21700 vs 18650 batteries

[No1up]

Just wanted to insight in to these batteries. Which ones are better, safer, more in tune for high speed 30mph+ ridding. 

[Tawpie]

That’s a tough one. So many variables. In general the 18650 form factor has been around a long time and the manufacturers are pretty good at making them, so in general one could possibly worry less with 18650s vs 21700 that are newer and have a relatively short history. The 21700 form factor was problematic for LG early on, and is suspected in the BG 900 wH troubles, but seems to have matured and has been reliable of late.

For speed, you need power (and volts) but how you turn the power into velocity is less about the cell geometry than it is how the entire system as a whole is designed. Yes, cell construction is a part of the equation, but the physical dimensions are only part of it.

The big advantage of the 21700 cell is its larger volume. It holds more chemicals so it stores more energy. And because it’s used by cars it gets a large portion of the R&D budgets so it continues to enjoy advances.

Edited June 5, 2022 by Tawpie

[OneLeg]

18650 has a volume of 16.54cm2

21700 has a volume of 24.25cm2

[alcatraz]

Good quality is more important than size. I'm very impressed with Sanyo/Panasonic NCR18650GA cells. They've kind of been the gold standard for a long time. There are many other good cells but I question if the track record is as good as these.

Also, the best cell won't be reliable unless there's a properly working bms.

[OneLeg]

I rebuilt net Fluke testers with 2P2S, and I've had a few GA's fail not sure why. I've never used them on a unicycle.

[Murdomeek]

You also need to compare max discharge ratings and parallel count in the battery pack design.

Compare the sherman vs sherman max.

Sherman: Panasonic GA 18650 24s10p = 10a max discharge * 10p = 100a

Sherman max: Samsung 50e 21700 24s8p = 9.8a max discharge * 8p = 78.4a

 

For such a big battery such as the sherman, both of those are fine for normal riding conditions. But if you try this calculation for a smaller 1400-1800wh battery, it matters more.  This is one of the reasons ewheels are offering higher discharge (more expensive) cells options for the buyer.