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.