Many people say that 21700 is a superior form factor, but I suspect that 18650 is still more efficient in terms of power and energy density.

Since the trivial solution of putting 2 cells in parallel (i.e. 2x volume) will give 2x both in power AND energy, a larger and more efficient form factor should give a boosted return in both measurements.

For 18650s, we have power-oriented cells such as VTC6 at (3000mAh, 30A), and capacity-oriented cells such as LGDBMJ11865 at (3500mAh, 10A)

The volume of 21700 is 1.466x of 18650’s, for it to be a more efficient form factor, we should see power-oriented cells at (4400mAh, 44A) and capacity-oriented cells at (5130mAh, 14.6A).

With my limited knowledge on off-the-shelf 21700 cells, it seems to me that their specifications are still substantially lower than these numbers.

What do you guys think?

If your numbers are correct (I’m not qualified to say), then 21700 probably aren’t as space-efficient as 18650.

But that isn’t always the right question, at least for me. 21700 is a newer format than 18650, so there could still be some headroom to grow.

More importantly to me, 1×21700 is still a lot smaller than 2×18650. I like having close to 5 Ah in my pocket, and there’s no way I’m carrying a 2×18650 torch. I have several 1×21700 formats that are comfortable in my EDC rotation.

Plus, I’m all about capacity, and the only 2x parallel 18650 I own is a cheap chinesium headlamp. I don’t know of any budget 2x parallel 18650 torches. It would be too big, anyway.

The Vapcell T50 aka Samsung 50S is rather good but I agree that Elon Musk oversold the 21700 as a miraculous format.

Very true, 21700 provides a slightly larger form factor that definitely has its practical value in single-cell applications.

Wow, Samsung INR21700-50S (5000mAh, 20A) those are some very good specs, leaning more to the capacity-oriented side but impressive maximum discharge current. So 21700 is indeed more efficient!

Well the very best 18650 is the Sanyo GA in terms of capacity and power. It doesn’t really have a 21700 counterpart though. The vtc6 however does. The Samsung 30T is the same capacity, but is capable of higher discharges. I tested at vtc6 vs a 30T in the FT02S xhp50.2, and the 30T pulled more current at the top end of ramp/steps and turbo, but equaled it at lower settings, so the difference there is negligible. However, looking at it in context, the largest capacity 21700 are still better across the board. They’re just a little larger (5 mm longer, 2 mm wider) so basically the same length as a protected 18650 and only 2 mm wider. The difference in size is negligible in my opinion. Unless someone can develop new tech for 18650’s I think the 21700 is the future.

The capacity and power of 21700s are in fact consistent with the weight increase over 18650s.

Comparing high capacity cells sanyo 18650 GA and samsung 21700 50G, there is a 1.46x difference in weight, so you might expect to see a 1.46x increase in capacity and a 1.46x decrease in internal resistance, and that is about what you see. 1.46×3.345Ah=4.87Ah which is close to the 4.83Ah of the 50G. To estimate the IR look at the difference in voltage between the 0.2A trace and the 10A trace. For the GA it is 0.4V and for the 50G it is 0.26V, which indicates a decrease in IR of about 1.5x.

There is a similar trend with more power-oriented cells like 30Q and 40T and molicel P42.

That’s until the T50/50S came along with a significant but incremental improvement

18650’s have been the standard for many years. As a result , there are more types available than 21700. That will all likely change.

3500mAh 18650s don’t actually reach 3500, that’s just the number they advertise./

You need to look at real tests.

https://lygte-info.dk/review/batteries2012/Samsung%20INR18650-35E%203500...

https://lygte-info.dk/review/batteries2012/Vapcell%20INR21700%205000mAh%...

At 1A:

Samsung 35E 18650: 251.9Wh/kg 711.7Wh/L

Samsung 50T 21700: 249.73Wh/kg 707.8Wh/L

At 5A:

Samsung 35E 18650: 231Wh/kg 652.7Wh/L

Samsung 50T 21700: 241.2 Wh/kg 683.7Wh/L

At 10-15A discharge the difference gets even bigger.

So basically at low currents they are nearly identical in energy density and specific energy, but at higher currents the 21700 cells perform much better.

The difference is you need fewer cells in parallel for higher capacity and much better high current capabilities.

It is also easier to assemble large battery packs when you need to use half as many cells.

The OPTOFIRE - 4.63Mcd aspheric LED flashlight The SYNIOSBEAM - 10Mcd recoil LED flashlight List of the farthest throwing flashlights

Enderman!

Very informative post,thank you.

You’re welcome!

I too found the 21700 batteries disappointing when they first started coming out, but now there are finally a few that match the capacity of the best 18650s, and their high current performance is even better.

I see no point to 26650 cells anymore, even the best 26650 is much larger and heavier for its capacity, which is at best close to 6000mAh.

The OPTOFIRE - 4.63Mcd aspheric LED flashlight The SYNIOSBEAM - 10Mcd recoil LED flashlight List of the farthest throwing flashlights

Power tool companies (Bosch, Milwaukee, etc) have adopted the 21700 format for their larger capacity battery packs. That is telling.

That’s likely because they can use fewer cells per battery pack so less welding and less things to go wrong.