eli5 why do electric vehicles use thousands of tiny 18650 cells instead of one big cell?

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Isn’t that incredibly wasteful in terms of space and materials? Why not just use a few large lithium ion cells?

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Batteries can only produce a small amount of voltage, maybe between 1-3 V depending on the battery. The actual voltage that you can get from the battery is significantly lower due to things like the certain parts of the electrochemical reaction being slow, etc. So even if the cell operates at a modest current, from P=IV the power output for the single cell will be pretty low. So you need to amass thousands of (if not more) cells to get enough power for your car and all its various systems. In order to use a few large cells, they would need to have ridiculously high voltages, but those reactions don’t exist as far as I know.

It’s cheaper to manufacture. You have to consider how much of the product goes to waste, time of manufacturing, AND the tooling required before you even get to the bills of materials

So the batteries are made with what are essentially layers of battery chemicals that are rolled into a tube. With a prismatic (rectangle) battery you still have to roll them, just in a rectangle shape, or stack multiple independent sheets that need connected together with tabs. This adds corners/edges that are nonuniform and lead to a higher rate of manufacturing defects and sensitivity to deformation. Especially when in a big block instead of a narrow cylinder.

So basically round is cheaper and easier to develop for high current, high capacity, reliability, resistance to deformation, etc. And 18650s just a really convenient size for general purpose so it’s had a lot of manufacturing tailored around it.

Note that lots of EV batteries use prismatic (blocky) batteries. But Tesla which uses the cylinders has a big focus on fast charge, fast discharge. And for that you need high quality, lots of surface area, and watercooling.

Most big batteries are just a lot of little batteries stuck together. I generally agree that electric cars are wasteful, but one big lithium battery would have about the same size and material demand as a bunch of little ones of the same spec anyway. Handling a bunch of 18650s is undoubtedly safer than handling big pieces of lithium, and individual batteries can sort of contain their own internal issues without damaging the entire assembly.