, 21700 and 20700 Cells
What Do Battery Numbers Mean? A Guide to Efest 18650, 21700 and 20700 Cells
A clear UK 2026 guide to vape battery number codes. 18650 + 20700 + 21700 + 26650 dimensions + capacity + use case + sustained vape mod form factor compatibility + sustained UK Trading Standards monitored. UK 18+ verification required.
The verdict
Battery sizes 2026Numbers = dimensions. Match to mod.
Diameter × length × cylinder shape + sustained correct fit.
4
Common sizes
~3.7V
All cylindrical
Vape battery numbers describe physical dimensions of cylindrical lithium-ion cells using simple diameter + length + shape coding + sustained UK Trading Standards monitored. Number coding system: first 2 digits = diameter in millimetres + next 2 digits = length in millimetres + last digit (typically 0) = cylindrical cell shape. 18650 example: 18mm diameter × 65mm length × 0 (cylindrical) = 18mm × 65mm cylindrical lithium-ion cell + sustained most common vape mod battery size + ~£8-15 per Efest IMR 18650 UK retailer + sustained ~2100-3500mAh capacity range. 20700 example: 20mm diameter × 70mm length × 0 (cylindrical) = 20mm × 70mm cylindrical lithium-ion cell + sustained intermediate size + ~£10-18 per Efest IMR 20700 UK retailer + sustained ~3000-3500mAh capacity range + sustained newer mid-power vape mod compatibility. 21700 example: 21mm diameter × 70mm length × 0 (cylindrical) = 21mm × 70mm cylindrical lithium-ion cell + sustained higher-capacity size + ~£12-20 per Efest IMR 21700 UK retailer + sustained ~3500-5000mAh capacity range + sustained latest sub-ohm vape mod compatibility. 26650 example: 26mm diameter × 65mm length × 0 (cylindrical) = 26mm × 65mm cylindrical lithium-ion cell + sustained large form factor + ~£12-18 per Efest IMR 26650 UK retailer + sustained ~4200-5000mAh capacity range + sustained niche large mod compatibility. Form factor selection by vape mod: 18650: most common older + entry-level vape mods (pre-2024) + most widespread UK retailer availability. 20700: mid-range 2024-2026 vape mods + sustained intermediate capacity + amp delivery. 21700: latest sub-ohm vape mods (2024-2026 increasingly standard) + sustained highest capacity + sustained higher amp delivery. 26650: niche large vape mods + sustained niche use case. Critical fitment rule: never use wrong form factor + 21700 won't fit in 18650 mod + 18650 too small for 21700 mod + always check vape mod manual for required form factor + sustained correct fit required + sustained reliable operation + sustained safe operation. Mooch independent testing context: independent battery tester Mooch (Phil Brackett) tests Efest CDR claims across all form factors + finds real-world CDR ~10-20A vs ~20-35A printed claims + sustained importance of using tested CDR for safe operation + sustained vape community resource. UK Tobacco and Vapes Act 2026: Royal Assent 29 April 2026 + framework restrictions + sustained UK 18+ verification + sustained UK Trading Standards monitored battery sales + sustained UK consumer protection. NHS cessation pathway alignment: vaping less harmful than smoking but not zero-risk; the NHS recommends vaping only for adult smokers switching from cigarettes. UK 18+ verification required.
Battery sizes in figures
4sizes
Common Efest form factors
18650 + 20700 + 21700 + 26650 + sustained UK retailer availability + UK Trading Standards monitored.
~£8-20per cell
Across form factors
18650 = ~£8-15 + 20700 = ~£10-18 + 21700 = ~£12-20 + 26650 = ~£12-18 + sustained UK retailer pricing.
~2100-5000mAh
Capacity range
Across all Efest IMR + Hybrid INR form factors + sustained UK Trading Standards monitored.
Efest battery sizes table UK 2026
Each form factor mapped to dimensions + capacity + amp range + cost + best use case + sustained UK retailer availability + UK TPD compliant.
