Porsche Taycan: Changing the EV Performance Durability Standard

Porsche is known for track cars. Tesla made electric vehicles quick, but they had no thermal durability. It’s well documented that the Model S can only handle a lap or two around a road course at pace before the battery overheats. The Model 3 is better as it has an improved battery cooling system, but it still derates power due to excessive battery temperatures. Porsche knew if it brought an EV to market, not only did it have to be quick, but it had to be able to withstand extended high-power usage. Being able to drive hard for a long time also translates into being able to charge faster because both require sustained high power and keeping the battery from overheating. As it turns out, it requires a full system level approach.

First, we’ll examine the basic EV architecture. Obviously, there’s the battery pack which is mounted in the floor. It’s the heaviest single component of the car, so it keeps the center of gravity nice and low. Every EV has a Battery Management System which is the brains of the battery pack. From the looks of Porsche’s design, the BMS box also contains the electrical contactors (on/off switches) and fuses. The battery is connected by high voltage cables to the motor inverters and High Voltage Distribution Box. In the case of the Taycan, it has front and rear AC motors. The battery pack puts out DC current, so the inverters that are attached to the motors converts the DC battery pack current to AC current for the motors. EVs typically have a High Voltage Distribution Box which is like the central hub for all the electrical power. Power goes in/out of the HVDB from the battery pack, charge ports, and every electrical device on the vehicle. It looks like the DC on-board charger is part of the HVDB. All of the high-power electronics like the inverters and HVDB are typically liquid cooled. In some of the super high-power charger concepts out there, the charger handles are liquid cooled.

The Taycan has two cooling systems from the looks of it. One is glycol coolant based and the other is refrigerant based. The coolant loop uses the radiator in the left front corner of the car and it’s used to cool the motors. The A/C refrigerant system has the condenser mounted in the right front corner of the car. The refrigerant goes from the compressor to the condenser and then to what I think is a combo expansion/distribution valve. The expansion valve part of it does what expansion valves do in A/C systems which is turn the high-pressure warm refrigerant into low-pressure cold refrigerant. That cold refrigerant is then sent to either the evaporator in the cabin to provide cold air to passengers or to the battery pack. The batteries sit on top of these long cooling plates which have the cold refrigerant flowing through them. In a perfect world, all the battery cells would be kept the exact same temperature. However, in the real world, that’s hard to implement. In the case of Porsche’s design, the battery cells on the left side of the car will stay cooler than the cells on the right side which implies the hotter right-side cells won’t last as long. However, there might be some tricks in the BMS system with regards to cell balancing and whatnot to mitigate the issue. This battery cooling system with refrigerant and cooling plates is the same concept as used in the BMW i3. There looks to be a high voltage heater which can be used to warm up the battery pack in cold weather.

This is the exploded view of the high voltage battery pack. The battery pack enclosure structure is extremely stiff with all those cross beams. It has to be stiff because Porsche is using it as a stressed member like how engines are often used as stressed members in open wheel race cars and motorcycles. Plus, the cross beams are used to transfer loads in the case of a crash. So now I’m going to get into why I think the cooling system is only mediocre. First, there’s nothing innovative about the cooling system as it’s basically a copy of the BMW i3 design which has been out for quite a while. The goal with battery cooling is to cool the batteries (duh). The Nissan Leaf saw massive battery life degradation because they don’t have a cooling system. So, all those Leafs in Arizona had their batteries lose capacity quickly because it’s so hot there. Li-ion battery cells like to live around 30C temperature. Once they hit that 45C-50C temperature area, they may start to lose capacity.

