Hi. First, I’m enjoying the game immensely - thank you very much! 
Ok, so I would suggest that the Fuel cells could stack in a way similar to the EMP cells. Right now it doesn’t really make any sense to pick up the smaller fuel cells, since you can’t have any more of them than the larger ones.
Furthermore, I would suggest that both Fuel and EMP cells would stack like the following:
Small cell: 5 stacks (like EMP is now)
Medium cell: 3 stacks (EMP is 2 now)
Large cell: 2 stacks (EMP is 1 now)
I’m not sure where Car battery/Gas tank fits into the above. 
In this way, I think it scales nicely with the power of the cells compared to how many you can hold.
