Different Sizes of Cells
Nearly all Lithium Ion cells come in a "can" or a tube shape, and there are many sizes to choose from. You can know how big a cell is by it's name, for example a 18650 cell is 18mm in diameter and 65mm in length. If you've heard of 20700 cells then you can apply this rule to know it is 20mm in diameter and 70mm in length. So, does being 11% wider and 8% longer with 30% more volume make it better or not? Let's find out!
Size Limits and Possibilities
There are many applications where size is very important. For example, our Airsoft packs already fit very snugly in most AEGs, so a 10% size increase is a non-option for a majority of rifles. However, for the most part, our flight customers don't have this limitation. They typically use packs built with many cells and have more options in fitting their battery packs. For these customers, final performance is king. In addition, they may have a few mm left to use up but not enough for another 18650 cell, increasing to 20700 may allow them to use up all the space available.
Apples to Apples Performance Comparison
It should come as no surprise that 20700 cells typically have higher capacities and discharge rates than 18650 cells as they are simply bigger cells. However, this isn't comparing them equally because they are also heavier and larger, so we need a way to measure these cells at an equal level. Enter in Gravimetric and Volumetric densities. Gravimetric Energy Density is a measurement of weight efficiency, how much energy does a battery have for how much it weighs. It's typically measured in Watt Hours per Kilogram (Wh/Kg). Volumetric Energy Density is a measurement of space efficiency, how much energy a battery has for how much volume it occupies. It's typically measured in Watt Hours per Liter (Wh/L).
An Example: NCR18650GA vs NCR20700B
So, let's take two leading cells and compare their Wh/Kg and Wh/L as an example:
|Wh at 15W Discharge||10.571||13.329||+26%|
We can see that despite these two cells being different sizes, final power for weight and volume are very similar. If we compare the NCR20700B to the NCR18650GA and the US18650VTC6, then it doesn't have a place where it's an ideal candidate:
So, what is the verdict? Which size is better? Unless a few millimeters matters to you, what it really comes down to is the cell's build quality and performance. Right now, most of the R&D and production optimization efforts are happening on the 18650 cell as it's essentially the standard size, so it's going to typically be the best and least expensive option. However, as electric vehicles grow in usage (we're looking at you Tesla!) we'd expect the 20700 to increase in performance and eventually take the lead for both Gravimetric and Volumetric densities. When will this happen? We don't know, but we're always watching and testing cells and will make the switch when they are ready.