shark2k

So does that mean if they had the same number of batteries (6895 or whatever it is) with the 2900mAh that instead of it weighing 1000 lbs it would weigh 1300 lbs? But they would be able to get the same performance from the 2900mAh batteries at a lower weight, right? My scenario would give a better performance, correct?

You guys seem to be really smart on this forum with your mathematical calculations and understanding some of the science behind this, so don't be surprised to see me asking it to be broken down in layman's terms every once in a while. Of course, that's not to say this is one of those times, I'm just making sure I understand.

-Shark2k

TEG

First of all it is 6831! All true Roadster fans have that number memorized.

I think the "30%" increase is increase in energy storage capacity not weight. An ESS (lately referred to as stimply "the battery pack") with 2900mAh cells would likely weigh only slightly more than one with 2000,2200 or 2400mAh cells.

The idea of reducing the number of cells (and thus size, and weight) in the ESS would be if you wanted to improve performance (and possibly reduce cost) by shrinking the ESS yet keeping the same pack capacity. So you could play it either way -
#1: The same size ESS with the same number of higher capacity cells with more range, higher cost, and similar performance.
#2: Smaller ESS with less cells (but each is higher capacity), same range, lower cost, better performance.

Ultimately it would be nice if Tesla were able to offer 2 or 3 options of different ESS configurations to customers based on their driving styles.

We heard rumors of a track ESS under development...

graham

Yes! More options would be better for everyone!

However, as dpeilow pointed out a significant shift in battery weight may require new crash testing - which I can't see Tesla doing in the near term. That could be ignored for a track car, though...

TEG

To do the math:

11sheets*621cells/sheet=6831cells ... 6831 cells * 2200mAh/cell * ~3.7V/cell = ~55kWh capacity
8sheets*621cells/sheet=4968cells... 4968 cells * 2900mAh/cell * ~3.7/cell = ~53kWh

(So a slight reduction in capacity if you did it that way)

shark2k

I'm sorry. I fail. I've just been reading about projectors a lot lately and home theater stuff so I've been racking my brain. I was only 64 batteries off .

But, thanks for the clarification. That actually makes more sense and I don't know why I didn't think of that instead. I agree that the different options would be nice so it could match the driving style more closely.

-Shark2k

Joseph

Eberhard's Tesla blog post "Balance."

"Ultracapacitors are not plug-in replacements for batteries mainly because their energy density is so low. Today’s ultracapacitors store around 1% of the energy of today’s batteries...

Another difficulty with ultracapacitors is that their voltage drops steeply (exponentially in fact) with state of charge. This means that we would need a fairly sophisticated high-power switching power supply to normalize the voltage to the motor, which isn’t impossible, but is definitely tricky and costly.

None the less, I do think that new high-energy ultracapacitors may one day be the right answer for electric cars, just not now. Nobody makes ultracapacitors with anywhere near the energy density or pricing needed..."

If remember correctly, Martin is an electrical engineer and Musk's (whom we all know is very ambitious) thesis for his Ph.D is related to ultracapacitors. If these two highly technically qualified folks decided that ultracapitors aren't going to be in the Roadster, created for the sole purpose of being the superlative of the electric automotive world, I really doubt ultracapitors are ready yet.

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TEG

Yeah, if nothing else Tesla started with battery experts on board.

Tim

Joseph,

Martin's reply has been against using Ultracaps as in situ replacements for batteries. This is an entirely different argument than using them in addition to batteries. I'm not a crazy eestor fanatic or ultracaps save the world guy. They don't have the energy density to replace batteries, but they do have enough to store the amount of energy received by 5 seconds of regen due to braking at rates significantly higher than capable of by the ESS(which is limited to about 0.5C due to LiCo chemistry). They have enough to store ten seconds at 70kw which would be enough to cover almost all acceleration on the test gear track and would consequently increase battery range by increasing regen and decreasing high C discharge capacity losses.

Trust me, DC-DC boost buck power supplies aren't that difficult. The prius has one right now, and the Tesla already is utilizing a buck circuit for constant torque(V/Hz), so its more of an evolution than a redesign. He's right as to the large voltage change involved, but sizing the cap pack to 1.20 of rated energy storage(ie add another 10lbs of ultracaps) would make the voltage drop tolerable.

There are lots of reasons to have skipped a cap pack in the first iteration from a business standpoint. It is extra engineering work, and it does add weight and cost, and wouldn't increase distance for cars driven already with the intent to maximize range(which is to say, EPA numbers aren't going to change), but for a track car, and for real world driving where people actually utilize brakes it would make a huge difference. They also skipped V/Hz initially despite the fact that I don't think you can buy an inverter today that doesn't use that as its default operating mode. This doesn't mean that it was due to a technical problem with it, but rather was due to the constraints of producing an actual working car. I think arguments to authority miss the point, which is that there are numerous reasons for leaving this stuff out other than technical feasibility. Hopefully now that their EEs aren't putting out fires in simply making the Tesla work, they have time to optimize the existing drive train.

Which brings me back to my original statement that the easiest way to double track range(test gear style track) would be to swap to 2900mah cells and include a cap pack to capture regen. Of course, they still need better thermal management of the motor, but that's another issue entirely.

Tim

JRP3

I think with the lighter pack range should increase somewhat due to less weight to move around.

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Tim

Yep, adding lightness helps everything. Probably another 20 miles or so. The only downside is cost.