They arrived today. 3,000 LG Chem MJ1 cells. These are 18650 Li-ion cells with a nominal voltage of 3.6 V and a capacity of 3.5 Ah. That’s 12.6 Wh in each cell. 37.8 kWh in the whole pile of batteries. Sound like a lot? Well it’s about the same amount energy as a gallon of gasoline. A gallon of gas weights about 6 lbs, whereas that pile of batteries weights 330 lbs (150 kg). And it’s not even enough batteries. I’ll likely need to get about 2,000 more.
Detructive Battery Testing
Next week I plan to start destroying some cells. I’ll over charge them, drain them too quickly, heat them too much, whatever it takes to make them explode, melt down, of fail in some way.
Why would I be so wasteful and destructive?
For the most part, these cells are pretty safe. The chance of a single cell failing catastrophically under normal use is quite low. In pack of ~5,000 cells, the chance is 5,000 more likely. For the sake of safety I need to assume that cells will fail during flight. The battery packs I build need to accommodate failures without damaging their function and more importantly, preventing a chain reaction. So I need a solid understanding of how these cells fail and what options I have to control the failures.
A couple weeks ago I finished modeling and printing Phil’s (How Flies the Albatross) Faraday First plane. With this design Phil was considering high speed passenger transportation. It’s got a blended wing shape, but more interesting are the control surfaces. They are totally unconventional. On the outer edge of the wings are what look like winglets, but they are rudder-like control surfaces. They rotate about their vertical axis inducing roll. There are no ailerons! The ventral fin on the bottom also rotates, yielding yaw. And come to think of it, I’m not sure what his plan is for pitch control. Hmm.