Tesla’s system is more complicated but adds flexibility, more operating modes and failure back up
Tesla, like GM, has two main cooling loops: one for the battery and one for the high voltage power electronics. The Bolt EV’s system is simple. The two cooling loops operate independently. They are not connected. Tesla’s system in both the Model 3, S, and X allow the motor/ power electronics cooling loop and the battery cooling loop to be connected in series via a fancy four-way valve (ref and see figure below).
The fact that there’s an additional valve connecting the two loops in series OR in parallel is a bit more complicated. However, that added complexity allowed Tesla to eliminate the battery heater in Model 3. With the two loops connected in series they can use waste heat from the motor and power electronics to heat the battery in cold weather. GM cannot, but as my partner Keith Ritter has pointed out it could be easily added to GM’s system via the addition of another heat exchanger between the two loops.
On the other hand, GM has a whole dedicated glycol loop for cabin heating while Tesla does not. So there are three glycol loops in the Bolt EV but only two in the Tesla. Tesla heats the cabin directly with a high voltage resistance heater that just dumps heat into the cabin air. GM on the other hand uses the resistive heater to heat the glycol loop and then uses that warm glycol to heat the cabin air.
Professor Kelly offered some explanation for GM’s using an additional glycol loop and heater in his video @ 28:59: better control. We think there’s another explanation. GM didn’t want the high voltage heater exposed directly to the cabin for safety reasons. (via GM engineer Wop on Tour). So, while GM’s system is simpler in that it doesn’t have the four-way valve connecting the two glycol loops, it's more complicated because it needs a dedicated battery heater and because it has another glycol loop dedicated to cabin heating. Tesla dumps the heat directly to cabin air and eliminates the glycol loop.
Here’s a diagnostic mode schematic of Tesla’s Model S cooling system operating with the power electronics loop and the battery cooling loop interconnected. In this schematic the front radiator is bypassed because we are scavenging heat from the motor and power electronics:
Tesla Model S cooling systems connected
With the cooling loops connected in series, the glycol coolant goes through the battery first, then the motor and power electronics. Since the Bolt EV cannot run with the battery loop and power electronics loop connected, it can’t scavenge heat from the power electronics. GM’s system is not as efficient.
What other operating modes does connecting the 2 loops allow?
- In the event of air conditioning compressor failure, Tesla can cool the battery and power electronics with the front radiator. To picture this mode, look at figure 1 and imagine the front radiator NOT being bypassed. In the Bolt EV, failure of the AC compressor would result in battery over temperature in hot climates. About all GM could do in the event of AC compressor failure is keep recirculating the battery coolant. So, at least GM’s battery would be uniformly hot. Please note that we don’t have confirmation from Tesla that this mode actually exists. We have postulated that does based on Tesla’s schematic only with front radiator NOT bypassed.
- Track mode in the Model 3. Again, we are speculating a bit here. Tesla has not verified the specifics but it can be postulated from figure 1. In this mode we DO NOT bypass the front radiator. With the two loops connected, we run glycol first through the front radiator and then through the AC chiller to super-cool the glycol prior to going through the battery and power electronics. In addition, we know that when track mode is switched on, all the fans in the cooling system come on and pre-chill the pack and power electronics.
Tesla’s cooling system is a bit more complicated because of the added four-way valve that allows the power electronics and battery loop to be connected together. However, this added bit of complexity opens up additional operating modes that make the vehicle more energy efficient (waste heat used to heat battery), allows elimination of the battery heater and provides back up in the event of air conditioning compressor failure.
Which system is better?
Let us know in the comment section.