Tesla uses 18650 sized cylindrical cells, lots of them. This cell size was originally designed for laptops and was driven by the dimensions of the disk drives used at the time. As computers became smaller, so did the form factor for cells. For computers, 14650 cylindrical entered the picture for a while (as drives became flatter) and now flat pouches and prismatic. This left a large overcapacity with 18650 equipment which in turn drove pricing down.

The volumetric and gravimetric energy content of the Kokam cells used in the FES vs. typical 18650 is higher. In a battery configuration, cylindricals waste space. In a rolling platform, the weight/size disadvantage of the 1850 could be tolerated but not so much with an aerial platform.

Additionally with the small 18650 cells, you need a lot of them to get to a desired energy content for a car or airplane battery. That means more BMU (battery management) again increasing weight, size, complexity, and opportunities for failure.

The battery solution (and PC diagnostics) for the FES system is well thought out given the tradeoffs. I am eager to see the final design that the GP guys are going to employ for their gliders. I certainly hope they make their batteries easily removable for charging and storage outside the glider. The volume, mass, and energy content of the proposed GP battery is on par with the FES battery although the dimensions are more long and skinny too fit in the wings.

Danny Brotto