Update: After trying 5+ different kinds of 18650 lithium batteries, from high spec LG HG2 to no-name generics I’m pretty certain that 18650 is not the way to go. The weight to energy to cost ratio is not where it needs to be, if I could find 3000mah 5amp drain cells for $1.50 each, it would be one thing, I cannot. As it stands the best 18650 I tested had 1600mah between 4.13 and 3.3 volts at a 1.5amp discharge rate, and weight 42 grams and costs $1.75. Fitting out the pack like I showed in a previous post with 4700 cells total ends up at a specific energy of 7grams/watt hour and the cells would cost $8100 for pack energy of 28kwh.
Now the update is the EBL 26650 cells I got my hands on. these guys provide 4.8AH between 4.13 and 3.3 volts are rated at 1200 cycles, which is 3x longer in shallow cycles, so its there. I am hoping that I can get these for $3/each (currently $4.35/each from ebay), and in the high density arrangement I came up with, which I think is really cool, I can get those numbers to move in the right direction. The specific energy would be 5.0gram/watt hour, and a pack energy of 47.5kwh and a cost of $7800 in cells. There is a lower density layout that is 37.5kwh and $6K.
My next steps are to find a high volume-low cost supply for these batteries and again, make one module and test the cooling approach I talked about before. I didn’t have a chance to test it on the 18650 because I didn’t want to waste the copper bus bar materials since the cells could not have worked.
I did find a new tool to help investigate cells, I found that if I X-ray them I can see how many wraps they have in the cell, how big the center pin and from that the square area of the electrodes. The 18650 that performed the best had a center pin diameter of 6.5mm, and 19 wraps of the electrodes, this means each wrap is .29mm thick and it has 13,000mm^2 of area. Then 26650 has a center pin diameter of 3mm and 31 wraps for an electrode area of 26,250mm^2, twice the 18650! And the energy works out to 5mm^2/milliamp hour for the 26650, and 8.3mm^2/milliamp hour for the 18650. The EBL 26650 also has a thicker layers at .37mm, so that might indicate durability or other robustness.
Also I really could use an empty battery shell to do fitment work on, if anyone knows of one in the SE Michigan area, please PM me. Lots of people sell the leaf modules, but the cases must be scrapped.