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Energy density can either be based on weight or volume. In the case of energy density by weight we need to know the energy of one cell (usually in watt-hours) and the weight (usually in kg). Similarly, in the case of volumetric energy density we need to know the energy of the cell and the volume.
We will calculate the energy density based on usable energy.
We will calculate the cell energy density based on usable energy= usable energy density of the pack divided by the number of cells.
As Jack Rickard says, usable energy density is a “squishy” number, and we agree. We’ve had lots of usable energy numbers bandied about. They started out at 78.3 kWh based on a Tesla/EPA test (ref). which went along with a range of 334 miles (ref).
Since then, the usable energy numbers have come down. Ingineer hacked into the LR Model 3 computer and found a “full pack energy” value of 76 kWh’s (ref). This 76 kWh number agrees with Jack Rickard’s estimate of a usable energy of the pack equal to 76 kWh and a cell amp hour rating of 4.8 a-hr (@13:33, 47:50 and 51:31 here).
However, it seems that in practice Model 3 owners are only getting around 72-74 kWh’s out of their packs. (reference Bjorn Nyland videos for Model 3 LR, 19” sport wheels, 310 mi range, 72 kWh (@16:15ref), LR AWD w/aero wheels, 347 mi range,74 kWh (@ 2:02:21ref) and Performance model 286 mi, 72 kWh (@12:19 ref). The highest number we can find is a from Ben Sullins, who ran a Model 3 LR to shut down and got 75 kWh’s (@4:55ref).
We have usable energy density numbers from 78.3 kWh’s at the high end, down to 72 kWh’s on the low end. We used 76 kWh’s in our calculation based on Ingineer and Jack Rickard. There are 4416 cells in the Model 3 LR pack, so that puts the usable energy density of one 2170 cell at 17.2 watt hours and the energy density equal to 76/4416/.070=
The Volume of one 2170 cell is .0242 liters (21mmX 70mm cylinder), resulting in a usable energy density by volume of:
How do these number stack up against the Chevrolet Bolt EV?
According to JeffN over at Electricrevs, the usable energy of the Bolt EV pack is 57 kWh’s (ref). This is the number that is stamped on the inside of the pack by GM. In practice, Bolt EV owners seem to be getting this number, some maybe 58-59 kWh’s (ref). We will use 57 kWh’s in our calculation. There are 288 cells in the Bolt EV pack, so that puts the usable kWh’s of one cell at 198 watt hours.
The weight of one Bolt EV cell is an estimate. The closest we have is from a couple of Chinese cell specs for a cell that is supposedly a Bolt EV cell= 820-850 grams (ref).
We will use the median number in our calcs= 835grams. Giving the usable energy density of one Bolt EV cell=198/.835=
So, the Bolt EV gravimetric (weight) energy density is less than Tesla’s 2170 by around 4%. Not a bad showing in our opinion. Especially since Bolt EV owners seem to be exceeding GM’s rated pack minimum energy and Tesla Model 3 owners are not. Considering that, we could almost call it a draw on the Bolt EV versus Tesla 2170 energy density by weight.
The volume of one Bolt EV cell is also an estimate. We will use dimensions of 270mmX100X 16.5=.446 liters (electricrevs ref). This may be an optimistic number. We can also calculate it from John Kelley’s video. He measured the width, height, and depth of module 1 and 10 in his Bolt EV battery reassembly video (ref@44:00:00) There are 60 cells in those two modules. So, dividing by 60 we get the dimensions of one Bolt EV cell as 16mmX 108X343= .592 liters (vs the .446 liters we used in our calcs). However, the 343mm length number includes a circuit board, so these volume numbers may be a bit high. We would have to subtract some for the circuit board. JeffN says he knows of someone with a disassembled Bolt pack and may provide us with an update on both weight and volume later.
That puts the volumetric energy density of one Bolt EV cell at 198 /.446=
*Quibbling about the correct volume to use for Bolt EV is a bit pointless since it is quite a bit lower than Tesla’s volumetric energy density= 711 watt-hours/liter by 38%.
We think Jack Rickard used an older 18650 cell in his calcs (Ref@51:31). He stated that the 2170’s energy density was better than the 18650’s.
However, our calcs indicate that the P100D 18650 cell has about the same energy density by weight and volume as the new 2170. This implies that all Tesla did with the new 2170 cell was package their best 18650 cell into a larger 2170 cylinder.
Here’s our numbers for the Tesla Model S P100D, 18650 cell:
Usable pack energy=98.4 kWh’s ( Jason Hughes number from Tesla’s computer), 8256 cells/pack=11.9 watt hours usable.
Weight=47.5 grams based on 18650B Panasonic cell spec. Jack quoted 45 grams. In that case, the energy density gets even better.
That puts energy density by weight for one Tesla 18650 P100D cell at 11.9/.0475=
And volumetric energy density at 11.9/.0165 liters=
Like we said, basically the same as the 2170, which further indicates that Tesla just repackaged their best 18650 cell into the 2170 format.
Below is the final comparison:
Tesla 2170 cell Bolt EV P100D 18650
Usable energy density, wh/kg 246 237 250
Usable energy density wh/liter 711 444 721
rThis article originally appeared on Inside EVs.