Why 18650 became the vape standard
18650 form factor became sustained vape mod standard ~2010-2018 due to balanced size + capacity + amp delivery + UK retailer availability + sustained vape mod compatibility. Industry origin: 18650 cylindrical lithium-ion cells originally developed for laptop battery packs ~1990s + sustained mass production + sustained low manufacturing cost + sustained widespread availability + sustained known-spec performance + sustained foundational vape battery format. Vape mod adoption ~2010-2014: early sub-ohm vape mod era required high-discharge lithium-ion cells + 18650 already widely manufactured for laptops + sustained ready availability + sustained vape mod manufacturers built mods around 18650 form factor + sustained format standardisation + sustained UK retailer adoption. Why 18650 size works for vape: 18mm diameter fits comfortably in handheld vape mod + 65mm length manageable + ~2100-3500mAh capacity reasonable for daily vape use + ~10-20A real-world CDR sufficient for mid-power vape mod operation + sustained balanced size + capacity + amp delivery + sustained ergonomic vape mod design. Sustained UK retailer availability: 18650 most widespread UK retailer availability + sustained variety of brands (Efest + Molicel + Samsung + LG + Sony) + sustained customer choice + sustained competitive pricing + sustained UK Trading Standards monitored + sustained UK TPD compliant operation across multiple brands. Continued 18650 relevance UK 2026: many UK vape mods continue using 18650 form factor + sustained legacy vape mod compatibility + sustained UK retailer availability + sustained mid-range pricing + sustained reliable operation + sustained UK consumer choice + sustained UK Trading Standards monitored alongside newer 20700 + 21700 form factors. UK TPD compliant. For best Efest cells see best Efest batteries 2025.
21700 emergence as new standard
21700 form factor emerged as sustained latest sub-ohm vape mod standard ~2018-2026 due to higher capacity + higher amp delivery + sustained newer mod design optimisation. 21700 industry origin: 21700 form factor originally developed for Tesla electric vehicle batteries ~2017 + sustained mass production + sustained manufacturing maturity + sustained spillover into vape mod + power tool + electronics markets + sustained ready availability for vape applications. Why 21700 suits modern sub-ohm vape mods: 21mm diameter slightly larger than 18650 (3mm increase) + 70mm length slightly longer (5mm increase) + sustained ~40-50% higher capacity + sustained higher amp delivery vs 18650 + sustained latest sub-ohm vape mod design optimisation + sustained ergonomic mod fit. Capacity advantage detail: typical Efest IMR 21700 ~3500-5000mAh vs typical Efest IMR 18650 ~2100-3000mAh + sustained ~40-50% capacity increase + sustained extended vaping time per charge + sustained reduced charging frequency + sustained battery life per charge cycle. Amp delivery advantage detail: typical Efest IMR 21700 ~25-40A real-world Mooch-tested CDR vs ~10-20A for 18650 + sustained higher amp delivery for sub-ohm vape mod + sustained safer high-wattage operation within tested CDR + sustained reliable sub-ohm vape mod operation. Sustained UK retailer availability growth: 21700 increasingly standard at established UK retailers (UK Companies House + VAT registered + UK Trading Standards monitored) + sustained variety of brands + sustained customer choice + sustained competitive pricing + sustained UK consumer protection. Industry transition timeline UK 2026: 18650 still widely used for older + entry-level vape mods + 21700 increasingly standard for latest sub-ohm vape mods + 20700 intermediate niche (less widespread) + 26650 large niche (specific older mod compatibility) + sustained mixed form factor era + sustained UK consumer choice. UK TPD compliant. For mod matching see match battery to vape mod.
Form factor selection guide
Choose correct form factor by checking vape mod manual + matching dimensions + matching capacity to use pattern + sustained UK Trading Standards monitored. Step 1: Check vape mod manual: vape mod manuals specify required battery form factor + sustained correct form factor required + sustained reliable operation + sustained safe fit + never use wrong form factor (21700 won't fit in 18650 mod + 18650 too small for 21700 mod). Step 2: Identify your form factor need: Older vape mod (pre-2024): most likely 18650 + sustained widespread UK retailer availability + ~£8-15 per Efest IMR 18650. Mid-range vape mod (2024-2026): likely 18650 or 20700 depending on mod design + sustained sustained UK retailer availability. Latest sub-ohm vape mod (2024-2026): increasingly 21700 + sustained higher capacity + amp delivery + ~£12-20 per Efest IMR 21700. Large or niche vape mod: possibly 26650 + sustained niche UK retailer availability + ~£12-18 per Efest IMR 26650. Step 3: Match capacity to use pattern: Light use: 18650 ~2100-3000mAh sufficient + sustained adequate battery life. Moderate use: 18650 ~3000mAh+ or 20700 ~3000-3500mAh recommended + sustained reduced charging frequency. Heavy use: 21700 ~4000-5000mAh recommended + sustained extended battery life per charge + sustained reduced charging downtime. Step 4: Pair with appropriate Efest charger: Efest LUC + XSmart smart chargers compatible with all form factors + sustained safe charging + 4.2V auto-cutoff + sustained UK retailer availability. Step 5: Buy from established UK retailer: UK Companies House + VAT registered + UK Trading Standards monitored + sustained UK consumer protection + sustained authenticity verification + sustained UK retailer warranty support. UK TPD compliant.
