Another common argument I hear for designing a HV system over LV is the cost due to thicker wires. Do you have a comment on cost conceptions?
At the time of filming this, copper is $10.58 per kg and aluminum is $3.80 per kg. For example, I made two cables with ring terminals of the same length, but different gauges, weighing in at 45g and 130g, respectively. I simulated a system pulling 100kW from the motor controller. The copper at 28s is 96¢ and there is a BMS intrinsic cost of ~$40 for cell management. In the lighter cable system, there is a BMS intrinsic cost of ~$120 for managing 96s cells because the chips need to be put into series. These chips can also be difficult to procure due to global supply chain shortages.
If you have the ability to package your battery near the inverter and the inverter near the motor, then the bus length can be very short. If you do a sloppy job or have a challenging application like certain kinds of electric aircrafts, and you have 4ft of cable, copper is $8.63 for 28s and $2.88 for 96s. The cost difference is $5.75 in copper to be 28s from 96s, but the BMS difference is around $70 cheaper. The battery and motor are agnostic to voltage. However, the motor controller is sensitive to voltage and operates in different ranges, so there are cabling differences. People may be concerned with reducing the cost of the harnessing, but the additional overhead from the BMS savings makes up for it. Plus, you have the added advantage of human safety.
But it does seem as though there is a weight cost?
In an EV, for most applications, you can package the powertrain system in a short distance, so the difference in mass is only proportional to the length the cables have to span. As your difference in packaging height gets tighter, the cost difference becomes negligible. Even most airplanes can be low voltage as they don’t have the propeller separated from the battery.
Based on what you’re saying about these wires, it sounds like a low voltage system could work for a variety of applications.
That’s right. From high powered to low powered, it doesn’t matter how much energy the system has. It just depends on how far your energy storage system is from the source of use in the application. LV can very likely improve cost, safety, and longevity. There are always exceptions where people need to have the battery 100ft from the motor controller. Still, low voltage would have been appropriate, cost-saving, and an improvement in efficiency for most situations that we have encountered.
Another common piece of feedback I get is about charging methods: “I need a HV system for quick charging.” Is it true you need higher voltage to charge faster?
It’s up to the ampacity of the cabling. From a power electronics perspective at the charger, it couldn’t care less if it’s 500V or 50V; it is how the transformer’s magnetics are wound, the size of the busbar, and the capacitors that make the difference. You pay per kW for DC charge infrastructure. There are a lot of infrastructures that only support 350A. If you’re a 100V system, that means you’re getting 35kW charging, which isn’t that great. If you look at a system like the Tesla supercharger, we have measured our own charge current at over 740A into that charge connecter, at which point the 100V system has a 74kW charge rate, which is pretty good. That is faster than most existing companies DC fast charging options. Therefore, there is no need for a giant connector or an impractical to make connector because this is from the same existing Tesla supercharger connector. The beauty is that the existing supercharger connector uses a pair of 12mm RADSOK pins. It’s only using 1 pair for 740A. If you made a 100V system that charged at the same speeds as the Tesla supercharger, with a 3 pair of those pins in parallel, you would have a charge connector which you could package really small, with higher mate insertion and higher extraction force on the connector. Your cable would be stiffer to manipulate, but it would still work fine in that situation.
Are you suggesting a new kind of connector?
If you could start from scratch, this would be a great foundation to start from a connector capable of 3 Tesla pins and 2200A. A 110V system with 220kW charging would be a dream. That would be a plausible solution for fast charging. Interestingly, a lot of existing infrastructure for motor and controller chargers is based around 1980’s era industrial VFD drive technology. It was groundbreaking at the time, but they ran these voltages based on industry 480 3-phase and IGBT switching range optimized for 800-1200V spikes. This is the voltage range used today in cars, but that range was selected at a time when you were looking to mount your industrial equipment in different locations around your factory. If you wanted to get a MW from point to point, hundreds of yards away, then 480 3-phase was a great option that could easily carry all the power you needed long distances with minimal cabling losses. This enabled a bunch of inverter and drive parts, existing high current IGBT, and modules built around this technology for this application. But it wasn’t designed for a battery-powered vehicle. Whenever MOSFET technology gets lower RDS on resistance, the ideal operating voltage for an electric vehicle drops. IGBTs have the same intrinsic forward voltage they have had since the 80’s due to the properties of silicon, but for MOSFETs, every time silicon lithography etching can make finer resolve structures and surface features, or the MOSFET can have higher conductivity per unit area, this is why we see MOSFET’s continue to drop in RDS on resistance.
