It wasn't the biggest news to drop this weekend, but the Financial Times and Wall Street Journal both confirmed the rumor that Nikola signed a contract with Romeo Power to use their batteries in the Tre. Here is a (long) thread explaining why this is so important.
Not only does this represent yet another lie from Nikola (Trevor stated on several occasions that the Tre would be using Nikola batteries) but it also has forced them to make a grave design error. This thread will be a little technical, but I'll try to explain this crucial error.
So, a little background on battery design. When you are designing a battery you have two separate things that you have to account for: capacity, which gives you range, and voltage, which allows you to provide necessary power. You get both of these by connecting together cells.
There are two different ways you can connect cells: In parallel, which increases capacity while keeping voltage unchanged, or in series which increases capacity AND voltage. An automotive scale battery will have cells connected in parallel and then those groups in series.
The problem comes when, like Nikola, you are stuck using off the shelf modules: For these, the cell groups are already set for you and you are stuck with them. If it isn't what you need for your battery design, you're stuck. This is why custom battery design is crucial.
In the case of Nikola, the Hermes modules they are using from Romeo Power have a capacity of 10 kWh and have a voltage of about 100 V when topped off.
To power their truck, they need 800V, so that's 8 modules connected in series.
But this sets the capacity for them: 8 modules of 10 kWh each comes out to 80 kWh. While that is a great number for the passenger vehicles that Romeo presumably designed these modules for, it isn't nearly enough for a semi truck. The Tre needs 720 kWh--9 times as much!
So they need 72 Hermes modules. But they can't just put them in series or the voltage will be 7200 Volts, far too much to be safe in an EV. So it is impossible to properly design a battery pack using these modules, and Nikola was forced to improperly design one. VERY improperly.
There is one other way to connect battery cells, and this is the "parallel strings" method. In this method, you connect your groups in series to get your voltage, and then connect THOSE strings in parallel to get your capacity. THIS IS A REALLY BAD IDEA AND SHOULDN'T BE DONE.
If anything goes wrong with one of the parallel strings, the other strings can rapidly feed electricity into it, causing catastrophic failure of the pack or even fire. You have to have extra fuses, extra switches, extra management system, and exponentially more exotic software.
All of this extra complexity causes more failure points that can result in the loss of the vehicle. Not doing parallel strings is one of the first things you learn as a battery designer. There are any number of articles cautioning against it.
https://www.orionbms.com/manuals/pdf/parallel_strings.pdf
The thing is, on rare occassions people might parallel two strings and be super careful with their BMS and safety circuits. BUT NO ONE WOULD EVER PARALLEL 9 STRINGS, THIS IS INSANE.
But Nikola has no choice because they don't know how to make their own batteries.
