The Complete Guide to 18650 and 21700 Battery Cells — Chemistry, Brands, Capacity
Everything about 18650 and 21700 lithium cells: chemistry types, real capacity, discharge rates, top manufacturers, fake detection, storage and recycling.
Contents
Lithium cylindrical cells in the 18650 and 21700 formats are the building blocks of virtually all modern rechargeable battery packs: from laptops and power tools to robot vacuums. At the SATER service centre, we work with these cells daily — repacking power tool battery packs, replacing cells in robot vacuum batteries, and assembling custom battery packs to order.
This article is a detailed guide for anyone wanting to understand the subject: from physical dimensions to spotting counterfeits.
What the Numbers Mean: 18650 and 21700
The format name is its dimensions in millimetres:
- 18650 = 18 mm diameter × 65 mm length (the 0 denotes cylindrical form)
- 21700 = 21 mm diameter × 70 mm length
The 18650 format appeared in the early 1990s and became the de facto standard. Sony was the first to mass-produce 18650 cells for laptops. The 21700 format is newer, developed by Tesla with Panasonic for electric vehicles (Model 3, 2017). Thanks to its greater volume, the 21700 holds 35-50% more energy.
Format Comparison
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Chemistry Types
Not all lithium cells are alike. The cathode's chemical composition determines key characteristics: capacity, discharge rate, safety, and lifespan.
INR — Nickel-Manganese-Cobalt (NMC)
The most common type in modern cells. A balance between capacity, current delivery, and safety.
- Typical 18650 capacity: 2500-3600 mAh
- Discharge rate: medium-high (10-35A)
- Examples: Samsung 25R (2500 mAh, 20A), Samsung 30Q (3000 mAh, 15A), LG HG2 (3000 mAh, 20A)
ICR — Cobalt (LCO)
High capacity but low discharge rate and elevated thermal runaway risk. Requires a protection PCB.
- Typical 18650 capacity: 2600-3400 mAh
- Discharge rate: low (2-5A)
- Examples: Panasonic NCR18650B (3400 mAh, 4.87A)
IMR — Manganese (LMO)
High discharge rate and safety at the cost of lower capacity.
- Typical 18650 capacity: 1500-2100 mAh
- Discharge rate: high (20-30A)
IFR — Iron Phosphate (LFP)
Maximum safety and longevity (2000-5000 cycles), but lower voltage (3.2V instead of 3.6-3.7V) and less capacity.
- Nominal voltage: 3.2V (not interchangeable with INR/ICR!)
Important: IFR/LFP cells CANNOT replace INR/ICR without reconfiguring the BMS — the voltage differs.
Capacity vs Discharge Rate
CDR vs Pulse Current
- CDR (Continuous Discharge Rating) — the maximum current a cell can deliver continuously without overheating. The most important parameter for power tools.
- Pulse current — short-term (typically 1-10 seconds) maximum current. Many manufacturers advertise this figure, which can be misleading.
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Top Manufacturers and Models
There are dozens of "brands" of 18650/21700 cells on the market, but only a handful of genuine cell manufacturers.
Sony / Murata: VTC5A, VTC6, VTC6A — the gold standard for high-drain cells.
Molicel (E-One Moli Energy): Canadian manufacturer, supplier to Tesla and Dyson. P26A, P28A, P42A — outstanding current delivery.
Samsung SDI: 25R, 30Q, 40T, 50E — wide range from all-rounders to specialists.
LG Energy Solution: HG2 ("Chocolate"), MJ1 — popular and reliable.
Panasonic / Sanyo: NCR18650B, NCR18650GA — capacity record holders.
How to Spot Fakes
The 18650 market is flooded with counterfeits. "UltraFire 9800 mAh cells" for €2 are a scam. It's physically impossible to fit 9800 mAh into the 18650 format (the real maximum is around 3600 mAh).
Weight Test
The quickest check. A genuine 18650 cell weighs 44-50 grams. A fake typically weighs 25-35 grams. Kitchen scales with 1g accuracy are all you need.
Capacity Test
Fully charge the cell (4.20V) and discharge to 2.5V through a charger with capacity measurement (LiitoKala Lii-500, XTAR VC4). If actual capacity is less than 80% of the claimed figure — it's a counterfeit or a worn cell.
Visual Inspection
- Originals have laser-engraved markings, even wrapping, crisp font.
- Fakes — blurry printing, uneven heat shrink, no QR code.
- Check the source — buy from trusted suppliers.
Cell Storage
- Temperature: +10-25°C (ideally +15°C). Not in the fridge (condensation!), not in heat.
- Charge level: 40-60% (3.6-3.8V). Don't store fully charged (4.2V) or fully discharged (below 3.0V).
- Container: plastic case to prevent contact short-circuits (pockets, keys, coins).
- Shelf life: with proper storage, cells lose 2-3% capacity per year.
Recycling and Safety
Lithium cells are hazardous waste. They must not go in general rubbish.
- In Latvia: drop off at battery collection points in shops (Rimi, Maxima, Euronics) or at Getliņi.
- Damaged cells (swollen, damaged wrapper, getting hot) are particularly dangerous. Place in sand or vermiculite and take to a specialist collection point.
At the SATER service centre, we build battery packs from tested cells (Sony, Molicel, Samsung, LG, Panasonic) and properly recycle old cells. If you need a power tool or robot vacuum battery repack, or a custom battery assembly — get in touch.
Repair path
Where to go next if this fault is repairable
Related SATER service, brand and fault pages help you understand the repair route and get the device into the right diagnostic flow.
Frequently Asked Questions
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