Does voltage have anything to do with cooling the battery?
No – just the C rate of the pack. Say you have a 100kWh pack in your car - if you discharge at 100kW, you’re burdening your pack at a 1C rate of discharge load. Whether this 100kWh pack was one huge series string or one huge parallel-group, if you pulled 100kW from it, it would produce identical heat per cell. What is different is that when you have high current density and high current bussing, your possibility of imbalance in current in balance are magnified. Your bussing design needs to allow symmetrical sharing of current in a validated current path to ensure that you don’t have bussing hot spots from non-uniform current density, which can be difficult to see with your eyes.
Feedback we commonly get is that “Everyone is doing high voltage, so we need to do it too.” There is a sentiment that people are more afraid of high current than high voltage because they feel high current is inherently not safe.
A dozen years ago, I had the same misconception. I’m almost embarrassed to admit this, but I wanted to build a 600V electric bicycle with small RC hobby packs. I would have done it had there been controller technology that was cost-effective and sized correctly. But this was my own misconception, even with EE training. When you initially consider the rule of I2R, then smaller A numbers = good. It’s an easy mistake to make. But 25+ EV builds later, I like to design them 16s – 20s.
So you are more concerned about voltage than current? What’s the difference between taking a high current shock or a high voltage shock?
Your skins dielectric potential prevents you from being able to take high current when you’re at lower voltages. The only way you can take high current is by touching high voltage. That’s the paradox. When humans take high current that stops their heart, it’s when they are 30-100mA range. But the cool news is that your skin’s resistance is generally protective up to 90-120V DC. With that voltage, you may feel a small current path and the tickle and may be uncomfortable, but it would be nothing imposing dangerous stress on your heart or respiratory system. They say touch-safe is below 60V, but even then, they are a little conservative, I think, having touched 120Vdc hundreds of times without much discomfort. I have unfortunately touched over 200V and luckily survived to tell the tale. That voltage causes your ribs to displace, tears your muscles and is extremely uncomfortable. Not great. I reevaluated my personal safety rules in high voltage applications after having a shock from over 200V.
That’s the danger of having high voltage systems. Life safety is obviously very important, and industries take steps with that in mind. But there is an insidious danger when it comes to high voltage – dielectrics, adhesives, and coatings break down under electrostatic stress and time. If you don’t validate your materials under voltage stress, you’ll get to find out when you build your pack if they last under voltage stress. You may say, ‘I measured the resistance on this adhesive joint, and its 100 MΩ.’ Often times it is not even as good as this, but the crazy thing is that even 100 MΩ with 1V is 6.23/1013 electrons – that is hundreds of trillions of electrons flowing across your insulator. This electron flow drives the breakdown of the dielectrics. It’s what carries metal ions through it. When testing through the base plates, you must run each polarity orientation between the materials and the interface because there is only one polarity that migrates towards a breakdown. One is pulling aluminum up, and one is trying to pull Nickel from the can.
There are a lot of things to consider when designing a high voltage system.
Absolutely. Let’s say you go with 500-1000V – everywhere you have a sharp edge on a conductor, the air breaks down spontaneously from electrostatic field forces. These make reactive species to break down plastics and dielectrics. Even when it isn’t breaking the air down, it’s inside your plastics imposing stress that drives decomposition with time. This is something I felt the most let down about from my training. They treat dielectrics as though it is an unimportant insulator you can ignore. It’s not. Polymer science is a weird one. We are lucky to have Bryan, our in-house polymer expert, on our team.