Tesla Semis: Charging Ahead
John Ewbank posted on January 29, 2018 |

Finally, Tesla has revealed its Semi, but much speculation still surrounds it, primarily about the battery. In this article, I hope to provide some insight into how they can build such a beast without remaking the wheel. Tesla won’t require a revolutionary battery or charging technology; it just needs a few good old-fashioned optimizations. Tesla’s battery and performance claims are legitimate, and I will show you why. But first, let’s outline what we know about the Semi.

What We Know About the Tesla Semi

  • Four independent motors
  • 300- or 500-mile range
  • Less than 2kWh/mile energy consumption
  • 0 to 60 mph in 20 seconds with 80,000lbs
  • Charging adds 400 miles of range in 30 minutes

What Can We Learn from These Specs?

With a few bits of math, we can learn quite a lot. But as always, the devil is in the detail.

If the Tesla Semi uses 2kWh to travel a mile, then going 500 miles at 2kWh/mile would require 1,000kWh (1MWh) of power. The conditions for using 2kWh/mile are unclear, so let’s assume it’s a value that’s representative of typical use.

Elon Musk claims the Semi can add 400 miles of range (800kWh) in 30 minutes. Charging at that rate would require a 1,600kW (1.6MW)charger.

Charging a single battery pack at 1.6MW is impossible, as I explained in this previous article. With powerful chargers, you encounter two primary limitations:

  • At over 1,000V, electrical arcing becomes a problem. This is where electricity flows through an insulator, like air, and goes where it’s not meant to—shocking behavior.
  • Currents over 500A cause the contacts between the charger and the vehicle to burn. This limits our charging speed through a single charger to a theoretical 500kW, but more realistically around 400kW.

So How Would the Tesla Semi Charge at 1.6MW?

It’s all about how it builds its battery pack. If you have one giant battery, you would have to charge it at 1.6MW, but if you have separate batteries, then each can be charged at a lesser rate.

There is evidence to suggest this is what Tesla has done. This video shows the Semi charger has 10 plugs: eight large plugs in the white section and two smaller plugs in the black section (hidden on the right-hand side). The eight large plugs are for charging, while the two smaller ones are likelyearths. My supposition is that the eight plugs are four positive and four negative terminals, which connect to four battery packs. Though it’s possible that positive and negative terminals could be contained within each plug, this is unlikely due to their small size. Furthermore, the Semi has four independent motors, so one battery for each motor seems logical.

The simple explanation for how the Semi charges at 1.6MW is this: four battery packs charge from what is effectively four separate 400-kW chargers, which gives a total power of 1.6MW. However, that is very basic, and I’m sure many people would like to know a little more. So, let’s begin with a quick battery refresher. If you are familiar with batteries, or you want to get on without the theory, skip down to “How to Charge at 400kW.”

Battery Basics

Batteries, Current and Voltage

Current is a measure of the flow of electric charge, where charge is measured in coulombs (1 coulomb is equivalent to the charge of 6.24 x 1018 electrons). A current of 1 ampere is equal to a flow rate of 1 coulomb per second.

The definition of voltage makes a lot of people angry, so no doubt this definition will make someone furious. Voltage is electric potential energy per unit charge, measured in joules per coulomb. Essentially, the higher the voltage, the more energy each amp carries.

A battery is a device that supplies a certain voltage. The capacity of a battery, measured in amphours (Ah), reflects how long that battery can supply a steady current.A capacity of 1Ah means the battery can supply a current of 1A for one hour, 2A for half an hour, 3A for one-third of an hour, etc.

When you put multiple identical batteries in series or parallel, they can be thought of as a single cell, with the cell’s voltage and capacity dependent on how the batteries are connected.

Batteries in Series

Adding batteries in series causes their voltages to add, while their capacity stays the same. We can see here that three batteries in series cause the voltage to treble from 3 to 9V, while the capacity (measured in Ah) remains the same.

Batteries in Parallel

Adding batteries in parallel causes their capacities to add, while their voltage remains the same. Here the cell’s voltage remains the same while its capacity trebles.

