On June 19, 2015, the feature article on the GM-Volt website examined how the new Chevy Bolt was likely derived from the Chevy Sonic, and some of the comparisons between the Spark EV, Sonic, and Bolt.
So now the question becomes; what will Chevy use for a battery pack?
Since the Spark EV has a range of 82 miles, and utilizes a 96S2P battery arrangement, a 2.5 scale-up would give 205 miles of range, thus the new pack would be a 96S5P. Sounds pretty simple and straightforward.
However, let’s look at this arrangement. Figure 1 is an illustration of the Spark EV chassis.
Over the rear wheels, we can see the large composite housing that contains the batteries. Imagine the size of this battery system if we need to increase its size by a factor of 2.5! Not only would it take up most of the undercarriage, but it would raise the height of the floorboards by the better part of 12 inches. This doesn’t fit the mission that Reuss stated, “5-passenger with utility”. Figure 2 provides another look at the Spark EV battery pack.
Again, imagine trying to place 2.5 of these battery packs in the Bolt.
There has to be a better way.
One major step in shrinking the size of the battery pack is to increase the energy density of the individual cells. Apparently, LG Chem has been working on this for an EV. It seems that there are tradeoffs between energy density and power density. A smaller battery pack, such as the 18.4 kWh pack in the Volt, needs cells with medium power density, but that compromises the energy density potential. However, a larger battery pack that would be used in a 200-mile EV could have low power density cells (since there are many more of them). This would allow the designers to optimize the chemistry for high energy density.
This is discussed by LG Chem’s CEO Prabhakar Patil in this link, but also in subsequent linked articles.
In fact, in his interview, Mark Reuss states that it is more challenging to build a flat battery pack, and “…the battery chemistry and the development of the battery pack coincides with the ability to package something like this. This is quite different from the Volt.”
So it seems that we are seeing not only a different package (flat platform battery pack), but that the cell chemistry has been optimized for the application (long range EV).
Let’s now look at a video of the Bolt at this website.
Please see the Bolt EV Interiors B-Roll video.
At 1:13 in the video, and also at 2:02, we get a glimpse of the floor thickness. It doesn’t appear to be much thicker than that of a conventional car. At 2:28, we see an energy diagram on the main screen, and an X-ray view reveals what looks like a mattress under the passenger compartment. Unlike many hippie vans of the ‘60’s, this mattress is actually the battery pack (although not likely drawn exactly to scale).
To fit within the required volume, this mattress-sized battery pack might be about 6” high, 40” wide, and 70” long (estimate only). This equates to 275 liters. I’m not sure of any volume data on the Spark EV battery pack, however, the Gen 2 Volt’s 18.4 kWh battery pack displaces 154 liters (see Figure 3).
So even two measly Volt battery packs (28 kWh usable) could not fit in this space. So it seems that Reuss is correct, this new battery pack will be quite different from the Volt.
So how will GM construct this battery pack? How do you package cells (such as those from the Spark EV and the Volt) that are approximately 5 inches wide and 7 inches tall (see Figure 4) in only 6 inches of height? How do all the coolant lines fit in this small package?
Figure 4. Each of the Gen 1 Volt’s cells is a “building block” within the
larger battery module and pack. An individual cell is about the size of
a 5-inch by 7-inch (12.7-cm by 17.7-cm) photo frame, is less than a
quarter-inch thick and weighs nearly a pound.
So it is obvious that there needs to be not only improved chemistry that is geared towards higher energy density, but also there needs to be a more optimized cell geometry for the package. What would be the best configuration, and what would be the best cooling scheme?
As I searched the internet, I found that LG Chem has been working on a new battery system in cooperation with the DOE. Here is a link to a 2013 progress report.
There is a great deal of information on cell life and other improvements, however, I took great interest in their low profile battery pack that is cooled by refrigerant. This battery pack is shown in Figure 5.
The cooling system shown utilizes a refrigerant and refrigeration cycle, similar to an air conditioner. The cold refrigerant flows through a cold plate at the bottom of the pack. Between each pair of Li-Ion cells is a thermal fin, which conducts heat away from the cells and down to the cold plate. With temperature sensors and controls, the temperature of the cells can be maintained within a few degrees of optimum.
It also seems that there are distinct advantages to utilizing refrigerant in lieu of direct water cooling, including lower flow rates and reduced pumping losses. The following link on this type of cooling system from Parker Hannifin, states that pumping losses are only 10 percent of a direct water-cooled system.