Battery numbers key points
Numbers = diameter × length × shape
18650 = 18mm × 65mm cylindrical + 21700 = 21mm × 70mm cylindrical + 26650 = 26mm × 65mm cylindrical.
18650 = vape standard
Most common form factor + sustained widespread UK retailer availability + sustained mid-range pricing.
21700 = latest sub-ohm
Higher capacity + amp delivery + sustained latest sub-ohm vape mod compatibility + ~£12-20 UK.
Always check mod manual
Never use wrong form factor + sustained correct fit required + sustained reliable safe operation.
Choose Efest battery by + understanding number coding (diameter × length × shape) + matching to vape mod manual specification + selecting capacity for use pattern + UK 18+ verification at established UK retailer + sustained reliable operation. Vaping not risk-free; the NHS recommends vaping only for adult smokers switching from cigarettes. To explore Efest visit the Efest collection.
Shop Efest in correct form factor for your vape mod
Choose Efest in correct form factor for sustained vape mod compatibility + sustained UK retailer availability + sustained UK Trading Standards monitored. Battery numbers explained UK 2026: number coding = diameter mm × length mm × shape (0 = cylindrical) + sustained simple physical dimension code. 4 sustained Efest form factors UK 2026: 18650: 18mm × 65mm × cylindrical + ~2100-3500mAh + ~10-20A real CDR + ~£8-15 per cell + sustained most common older + entry-level vape mods + sustained widespread UK retailer availability. 20700: 20mm × 70mm × cylindrical + ~3000-3500mAh + ~20-30A real CDR + ~£10-18 per cell + sustained intermediate size + sustained newer mid-power vape mod compatibility. 21700: 21mm × 70mm × cylindrical + ~3500-5000mAh + ~25-40A real CDR + ~£12-20 per cell + sustained higher-capacity + sustained latest sub-ohm vape mod compatibility (2024-2026 increasingly standard). 26650: 26mm × 65mm × cylindrical + ~4200-5000mAh + ~15-25A real CDR + ~£12-18 per cell + sustained large form factor + sustained niche large vape mod compatibility. Why 18650 became vape standard: balanced size + capacity + amp delivery + sustained mass production from laptop industry + sustained widespread availability + sustained vape mod manufacturer adoption ~2010-2018 + sustained UK retailer availability + sustained continued relevance UK 2026. 21700 emergence as new standard: developed for Tesla EV batteries ~2017 + sustained ~40-50% capacity advantage vs 18650 + sustained ~50-100% amp delivery advantage + sustained latest sub-ohm vape mod design optimisation + sustained UK retailer availability growth + sustained customer choice. Form factor selection guide: 1) Check vape mod manual + 2) Match dimensions to mod requirement + 3) Match capacity to use pattern (light/moderate/heavy) + 4) Pair with Efest LUC + XSmart smart charger (compatible with all form factors) + 5) Buy from established UK retailer. Critical fitment rule: never use wrong form factor + 21700 won't fit in 18650 mod + 18650 too small for 21700 mod + always check vape mod manual + sustained correct fit required for sustained reliable safe operation. Mooch independent testing context: real-world tested CDR varies by form factor + ~10-20A 18650 vs ~25-40A 21700 + sustained importance of using tested CDR for safe wattage calculation. UK retailer warranty support: established UK retailers (UK Companies House + VAT registered + UK Trading Standards monitored) offer Efest battery + form factor guidance + UK 18+ verification + sustained customer education + sustained authenticity verification. UK consumer rights: faulty Efest battery within reasonable timeframe = retailer replacement obligation under Consumer Rights Act 2015. UK 18+ age verification: all vape battery purchases require UK 18+ verification at retailer; UK Tobacco and Vapes Act 2026 strengthens verification; never sold to under-18s. NHS cessation pathway alignment: vaping less harmful than smoking but not zero-risk; the NHS recommends vaping only for adult smokers switching from cigarettes; long-term complete cessation remains public health goal; NHS Stop Smoking Service offers free 12-week programme on 0300 123 1044 (England) with NRT typically around £100 total cheapest cessation alternative. UK Tobacco and Vapes Act 2026 received Royal Assent 29 April 2026 + framework restrictions + sustained UK 18+ verification + sustained UK Trading Standards monitored battery sales + sustained UK consumer protection. Vaping is not risk-free regardless of form factor: smokers switching from cigarettes only; never start vaping if you don't already smoke.
More on Efest
For best Efest cells see best Efest batteries 2025. For mod matching see match battery to vape mod. For mAh + amp ratings see capacity + discharge rate guide.
