Keep it Simple is the Rule of the Day
When people hear of “battery cooling” it can sound generic and one dimensional but there are subtle differences in how respective manufacturers manage an electric car battery’s temperature.
Is it liquid coolant? How is the liquid circulated through the pack, and most important of all what is the most cost effective system?
Let’s look at a few existing systems, and then conjecture a bit about the 200-mile 2017 Chevy Bolt’s battery cooling and gearbox.
Two similar but different systems are in the Chevrolet Volt, and Tesla Model S with a common denominator being both use active liquid battery cooling.
Tesla snakes a flattened cooling tube through its battery’s cylindrical cells resulting in a very simple cooling scheme with very few points for leakage.
GM’s Chevrolet Volt and Spark EV use thin prismatic-shaped cooling plates in between the cells with the liquid coolant circulating through the plate.
The BMW i3 cools the bottom of the battery case with refrigerant eliminating the liquid coolant entirely.
The Volt’s cooling scheme is very effective at its job of cooling, but it is complicated. The cells are encased in multiple plastic frames.
These frames repeat and are then stacked longitudinally to form the whole pack. The main feed line for the liquid coolant runs along the bottom edges of the pack. This main coolant passage is cast into each plastic frame and as the frames are stacked lengthwise the coolant passage is formed. Each inter cell cooling plate is fed off this main feed line.
The problem with this scheme is there are multiple potential points where leaks can develop since there needs to be a seal between each plate but we must point out that there doesn’t seem to be a lot of problems reported in production Volts. Tesla’s system is simpler and less prone to leaks since each battery module has one continuous cooling tube.
This “repeating frame” cooling system seems to have been abandoned in the Bolt. Here is an excellent video animation of the Bolt EV battery pack and power train.
At 1:04 minutes into the video we can see one three-cell group being removed from the pack. The active inter cell cooling plates that were used in the Volt are totally absent . Instead we see a passive plate which is wrapped around each cell. Keep it simple.
Where does the liquid coolant go? It does not appear to be between the cells as used in both the Volt and the Tesla.
Consider this high resolution slide of the Bolt battery pack.
Now look at a close up view .
In this photo we can see what appears to be liquid coolant connection fitting on the front of the pack. Inside the pack we can see liquid coolant tubes. We know they are liquid and not refrigerant tubes because GM has told us so in the early spec release.
The following “bottom cold plate description” is not directly from GM but is based on the author’s inferences from GM’s high resolution photo of the Bolt’s battery pack.
We can also see that the liquid coolant tube drops down to the bottom of the pack into a flat black plate. The authors believe this is a bottom cooling plate. Bottom cooling plates for battery cooling are not unprecedented. The BMW i3 uses it and GM had just such a system in a preliminary development configuration of the Spark EV when A123 was being considered as the supplier of the Spark’s battery.
The A123 pack and cooling system were used for the Spark EV 2014 model year and then GM switched to its own pack design based loosely on the Volt pack and using LG Chem cells for the 2015 model year.
Searching the web we find that the same supplier of the Volt’s inter cell active cooling plates also makes bottom cooling plates. These bottom cooling plates can be dimpled or channeled to take the liquid coolant. The ingenious part is that the cooling bottom plate can also be used as a structural member of the battery pack. The cooling plate could also double as the structural battery tray.
Simpler, lower cost and less prone to leaks.
The Chevy Spark EV uses a very compact gearbox arrangement called a coaxial gearbox. It is coaxial because the centerline of the electric motor is also the center line of the axle shafts. Another unique part of the Spark co axial gearbox was the planetary gear reduction set.
Tesla Model S, and BMW i3 however use simple parallel-helical single speed gear reduction gear sets. What do we see in the Bolts Gearbox?
It is still a co-axial gearbox. The drive shafts are on the same axis as the motor.
However, the gear reduction set is now a simple parallel-helical gear set like Tesla and BMW i3.
Simple and lower cost than a planetary reduction set that was used in the Spark.
GM followed the “keep it simple” principal in the Bolt without sacrificing performance. This is essential if we want lower cost EV’s for the masses that are also fun to drive.
George Bower is a retired mechanical engineer with over 20 years experience in gas turbine power systems.
Keith Ritter is a mechanical engineer, and licensed professional engineer with over 35 years of experience in heating ventilation and air conditioning systems.