The lower flow rates should equate to lower volume in the battery pack (less fluid to move). Another advantage would be the ability to maintain the cells at ~ 70° F, even when ambient air temperatures are much higher.
Further in my search, I located a patent application by LG Chem that seems to be related to this battery pack system.
If you click on the link “Download PDF 20140322563”, you can view the entire patent application and its associated images. Figure 6 below is a basic schematic of the system in the LG Chem patent application.
So, these low-profile battery systems could be placed in the Bolt’s skateboard chassis to provide the needed electrical energy, however, air passageways would be required for the necessary cooling air. This could add a great deal of duct work, which adds more volume.
But, if this battery pack could be integrated with the Bolt’s heating and air conditioning system, there are some significant advances that can be achieved.
So let’s look at how the Bolt’s heat/AC could be configured. See Figure 7.
This heating/AC system (air module) utilizes dampers to control the temperature and air flow in the car. Recirculated inside air and outside air is mixed and delivered to the blower. The position of the inlet damper determines the mix of inside and outside air. The heater core damper determines the amount of air that goes through the heater core. Other dampers determine which car vents supply the air. A similar air module could be employed in the Bolt.
So now imagine an integrated heat/AC and battery cooling system.
For the battery pack, the low-profile LG Chem configuration (seen in Figure 5) could be utilized, using the most energy dense cell chemistry for this application. However, the packs would include only the cells, thermal fins, cold plates, and any necessary electronics; no refrigeration system or equipment would be included. This power-dense battery pack would have external connections for “refrigerant in” and “refrigerant out”.
Now, let’s see Figure 8 for a schematic of a possible integrated system.
As seen in Figure 8, a motor driven compressor is utilized in the refrigerant system. This could be a variable speed motor whose speed is modulated to meet the demands of the system. As I describe this system, I will use four different temperature nomenclatures; hot, warm, cool, and cold.
The refrigerant is compressed in the motor driven compressor, and exits as hot refrigerant (S1). This hot refrigerant is directed to the heater core in the Bolt’s air module. This can now provide heat to the Bolt’s cabin, if necessary. The hot refrigerant continues from the air module via S2 to a radiator at the front of the car, probably located directly behind the Bolt’s front grill. After being cooled by the incoming air flow across the radiator, the warm refrigerant is directed to the expansion valve.
After expanding through this valve, the refrigerant is cold. From here it goes to the ambient air evaporator. In cool weather, this evaporator will draw heat from the ambient air. Next the refrigerant goes to the air module. Here the cabin evaporator can absorb heat and provide cool air to the cabin. The cold refrigerant then continues to the battery pack to cool the battery cells and then to an evaporator at the power unit. This would cool the power electronics and also the oil used in the motor/final drive. The cool refrigerant would now return to the motor driven compressor.
On hot days, the motor driven compressor would be working at higher load, and the heater core in the air module would be isolated. After being cooled in the radiator, the cold refrigerant exiting the expansion valve would bypass the ambient air evaporator (or this evaporator could be isolated, as we don’t want to add heat to the refrigerant in this mode). The cold refrigerant then goes to the cabin evaporator to provide cool air to the occupants, and then goes to the battery pack and power unit to provide additional cooling.
On cool days, the hot refrigerant from the motor driven compressor provides heat to the occupants through the heater core. The ambient air evaporator draws heat from the air and then continues through the system collecting heat from the battery pack and the drive unit (cabin evaporator is isolated). So now the system is acting like a heat pump, deriving heat not only from the ambient air, but also from the battery pack and the drive unit.
On very cold days (less than 30° F), there will likely be a need for resistive heating elements.
In summary, Mark Reuss states that packaging the Bolt’s flat battery pack is more challenging than that of the Volt, and also states “…the battery chemistry and the development of the battery pack coincides with the ability to package something like this. This is quite different from the Volt.”
Therefore, it seems that to package the ~ 50 kWh of battery capacity into the limited volume skateboard chassis of the Bolt will require a more energy dense battery chemistry as well as an innovative battery cooling system. It appears that LG Chem has been working on such a system for several years with funding from the US Department of Energy.
By utilizing this new “cold plate” battery cooling system, and integrating the system with the Bolt’s heat/AC system, the Bolt could not only have a smaller and lighter weight battery pack, but may also have a system with lower coolant flows, smaller radiators, an integrated AC system, and a heat pump extraordinaire!