Looking at the heat conduction path from the battery cell to the battery cooling plates, there are many interfaces which means many resistances; each interface is a resistance to thermal heat conduction. Along with that, there’s a lot of thermal mass/capacity to manage too. It looks like the heat conduction path is: cooling plates > battery enclosure > battery module > battery cell. Between the cooling plates, battery enclosure, and battery modules are likely a heat conduction paste or thermal gap pad creating more interfaces. A paste or gap pad is required because all of those surfaces are not perfectly flat, so you need the paste/gap pad to take up the imperfections to increase the contact surface area. Porsche is using LG pouch cells inside the modules, so there’s a question in my head about the heat conduction path between the battery module shell and the cells inside. Anyway, we’re likely not going to find out for a while until the car goes on sale and a company rips one apart. From the looks of it, the cooling plates control the temperature of the battery enclosure which acts as a massive thermal sink for the battery modules. If this is the case, it should help prevent the battery cells from increasing in temperature too rapidly.

We have to look at Tesla’s cooling schemes because they are the benchmark. With EVs, how long and hard you can drive also translates into how fast you can charge. It’s the same situation in both, how long the battery pack can handle high kW of power without overheating. When driving hard, it’s kW leaving the pack. With charging, it’s kW entering the pack.

Tesla’s basic design is using an aluminum cooling ribbon that snakes between the cells in the module. A thermal gap pad covers the cooling snake to make contact with the side of the cells. The snake is roughly half the height of the cell and only makes contact with a small percentage of the circumference of the cell. The first-generation Model S battery module only had one snake going through the entire module. This is really bad. As the coolant enters the snake at the beginning of the module, it starts to pick up heat. By the time the coolant has reached the end of the module, the coolant is much warmer. Therefore, the battery cells at the end of the snake are much hotter than the cells at the beginning of the module. As Nissan Leaf owners found out, the hotter the cells get, the shorter they live. So within the first generation Model S battery module, one could expect the battery cells at the end of the cooling snake to have less capacity over time vs the cells at the beginning. So this design has a lack of contact surface area for heat conduction along with poor cell temperature distribution. It’s still better than a Nissan Leaf. Tesla made an upgrade to the Model S module by fitting two snakes in there which probably improved the cooling about 50% and reduced the difference in temperature between the hottest and coldest battery cells in the module which increases battery life. The Model 3 battery module design has something like 7 snakes all running in parallel straight down the module. Hence why the Model 3 has much improved performance on track and with charging. Porsche’s stated goal with the Taycan was repeatable performance use as they know that’s where Tesla falters. Quicker charging is also a result of having a system that can be driven hard for a long time. As I stated, I don’t think there’s anything impressive about the Taycan cooling system. So, there has to be something else going on that allows the Taycan to driver harder and charge faster than anything else on the market.

This is where we have to step back and look at the entire system. In this case, it seems Porsche going to the 800V system is the primary differentiation from everyone else. Doing an 800V based vehicle architecture must have been at great expense to Porsche because all the existing EV powertrain parts out there are for 400V systems. Therefore, Porsche ended up designing a lot of new parts instead of being able to use more off-the-shelf parts. To understand where 800V versus 400V comes into play, we have to become familiar with Joule heating (also known as Ohmic heating).

Any electrical conductor has an internal resistance. For example, copper has a lower electrical resistance than aluminum. When you push electrical current through the conductors, some of the current turns into heat due to the resistance of the conductor. This is Joule heating. Remembering our basic equations about DC electrical current, Power = Volts (V) x Amps (I). Volts = Amps x Resistance (R). P = V*I, V = I*R. After a little rearranging, P = I^2*R. Battery cells have an internal resistance too. So, you can see that the heat (power) that the battery cooling system has to reject from the battery cells is a function of the current squared times the resistance.

I found numbers for the quantity of cells and capacity of the Taycan and Model 3 battery packs. Accuracy may be off a little, but it clearly illustrates the benefit of Porsche using the 800V system. A quick note, ‘400V’ and ‘800V’ are not true numbers. Li-ion cells have a nominal voltage around 3.6-3.7 volts, a peak voltage of 4.2V and a end of charge of around 3.0V. So, the nominal voltage of the Model 3 pack is around 360V and the Taycan around 720V. I found numbers for the time to charge for the stated range of State of Charge (SOC) and calculated the average kW and therefore amps. I tried to find similar SOC range to minimize the variables. Anyway, notice the Taycan charges at about 50% higher power than the Model 3 but at a current that’s about 1/3 less. Because Joule heating is to the square of the current, the heat the Taycan has to reject compared to the Model 3 is about half. So, even though the Taycan doesn’t have a stellar battery thermal system, it only has to reject about half the heat compared to the Model 3.

