Friday, May 22, 2020

IoT Platform Endurance

The Tesla-Panasonic Lipo cells are generally rated at 230-240Wh/kg energy density. The prerequisite for that number is halving discharge current and halving voltage from high performance applications, and the cell volume is there by more than doubled. Actual driving adds cooling/control and discharge limit, and the packs have equivalent 150 energy density.

The lithium manufacturing distillation electricity/fuel cost grows hyperbolically in relation to purity and energy density. For an all-around IoT platform, the cell energy density should be 180Wh/kg , saving the reserve ion for extended life and lowering maintenance cost.

The tattu 800mah 1-S cells here have such 180Wh/kg energy density. The actual capacity is 860mah. 3-S configuration is used, and mission time is 0.86Ah x 3 x 3.7V / 27.3W(cruise with HD video power) = 0.35h = 21 minutes. Each cell weighs 17.7 grams. Takeoff weight is 250 grams with older, heavier components. Total battery energy is 0.86Ah x 3 x 3.7V = 9.55Wh .

The GensAce 900mah airsoft riffle battery has actual capacity 860mah. 4-S configuration. Flight time is 21x4/3= 28 minutes. Each cell weighs 20 grams. Energy density 159. Takeoff weight is 250 grams.

The reason the DJI Mavic mini has very similar flight time of 23 minutes with HD video with 17.28Wh battery energy is because the disk area of our platform is much larger. DJI's 4 4-inch props have a combined 50 square inch area, which can be covered by a single 8 inch prop. Our IoT platform has a 19 inch prop with 280 square inches area, but the straight blades give a sloped wing loading halving the effected area to 140 square inches. We can calculate the relationship between disk area to fuel efficiency as 17.28 / (9.55x23/21) = 1.65 . So, the 140/50=2.8 disk area ratio gives roughly square root fuel economy of that ratio, 1.67. This is expected Newtonian physics. 

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