Understanding Tesla's lithium ion batteries

Guest Blog Post: George Hawley*

Tesla cars are powered solely by the electrical charge stored in batteries and are termed Battery Electric Vehicles or BEVs.  The reason for the existence of Tesla as a company is simply that Lithium ion batteries have the highest charge capacity of any practical battery formulation in history for the money, high enough to make BEVs practical. 

Above: A look at the new Tesla Model 3 (Image: InsideEVs)

The idea for using Lithium ion rechargeable battery cells was first proposed by a British chemist in the early 1970s.  There is an in-depth review of Lithium ion battery cell development in this Wikipedia article.  The television show NOVA (see below) devoted an episode to lithium ion cells in early 2017 that demonstrates the advantages and dangers associated with Lithium ion cells.  It is illuminating, if a little lightweight, technically.

Above: Nova's Search for the Super Battery (Source: PBS)

Battery cells are deceptively simple devices consisting of three basic components: two electrodes, the negative anode and the positive cathode separated by a chemical "soup", called the electrolyte.  When Lithium ion batteries are charged, Lithium ions are forced to migrate to the negative electrode where they are deposited.  During discharge the Lithium ions reverse direction for the Cathode.

Above: How a rechargeable lithium ion battery works (Image: How Stuff Works)

Tesla has been using 18650 cells manufactured by Panasonic in Asia in the Models S and X cars since 2013.  These are small battery cells, slightly larger than the standard AA cells.  The Tesla cylindrical cells are 18 mm in diameter and 65 mm tall.  The Panasonic design, perhaps with input from Tesla, is by some accounts one of the most robust formulations available today, yielding very long-lived, reliable performance in the harsh automotive environment.

Above: Panasonic's 18650 battery cells used in the Tesla Model S and X (Image: PBS)

The most popular battery pack supplied by Tesla contains 7,104 18650 cells in 16 444 cell modules capable of storing up to 85 kWh of energy.  In 2015 Panasonic altered the anode design, increasing cell capacity by about 6%, enabling the battery packs to store up to 90 kWh of energy.  More recently, Tesla engineers reconfigured the internals of the battery pack to hold 516 cells in each module for a total of 8,256 cells capable of storing a little more than 100 kWh of energy enabling the cars to enjoy a range of over 300 miles.

Above: Inside a Tesla Model S battery pack (Source: Electrek)

In order to further improve cell efficiency and lower costs Tesla has built a large battery factory in Sparks, NV near Reno called Gigafactory 1 that is now producing a new cell design called the 2170 because it is 21 mm in diameter and 70 mm high to be used initially in Tesla Powerwall home storage products and Powerpack utility storage products as well as the new Model 3 sedan, designed to be smaller and less expensive than the Model S.  The 2170 design is 46% larger in volume than the 18650 and 10-15 % more energy efficient than the 18650 cells, according to J. B. Straubel, CTO  of Tesla.

Above: Comparing the Model S/X 18650 battery cell with the Model 3 2170 battery cell (Image: DNK Power)

One of the key requirements for electric car batteries, especially on road trips, is that they need to be recharged relatively quickly.  Since batteries are direct current (DC) devices and home electrical service is AC, charging at home typically uses a 240 volt circuit supplying 40 amperes (about 10 kW of power).  The car has built in charging circuitry that rectifies the AC, converting it to DC.  Charging this way typically takes several hours.  Tesla has installed Supercharger DC charging stations worldwide that supply up to about 135 kW of power.  The DC bypasses the car's charging circuitry and charges the battery pack directly.  This is much faster, requiring 20 to 40 minutes typically.

Above: Tesla vehicles charging at a Supercharger station (Image: Motoring Research)

The Tesla battery packs using Panasonic 18650 batteries can charge no faster than this.  The maximum charging voltage for a Panasonic cell is 4.2 volts.  Panasonic specifies a maximum charging current of 2 amperes per cell.  Tesla allows charging current to be up to 4 amperes.  Therefore the maximum power that a Tesla battery pack can can use for charging is 4.2 X N X I where N is the number of cells in the pack and I is the maximum current allowed per cell.  For 85/90 kWh packs this is 7,104 X 16.8=119.3 kW.  For the 100 kWh packs it is 8,256 X 16.8=138.7 kW.  There is no way to charge faster without increasing the maximum charging current per cell which might hasten degradation of the cells or worse.

Above: Tesla Model S battery pack is positioned in the floorpan of the vehicle (Image: ExtremeTech)

All rechargeable battery cells degrade over time as undesirable side reactions take place in the cells that produce byproducts that block lithium ions from reaching the anode during charging.  Tesla battery packs are warranted against failure but not degradation.  Early indications are that 18650 cell degradation is very slow, losing only a percent or two of capacity per year at worst.  The cells are very resistant to degradation, apparently.

Above: Early indications show that Tesla Model S batteries have only degraded by around 5% after ~30,000 miles (Image: CleanTechnica)

The Tesla Model 3 cars will use the Gigafactory manufactured 2170 cells mentioned above.  The larger cells may be able to use more than 4 amperes of charging current which would hasten charging but, because the 2170 cells have more energy storage capacity than the 18650 cells, proportionately fewer will be needed to create a pack with a given kWh rating.  (N gets smaller, I gets larger). This means that higher power charging is meaningless for these battery packs.  The 4.2 X N X I relationship still applies.  It will be interesting to see how these new battery cells perform.

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*George Hawley has owned a Tesla Model S S85 and a Model X 90D. He also currently has a Model 3 reservation. Hawley worked in telecommunications primarily as a Product Planner for over 40 years.  He started with Bell Telephone Labs in New Jersey for 20 years and finished with several startup companies in California.  His career is described in his book, Tangled Wires, published in 2017.