Deep dive: The 2017 Chevrolet Bolt's electric powertrain - Roadshow Examining the guts of the all-new Chevrolet Bolt (pictures) - Roadshow Very detailed discussion of battery and drivetrain. Lots of comparisons to the Spark. "Many have started comparing the Bolt to Tesla's forthcoming Model 3. But unlike the Tesla, we actually know what's going on under the Chevrolet's skin."
The Bolt EV battery pack is about 60% more energy dense by weight and by volume than the Spark EV pack. It is also twice the volume. It is about twice as energy dense by weight as the Tesla S60 pack (Bolt is probably slightly better). I can't find the stats on the volume dimensions of the Tesla pack but my guess is that the Bolt pack matches or bests it there also. The actual Tesla cells are more energy dense than the LG cells but the S60 uses the same pack structure as an S85 so the packaging overhead is higher with an S60's smaller number of cells. The same likely applied to the Model 3 which will have an option to have its pack stuffed with extra cells.
Another article covering the same technical briefing. GM Engineers Discuss the 2017 Chevy Bolt’s Powertrain
I found some usable estimates for the Model S pack dimensions. The Bolt numbers are from official GM published specs. The Tesla numbers are the best estimates I could find (treat them as approximations) and are from a thread on TeslaMotorsClub.com. Model S60 Pack volume: 365 L Pack weight: 1,000 pounds, 454 kg Gravimetric density: 132 Wh per kg Volumetric density: 164 Wh per L Bolt EV Pack volume 285 L Pack weight: 960 pounds, 435 kg Gravimetric density: 138 Wh per kg Volumetric density: 211 Wh per L These numbers are better for the Bolt EV. The Bolt EV pack numbers are about 60% better density by weight and volume than the Spark EV. The numbers look worse for Tesla because they used the same pack size and structure for both the 60 and 85 kWh packs. The numbers for the S85 would be: Model S85 Pack volume: 365 L Pack weight: 1,200 pounds, 544 kg Gravimetric density: 156 Wh per kg Volumetric density: 233 Wh per L Here, Tesla does better but the Bolt still comes in a strong 2nd. It will be interesting to see what the Model 3 numbers look like since it is expected to have at least 2 pack capacities probably using a single pack size and structure like the Model S.
Curious - why the data focuses on the S60 when it's been out of production now for at least a year. Wouldn't the S70 be more apropos? .
Everyone seems to assume that the standard pack on the Model 3 will be 55-60 kWh so I think the S60 is a better comparison point until we eventually find out the specs on the 3's cells and new pack design. Presumably it will have better energy density but it will still likely have the trade off of using one physical pack size and structure for both the standard and optional bigger pack capacities. That means the standard pack will inherently be less efficiently packed and therefore will have more pack structure overhead than it really needs. The Bolt EV has only a single pack capacity and a pack that is already efficiently stuffed with cells.
But we also have to account for how much of the pack weight is also structural weight for the car itself.
Sure, if you had a smaller efficiently packaged battery then you would still need a floor in the remaining space not being used by the smaller pack. I don't know how much difference that would make. I don't think it's a big issue and it's worth doing it with a common physical pack size like Tesla is doing in order to use common parts, common design and assembly, and to have the flexibility for upgrading.
I was thinking about how the Tesla pack case forms part of the lower frame of the car. Are the weights being quoted sans the aluminum belly pan? The Bolt's pack case adds to the car's rigidity, but it doesn't appear to be an integral part of the car body.
I don't know if there is an additional underbody pan that goes on after the Bolt EV pack is installed. The pack itself is made up of several structural layers on the bottom to seal the pack but also to provide support, rigidity, and protection from under-car damage. As far as it being structurally integral to the body, I believe it is as integral as the Tesla pack is. GM has carefully modeled the rigidity and flexing of the pack and it was designed as part of the total body structure. It basically has to be since it takes up almost the entire width of the vehicle and takes up almost all of the floor space between the front and rear wheels. Here's an "exploded" view of the pack. I would imagine there is an additional flat pan not shown here which attaches underneath the pack:
