What the numbers mean
The two names are size codes. An 18650 cell is about 18 mm in diameter and 65 mm long; a 21700 cell is about 21 mm in diameter and 70 mm long. The larger cell holds more active material and, in most product generations, offers more capacity per cell, which means fewer cells for the same energy or more energy in the same number of cells. A cordless stick pack is built by connecting cells in series for voltage and in parallel for capacity, typically giving 21.6-25.2 V and 2,000-4,000 mAh in the pack, with a battery management system on top. Smaller packs of 7.4-14.8 V power handhelds and some robot machines carry 2,600-5,200 mAh at a lower voltage. The pack ranges are typical across the category and confirmed per model datasheet.
Side by side
| Factor | 18650 | 21700 |
|---|---|---|
| Size and fit | About 18 x 65 mm; more cells needed per pack for a given energy; flexible layout | About 21 x 70 mm; fewer cells for the same energy; slightly longer and wider pack tube |
| Capacity | Lower per cell in most generations; pack capacity built with parallel groups | Higher per cell in most generations; supports a given pack capacity with fewer cells or a longer runtime |
| Cost drivers | Cell count, nickel strip welding, BMS channels, holder, casing | Fewer cells and welds for the same energy; larger cell unit cost; casing sized for the larger cell |
| Supply | Widely available from many cell makers; many spare options | Increasingly common; fewer second sources for some grades |
| Certification route | UN 38.3 for transport, IEC 62133-2 for pack safety, UL 2054 or UL 62133 in North America | Same route; tests are on the pack and cell configuration actually shipped |
| MOQ reference | Packs are components bought by the thousand; machines 200-500 per model on public moulds | Same bands; a new pack form in ODM adds tooling and testing time |
| Fits which buyer | Established stick platforms, handhelds, buyers who need many cell sources | New designs where weight or runtime is the headline, brands that want fewer cells in the pack |
The table sets out structure, not specific cell data; capacity and discharge ratings of a given cell are read from its datasheet. Quantities and timing come from MOQ and lead time and are confirmed in writing for each program.
Runtime is energy divided by draw
Runtime in a cordless vacuum is the pack energy divided by the average power drawn. As an illustration of the arithmetic, a pack of 21.6 V nominal and 2,500 mAh holds roughly 54 Wh; a normal mode draws a fraction of the power that boost mode draws, so the same pack lasts many times longer in one than in the other. Larger cells let the plant choose between a longer runtime, a lighter pack or a smaller pack. Typical cordless sticks quote 30-60 minutes in normal mode, but the figure depends on the mode, the brush motor and the temperature. The battery runtime guide explains how to compare these claims. Ask for the runtime in each mode, with the head fitted and a stated floor type, not a single best-case number.
Discharge rate and heat
Capacity is only half of the story. The motor demands current in bursts, and the cell must deliver it without overheating or sagging in voltage. Cells are rated for a continuous discharge current, and cheap high-capacity cells often have a lower rating than lower-capacity power cells. In a vacuum with a brushless motor and a boost mode, a cell with the wrong discharge profile will heat up, trip the BMS or lose capacity quickly. Pack design adds to the picture: nickel strip thickness, spot-weld quality, spacing for heat, and a temperature sensor read by the BMS. The right question to a plant is which cell model is used, its continuous discharge rating, and how the pack was tested at the vacuum's peak current. See BMS and battery safety for the protection side.
Certification, transport and spares
Whichever cell is chosen, the pack route is the same. UN 38.3 covers transport testing and takes about 2-3 weeks as a reference; IEC 62133-2 covers pack safety; UL 2054 or UL 62133 applies in North America; the charger follows IEC or UL 62368-1 or 60335-2-29. Air freight of packs and machines with packs follows IATA rules and needs UN 38.3 test evidence and safety data sheets. A change of cell supplier is a change of the tested configuration, so it must be written into the agreement and re-tested where required. Vaculent holds no certificates; the documents come from the partner plant and are verified per project. The shipping note for lithium battery vacuums lists the paperwork.
Pack design: what changes with the larger cell
A 21700 pack is not simply an 18650 pack with fewer cells. The longer and wider cell changes the tube or block that holds the cells, the position of the BMS board, the heat path to the casing and the way the pack clips into the handle or the body. In a stick vacuum, where the battery sits behind the hand or at the base, a bigger cell can shift the balance of the machine and change how the wand feels in use. Weight is a second effect: fewer cells means fewer nickel strips and welds and a lighter holder, but each cell weighs more. Because the pack is a sealed, tested item, a switch of cell format on an existing model usually means a new housing, new pack drawings and a new round of tests. For this reason brands that plan a battery upgrade for the next model year do better by agreeing a common connector and a common voltage platform, so the old charger and the new pack remain compatible and the spare-parts business continues.
Which to choose
- Choose 18650 when the platform already uses it, when the pack form is fixed by an existing body, or when the buyer wants many cell sources and a proven supply chain.
- Choose 21700 for a new body where runtime or pack weight is the headline, or where fewer cells simplify the BMS and welding.
- Do not mix formats between production lots without a written change and a re-test.
What to specify
- Pack voltage and capacity, cell format and cell model with datasheet.
- Series and parallel configuration, BMS protections and temperature sensing.
- Runtime in each power mode, with the head fitted and the floor type stated.
- Test reports: UN 38.3, IEC 62133-2, UL 2054 or UL 62133 as the market needs.
- Replacement pack supply and pack compatibility across models.
Send the target market, runtime target and quantity through the quote form. For the machine-level choice read cordless vs corded vacuums.