3 ms·

Power density I can see. It would be harder to get the heat out of larger cylinders. But how could the 2170 possibly be less energy dense? You have more active

by yessql 10y ago

Power density I can see. It would be harder to get the heat out of larger cylinders. But how could the 2170 possibly be less energy dense? You have more active material per unit casing, so it almost certainly is more energy dense, based on geometry alone. Add the fact that you will have fewer connections, and the pack will be more energy dense as well.

owenversteeg 10y ago
The gains in energy density due to casing/connections are generally negligible. For reference, individual 18650s can range from 42 to 52 grams. In regards to "But how could the 21700 possibly be less energy dense?" the answer is "Cause batteries are magic." Batteries are weird; we know how they work, and how we should be guaranteed to improve energy density if we do x, y, and z. But when we do x, the batteries explode; if we do y, they only last a few cycles, and if we do z they heat up too fast. If it was my head on the chopping block and I had to explain why the 21700s weren't as good, I'd say 'heat', but that's about as useful as saying 'magic', to be honest, and this is because pretty much every time there's a problem with batteries, it's due to heat. High power discharge? Heat. Cycle life? Heat. Thermal runaway? Well, duh, heat. Chemistry works in the lab but not in reality? Heat. Eventually, you get tired of seeing heat as the source of all your problems and move to Siberia. But in all seriousness, as I said in the comment you're replying to, I think it's certainly possible Tesla's 21700s are better than the current max energy density, I just doubt that they'll get anywhere near Musk's claims of an energy density revolution. Also worth noting that Panasonic's best 21700s are less energy dense than their best 18650s. (Panasonic is supplying the battery tech.) Another example of 'batteries are magic'.

3 ms·

Power density I can see. It would be harder to get the heat out of larger cylinders. But how could the 2170 possibly be less energy dense? You have more active

by yessql 10y ago

owenversteeg 10y ago
The gains in energy density due to casing/connections are generally negligible. For reference, individual 18650s can range from 42 to 52 grams. In regards to "But how could the 21700 possibly be less energy dense?" the answer is "Cause batteries are magic." Batteries are weird; we know how they work, and how we should be guaranteed to improve energy density if we do x, y, and z. But when we do x, the batteries explode; if we do y, they only last a few cycles, and if we do z they heat up too fast. If it was my head on the chopping block and I had to explain why the 21700s weren't as good, I'd say 'heat', but that's about as useful as saying 'magic', to be honest, and this is because pretty much every time there's a problem with batteries, it's due to heat. High power discharge? Heat. Cycle life? Heat. Thermal runaway? Well, duh, heat. Chemistry works in the lab but not in reality? Heat. Eventually, you get tired of seeing heat as the source of all your problems and move to Siberia. But in all seriousness, as I said in the comment you're replying to, I think it's certainly possible Tesla's 21700s are better than the current max energy density, I just doubt that they'll get anywhere near Musk's claims of an energy density revolution. Also worth noting that Panasonic's best 21700s are less energy dense than their best 18650s. (Panasonic is supplying the battery tech.) Another example of 'batteries are magic'.