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Understanding Batteries – Why so many cells?


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Understanding  Batteries – Why so many cells?

Have you ever wondered why some EV batteries have thousands of cells? Why the Volt’s battery is so big even though it was “only” 16 kWh? Can you actually save about half the money if you have a 50 kWh battery instead of a 100 kWh pack?

(1)    Cells and batteries: Before we get started, let’s get the terminology right. A “cell” is the smallest element in a chemical charge storage device. A “battery” is a bunch of cells strung together. The 1.5V AA Duracell is a “cell”. A pack with four of them strung together for a total of 6V in your flashlight referred to as a “battery”.

(2)    EV Batteries: A lithium ion EV battery is typically made up of many individual ~3.65V “cells”. EV batteries string up many of them to achieve their high operating voltage of hundreds of volts. The Tesla Model 3 battery has 2,976 individual #2170 cells as 31 parallel strings with 96 cells a piece to achieve 350.4 volts. The Chevy Bolt strings up 288 individual cells in 3 strings of 96 for 350.4 volts.

(3)    C-rating: While we hear a quite a bit about battery capacity and voltages, we seldom hear about C-ratings. This is perhaps more important if you are looking to get a set of electric wheels to go fast. The C-rating is a battery’s discharge capability in relation to its capacity. A 1C 1000mAh battery can discharge at discharge its entire 1000mAh capacity in one hour. A 4C battery can discharge its 1000mAh energy in 15 mins. The Tesla Model 3’s 211kW (283 hp) reluctance motor will discharge the base 50kWh battery pack at a maximum rate of 4.22C.

(4)    Series or Parallel: When you connect multiple cells in a series (end-to-end) the voltage and capacity (kWh) goes up, but the discharge rating does not. Five 1C 1000mAh batteries in a series has 5 times the voltage and five times the energy storage capacity, but it is still a 1000mA battery. When you connect batteries in parallel, the voltage does not go up, but the capacity does and the discharge rating does.

(5)    Why so many cells in a battery: Put simply, you need enough cells in series to get to the voltage you need for the motor and you need enough of these series in parallel to get the discharge rating require so the battery doesn’t blow up. That Tesla Model 3 draws 211 kW / 0.35 kV = 602.9 A under maximum power. If you put 602.9 Amps through those tiny #2170 lithium ion cells, it’ll boil the electrolyte and set things on fire. That is what happens if you only have 96 tiny lithium ion cells in series to give you the operating voltage of 350.4V. But, if you divide that amongst 31 parallel string of 96 cells, each only has to see 19.4 Amps – tolerable if the battery is liquid cooled.

(6)    Many little ones or a few big ones: To make EVs go fast you need powerful motors and powerful motors need a lot of Amps. There are two ways to make batteries tolerate a lot of amps. One is to simply make them with larger electrode areas – the D sized cell tolerates more current than the AA cell for instance. The other is to use a lot of small cells in parallel. Tesla is an ardent advocate for lots of small cells, while GM is firmly in the “fewer bigger ones” camp. The Bolt’s 66kWh battery uses only 288 cells in 3 parallel strings of 96 cells. Its 149 kW motor puts 149kW / 0.35kV / 3 = 142 Amps through each of those large, slate like, GM prismatic cells. Boeing is also in the fewer and bigger is better camp with those Boeing 787 batteries that caught fire repeatedly early in the plane’s career. So I guess bigger is not good enough when it’s air cooled while sealed in a box. If you are wondering the good old 12V lead acid battery in your car has six nominally 2-volt cells in series with massive electrode plates to support hundreds of cranking amps.

(7)    Lower Capacity doesn’t necessarily mean fewer cells: As you can probably deduce by now, lower capacity batteries do not necessary have fewer cells. Why? Because you need a string of about 100 cells to get to 365 volts so you can run the kind of motor you typically need to move 3000 lbs to 5000 lbs worth of car, gays and guys’ stuff. However, unless each cell is capable of running the Amps the motor demands, you also need multiple parallel strings of cells for a high enough C-rating (discharge current tolerance). While total material and weight is certainly down to almost a third, the complexity may not go down that much. This is why batteries a third the capacity are usually not third the price.

I hope this helps everyone understand batteries better!

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