Batteries in Series and Parallel

Combining batteries in both series and parallel affects the voltage and capacity in the same ways, like below:

How to Charge at 400kW

Charging at 400kW is faster than any other battery pack on the market, so how will Tesla do it?

First, we need to consider the makeup of an EV battery. Tesla’s EV batteries are made up of thousands of cells; these are like AA rechargeable batteries, but bigger and better. To avoid damage and prolong their lifespan, these cells have maximum charging rates. Let’s say at their maximum charging rate each cell takes half an hour to charge. Therefore, as the pack functions as a giant cell, the time each cell takes to charge will dictate the charging time of the whole pack. The larger a battery pack, the more cells it will have. While having more cells means you can use a more powerful charger, it doesn’t mean the battery pack will take less time to charge.

Both small and large battery packs with the same number of cells will charge in the same amount of time, but a larger battery pack can take on more energy in a set time than a smaller one.

My Hypothetical Semi Battery Pack

We’ve done the theory, so let’s look at a theoretical Semi pack. Recall that if the Semi has a range of 500 miles and uses 2kWh per mile, it would need a 1-MWh battery. To get the 1.6-MW charging rate necessary, we need to break the battery up into four sections and charge each independently.

A 1-MWh battery pack divided into four pieces gives us four 250-kWh batteries. These 250-kWh batteries would be broken up into smaller modules, which then would be composed of individual cells.

In my scenario, each 250-kWh battery is made up of four 400-V, 62.5-kWh modules.

In the image above, the blue and cream halves of each battery represent two 62.5-kWh modules wired together in series, which doubles their voltage. Therefore, if these modules were 400V individually, like those used in the Tesla Model S, then their combined voltage would be 800V. This is an important step, and the reason for taking it is explained in the section below.

The green and cream halves of each battery are wired together in parallel and connected to the charging port.

The primary purpose of the image is to show how each battery could be wired into the charging port. Each set of batteries is wired to one set of terminals. I’ve only show the wiring of one battery, because adding more makes the image look confusing.

You’ve Doubled the Voltage. Why?

We know that Power = Voltage x Current. We also know that the maximum current we can put through a charger is about 500A and the maximum voltage is about 1,000V.

Each of the four battery packs must accept 400kW of power to add 400 miles of range in 30 minutes.

800V x 500A = 400kW

If the pack has a voltage lower than 800V, the current would have to be over 500A,which would burn the contacts between the charger and the vehicle. A voltage over 1,000V would cause issues with electrical arcing. The Semi could use more than 800V, but from my conversations with people in the industry, I think this is unlikely.This is on the cusp of what is possible with current charging technology.

How Much Would a 1-MWBattery Pack Weigh?

Around 5 metric tons, or 11,000lbs.

If we take the weight of a Tesla Model S 90-kWh battery pack, 540kg, and scale that up to a 1,000-kWh battery pack, we get 6,000kg. Then,to factor in the weight-savings of Tesla’s new 2170 cell, and the potential for better packaging in the Semi, let’s multiply 6,000kg by 0.85,to get about 5,000kg. This is a pretty arbitrary value, and the actual weight-savings may be greater, but the specifications of the 2170 cell haven’t been released—so take this with a shovel of salt. However, I think it is fair to say the Semi’s battery pack will weigh less than 6metric tons.

How Does that Compare to a Typical Truck Tractor?

A conventional long-haul truck tractor, like a Freightliner Cascadia, weighs about 20,000lbs.

Does this mean the Tesla Semi will be very heavy? Your guess is as good as mine, but it seems that the Tesla Semi will be a couple of tons heavier than a typical truck tractor.

In Review

Nothing otherworldly is happening here; Tesla is just making massive battery packs and putting them in a truck tractor.

To add 400 miles of range in 30 minutes, the Tesla Semi would need 1.6MW of charging power. It will achieve this by breaking up a 1-MWh battery into four 250-kWh batteries and charging each separately.


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