Okay, that isn’t really the whole picture. If you recall from high school physics again, wiring resistors (i.e. battery cells in this case) in series increases the resistance of the circuit by the sum of the resistances (R1 + R2 + R3…). If you wire resistors in parallel, the resistance is the summation of the reciprocals of the resistances (1/R1 + 1/R2 + 1/R3…).?Therefore, to get to 800V, Porsche has to wire twice as many cells in series doubling the resistance of a single 800V circuit of battery cells compared to a 400V. So what’s the answer?

Porsche’s pack level resistance may be lower because it has a magnitude fewer cells. I’m assuming the Taycan battery modules are essentially the same as those in the Audi e-tron which uses 12 pouch cells of 60Ah capacity each. The cylindrical cells in the Model 3 are around 5Ah each. Doing some quick math, the Model 3 needs 12 2170 size cylindrical cells to equal the capacity of one pouch cell in the Taycan. Maybe we can assume one massive cell has less resistance than 12 smaller ones wired together. Also, every battery cell has two electrical joints which means two spots of additional resistance. The Model 3 has about 10x more battery cell electrical joints. There are a number of other factors like the resistance of the HV connectors, the sizing of the HV cables and electrical busbars and things like that. But 10x more battery cells and joints is a big magnitude difference in the resistance summation game. The higher individual capacity of the pouch cells may be the significant contributor to the lower overall pack resistance compared to the Model 3 in addition to the system operating at 800V.

Another system level consideration is this gap between the battery modules in the pack. The gap is there to allow a place for the feet of the rear passengers. Passengers are the next heaviest thing in the car, so having people sit lower?reduces the center of gravity. It also allows the roof line of the car to stay lower for better aero. It’s just a pain in the ass for the battery hardware engineers to do the rearrangement of the battery modules, cables, cooling, etc.

Some other little meaningful features Porsche touts is the shape of the copper wires in the motors. To maximize the power density of an electric motor, as much copper as possible needs to be crammed in. To cram in more copper, square wire is used instead of round. I used some masterful Power Point shapes to illustrate the better utilization of space of the square wires.

You may be asking, what do I think is the best battery cooling scheme out there right now? Of what’s public knowledge, I’d go with Rivian’s concept of a cooling plate at the base of cylindrical cells. You’re going to point out to me that the contact surface area is low because it’s just the bottom of the cell touching the cooling plate and you’d be correct. Here’s the thing however, the conduction heat transfer coefficient for a li-ion cylindrical battery cell is not the same in all directions. It turns out, the conduction heat transfer coefficient is much better axially compared to radially. Tesla is getting heat out on the circumference of the cell, so radially. The cooling plate concept that Rivian has shown is grabbing the heat in the axial direction. So even for the same contact surface area, a lot more heat can be pulled out of the cylindrical battery cell from the bottom.

An important performance metric when it comes to EVs is range. The Taycan’s rated range is not impressive for the battery pack capacity; i.e. it has poor efficiency. Elon Musk has rightfully boasted that the competition’s modern EVs can’t even compete with the first-generation Model S in efficiency. The Taycan has a stated low coefficient of drag. So why the mediocre efficiency? One part of the puzzle could be the tires. Porsche makes track cars and fitted appropriate track size tires to the Taycan.

Has Porsche made a step change in EV performance? I believe they have. Tesla made EVs fast and gave them range. Porsche has taken the next step and made an EV that can handle sustained high-power usage, whether that be driving quickly or charging quickly. Don’t forget that Porsche came out with the 918 in 2013. The 919 started racing in 2014. Porsche definitely has some background in high performance powertrains involving batteries and high power electronics. The Taycan has already laid down an impressive lap at the Nurburgring. Once they hit the streets, we will see how it does in the real world in the hands of normal owners