Hey, so I wrote that comment :) > A 18650 is a size and voltage specification. As in, it's like calling a battery a AA or AAA: any AA or AAA fits into a device

Hey, so I wrote that comment :) > A 18650 is a size and voltage specification. As in, it's like calling a battery a AA or AAA: any AA or AAA fits into a device and makes it run, but some AAs and AAAs last a lot longer than other batteries in flat out runtime, or survive usage in abusive devices, or in extreme weather conditions, or so forth. 18650 doesn't mean anything other than a cylinder 18mm diameter by 65mm long; it doesn't imply a certain voltage or anything else. I've worked with 18650s that have voltages from 2V to 4.4V. Nonrechargeable 18650s, though quite rare, exist at even lower or higher voltages. I've got access to several spec sheets from Panasonic, who Tesla is pairing with, in combination with public spec sheets from existing 21700 cells. My guesses at Tesla's final 21700 battery specs are a combination of these spec sheets, my own knowledge about batteries, and some physical limitations (e.x. the theoretical max power density shrinks as the cell grows, and 21700s are roughly 50% more volume than 18650s.) > There are low grade 18650s just as there are high grade 18650s. Maximum capacity isn't the only way to measure a battery, but also it's maximum discharge rate and thermal characteristics under load; as in, larger capacity isn't always better in some use cases. Yep, that's why I addressed those things in my comment. Thermal issues are particularly interesting with Tesla's packs.