Tesla battery pack capacity confusion
August 9, 2022 8:16 AM   Subscribe

I'm confused about the Tesla battery pack specifications. My understanding is that when you add cells in series their overall capacity stays the same - it's only when you add them in parallel that you add the capacities. But the spec seems to be as if the modules that make up the pack are both in series and in parallel! Maths below the fold.

A Tesla module contains 6 series x 74 parallel 18650 cells. This gives 6 x 3.7V = 22.2V total, and 74 x 3.4Ah x 22.2V = 5.6 (rounded)kWh capacity. So far, so good! My calculations match the specs.

16 modules are wired in series to create the battery pack. I'm confused because the total capacity is given as 89-90kWh. This is 16 x 5.6 (89.6) but that would only work if the modules were connected in parallel! If they are in series then isn't it still 5.6kWh?

What am I missing here? I'm guessing it's to do with the configuration within the modules? Or just some weirdness in how the specs are given?

(This is to satisfy my curiosity, no actual battery use is intended. I don't even own a Tesla.)
posted by dowcrag to Science & Nature (3 answers total) 1 user marked this as a favorite
Best answer: A kWh isn't just another unit of Ah (amp hours). It includes the voltage. So if they are connected in series (which I have no idea) you'd see higher voltage and the same amp-hours, which would multiply together to give kWh.
posted by Lady Li at 8:27 AM on August 9, 2022 [1 favorite]

Response by poster: YES! I'd forgotten that the voltage is increased. Thanks!
posted by dowcrag at 8:33 AM on August 9, 2022

Best answer: To clarify, a (kilo)Watt is a unit of power, which is energy per time, and is equal to the amperage (how much current) times the voltage (something like how hard the current is being pushed). Kilowatt-hours is multiplying energy per time by time to just give you a unit of energy. With a given set of cells, the amount of energy in them is the same no matter how they are connected.

Batteries operate at a fixed voltage, so their KWh is directly translatable to Ah, but it's easier to measure the current draw directly, so their capacity is given in Amp hours so you don't need to d an extra step when you're trying to figure out how long they will last at a given current. If the batteries are in series, each will adding their voltage but drawing the whole current for the load, if they are in parallel, the batteries won't be adding their voltage, but will only be providing their fraction of the current and so will last longer. You're getting the same amount of energy either way, (disregarding fiddly things like changes to the efficiency of the cell based on current draw.)
posted by Zalzidrax at 10:01 AM on August 9, 2022 [1 favorite]

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