21700 mod | what moving to the bigger cell changes

Moving to a 21700 mod means accepting three extra millimetres of diameter and five of length per cell. On paper the gap looks trivial. In the hand it changes the depth of the mod, its weight, the atomiser that sits on it without overhanging, the charger on your desk and the cells you can slide in. Before switching format, it is worth knowing what those millimetres bring, and what they demand.

Should you move to a 21700 mod, the answer in three figures

Yes, if you vape on a single cell above 80 W or want longer between charges. Working from Molicel’s figures, a P42A 21700 stores 15.12 Wh against 10.08 Wh for a P28A 18650, 50% more energy, and handles 45 A of discharge against 35 A. In return it weighs up to 70 g against 46 g, and the mod grows by a few millimetres.

Stay with 18650 if your mod already takes two cells, if a compact format matters to you or if your atomisers measure 26 mm or less. For everyone else, a single-cell 21700 mod is currently the best compromise between size and safety margin.

21 × 70 against 18 × 65, what the millimetres change

A cylindrical cell’s name gives its dimensions. An 18650 is 18 mm across and 65 mm long, a 21700 is 21 mm across and 70 mm long. Molicel’s datasheets give the true maximums, 18.6 × 65.2 mm for the P28A and 21.7 × 70.2 mm for the P42A, and those are the figures a mod maker designs the battery bay around.

In volume the gap is larger than it looks. An 18 × 65 mm cylinder takes up π × 9² × 65 = 16,540 mm³, or 16.5 cm³. A 21 × 70 mm cylinder takes up π × 10.5² × 70 = 24,245 mm³, or 24.2 cm³. The 21700 therefore needs 47% more room.

Two mods from the Lost Vape Centaurus family show the result. The M100, built for an 18650, measures 92.7 × 38 × 26 mm. The N100, which takes a 21700, measures 97.6 × 38 × 30 mm. The width stays the same, but the height grows by 4.9 mm and the depth by 4 mm.

Those 4 mm of depth, the dimension that counts at the 510 connector, change how an atomiser sits. A 26 mm atomiser is flush on the M100, but on the N100 it sits 2 mm inside each edge, markedly inset by the Fumoir overhang checker. A 30 mm atomiser reverses the verdict, flush on the N100 and a marked 2 mm overhang per side on the M100. UK kits ship with 2 ml tanks, the legal cap for refillable tanks, and their glass can be narrower than the larger version, so measure the tank you actually own.

Run time and weight, worked out on real cells

The figures below come from Molicel’s product pages. Nominal energy is the nominal voltage of 3.6 V multiplied by the typical capacity. Molicel’s PDF datasheets quote slightly more, 15.5 Wh for the P42A and 10.3 Wh for the P28A, and the ratio between the two stays at 50%.

Molicel cell	Size	Typical capacity	Nominal energy	Discharge current	Maximum weight
P28A	18650	2.8 Ah	10.08 Wh	35 A	46 g
M35A	18650	3.5 Ah	12.6 Wh	10 A	48 g
P42A	21700	4.2 Ah	15.12 Wh	45 A	70 g
M65A	21700	6.5 Ah	23.4 Wh	26 A	74.5 g

One point is surprising. For the same energy, a 21700 weighs no less. At the quoted maximum weights, three P28A cells add up to 30.24 Wh for 138 g, and two P42A cells to the same 30.24 Wh for 140 g. The gain of a 21700 mod lies in the number of cells, not the weight, since one 21700 does the work of one and a half 18650s.

For run time, take 60 W of continuous firing. The P42A’s 15.12 Wh gives 15.12 ÷ 60 = 0.252 h, about 15 minutes of total firing time, or 13.6 minutes at 90% efficiency. The P28A tops out at 10.1 minutes, 9.1 at the same efficiency. These are theoretical ceilings, as the mod cuts out before the cell is empty.

Current draw, where the 21700 makes the difference

On a regulated mod the most demanding case is a nearly flat cell at around 3.2 V, with a cautious 90% efficiency. The current per cell is then the power it supplies divided by 3.2 × 0.9, which is 2.88.

