2017 Chevrolet Bolt EV (full specs/gallery here) - 0 -30 mph in 2.9 Seconds (InsideEVs/Tom Moloughney)
Keep it Simple is the rule of the day
GM's Volt, and Tesla's Model S both use an active liquid battery cooling system. Tesla snakes a flattened cooling tube thru their cylindrical cells resulting in a very simple cooling scheme with very few points for leakage.
Tesla Battery uses a simple flattened tube which snakes thru the cylindrical cells
GM Volt and Spark EV use thin prismatic shaped cooling plates in between the cells with the liquid coolant circulating thru the plate.
GM Volt uses active cooling plates between the prismatic cells: Courtesy General Motors
The BMW i3 cools the bottom of the battery case with refrigerant eliminating the liquid coolant entirely.
The Volt cooling scheme is very effective from a cooling point of view but it is complicated. The cells are encased in multiple plastic frames.
GM Volt uses multiple repeating frames stacked together: photo courtesy Karl Reque, Composites World
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 a see a passive plate which is wrapped around each cell. Keep it simple.
Bolt EV appears to use simpler passive inter cell cooling
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 .
Bolt EV Specs Via General Motors
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 the Spark EV when A123 was the supplier of the Sparks battery. This Spark bottom cooling plate was abandoned however when LG Chem was chosen as the battery supplier for the 2015 Spark in favor of the cooling scheme used in the Volt.
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.
Dana bottom cooling plate: Courtesy Dana Corp
Simpler, lower cost and less prone to leaks.
The Chevy Spark EV uses a very compact gearbox arrangement called a co-axial gearbox. It is co-axial 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.
Chevy Spark co-axial gearbox is compact (via GM)
Bolt EV gearbox is still co-axial
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.
Bolt EV uses simple parallel helical gears like Tesla and BMW (via GM)
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.
About the authors: George Bower is a retired mechanical engineer with over 20 years experience in gas turbine power systems.
Co-Author of the piece, Keith Ritter is a mechanical engineer, and licensed professional engineer with over 35 years of experience in heating ventilation and air conditioning systems.