At 100 W on a single cell, that gives 100 ÷ 2.88 = 34.7 A. The P28A and its 35 A have only 0.3 A in hand, while the P42A keeps 10.3 A. At 120 W, the ceiling of the Centaurus M120, the draw reaches 41.7 A, out of reach for the P28A and still covered by the P42A, though with a thinner margin.

Capacity is not power. The M65A stores 23.4 Wh, but its 26 A limit a single-cell mod to 26 × 2.88 = 74.9 W in that demanding case. In 18650 size, the M35A and its 10 A stop at 28.8 W. A cell is chosen first on its discharge current, read on the cell maker’s datasheet, never on a reseller’s packaging.

Chargers, sleeves and charging, the precautions specific to 21700

Changing format does not stop at the mod. It affects your charging kit, the cells already in rotation and the way you recharge.

A charger that takes 70 mm

A charger designed for 18650 cells will not necessarily take a 70.2 mm cell. Check that its manual lists 21700, and set a charging current no higher than the value the cell maker gives, 4.2 A for the P42A according to Molicel.

The 18650 sleeve

Vaporesso ships the Armour Max and Armour S with an 18650 sleeve stored inside the mod. It lets you run down your old cells, but the limit then becomes that of the 18650. At 100 W a P28A drops back to 0.3 A of margin, and the wattage is better turned down.

Charging in the mod or separately

A 21700 mod can charge its cells over USB-C, at 5 V and 2 A on the Armour Max according to Vaporesso. Lost Vape nevertheless recommends taking the cells out and charging them in a suitable external charger when the mod reports a low battery, and never leaving a charge unattended.

When to stay with 18650

The 18650 still has real arguments. On a twin-cell mod the load is shared. At 120 W each cell supplies 60 W, so 60 ÷ 2.88 = 20.8 A in the most demanding case, well below a P28A’s 35 A. And two P28A cells weigh 92 g against 140 g for two P42As, a 48 g difference in your pocket.

There is also the format of the slimmest mods, such as the Centaurus M100 and its 26 mm, which takes 26 mm atomisers without any overhang. And pairs of 18650s in good condition have no reason to head for recycling.

FAQ

Five questions that come up when changing format.

What should I do if the mod will not switch on after a cell change?

Lost Vape suggests checking the cells are the right way round, cleaning the positive and negative contacts, trying another set of cells in good condition, then plugging the mod into a charger to see whether it starts.

Are there twin 21700 mods?

Yes. Vaporesso, for example, specifies two 21700 or 18650 cells for the Armour Max, from 5 to 220 W. At 220 W each cell supplies 110 W, which is 38.2 A in the most demanding case, covered by a P42A’s 45 A.

Can I fly with 21700 cells?

Airline rules generally require spare cells to travel in the cabin with their terminals protected, never in the hold. Your airline’s own conditions apply.

Which mod for a first 21700?

A single-cell mod that also takes an 18650, so your existing cells stay useful. Vaporesso specifies both sizes on the Armour S and Target 100, Lost Vape on the Centaurus N100 and M120.

What should I do with a worn 21700 cell?

Tape over its terminals and take it to a battery recycling point, never household waste. A cell with a damaged wrap does not go back into the mod in the meantime.

21700 mod or 18650 mod, the right format for how you vape

The 21700 wins as soon as a single cell has to deliver more than 80 W or go longer between charges, because it offers 50% more energy and a current margin an 18650 no longer has at 100 W. The 18650 stays relevant on twin-cell mods and compact formats.

Either way, the cell maker’s datasheet decides and the charger has to keep up. The cell size is only one part of the choice, yet it is what fixes the size of a 21700 mod or an 18650 one for years.

Also in the lounge

The Fumoir writes for adult readers only. Vaping products usually contain nicotine, which is addictive, and nicotine vaping products are not for sale to under-18s in the UK. This editorial feature is not a buying recommendation.