It's the Batteries, Stupid!

[dpeilow]

Does that mean other manufacturers have to crash test every variant of a model with different ICEs?

If so, then this may explain a non-move to higher battery capacities. There is a small, but tangible difference in cell weight, which would translate to a few kilos or tens of kilos over the entire battery.

 

[malcolm]

Ah no. I was thinking about a major ESS redesign to capitalise on higher energy density allowing weight reduction.

I suspect Tesla may be penalised because of the "new tech" angle. but that's a complete guess

 

[dpeilow]

Maybe they could just put the fewer cells at the bottom of the ESS box, but keep the same overall form factor (to allow retrofitting to existing cars). That ought to be allowable.

 

[malcolm]

Yes 2200mAh to 2900mAh is a 30% increase. Does that mean they could knock out 3 of the 11 "sheets" in the ESS?

 

[graham]

My prognostication on "when will Tesla possibly re-crash the Roadster?"

We know it won't happen for the 2009 or the 2010 models, currently slated for Spring and Fall this year.

The 2011 model is probably out as that will be all about bringing Model S to market.

*possibly* the 2012 model could get some love? That would be my guess as the earliest possible.

 

[TEG]

...Can't find an acceptable discharge rate range...

That is a rather critical value for EVs. Unlike a laptop with a more uniform current draw, a performance EV is prone to fits of high current discharge (based on the disposition of the driver's foot!).

Tesla may not pick cells with the highest storage capacity because testing could show that some of the lower energy density cells can handle abusive discharge better in the long run (e.g.: after 100,000 miles of 'spirited' driving).

 

[malcolm]

Tesla may not pick cells with the highest storage capacity because testing could show that some of the lower energy density cells can handle abusive discharge better in the long run (e.g.: after 100,000 miles of 'spirited' driving).

That's a thought; if higher energy density cells exist toward the fringes of what can be achieved with Li-ion chemistry, does it become increasingly harder to make them stress-resistant?

In other words, within li-ion, is there a ceiling on EV applications and some commodity cells will just never be suitable?

 

[Tim]

That is a rather critical value for EVs. Unlike a laptop with a more uniform current draw, a performance EV is prone to fits of high current discharge (based on the disposition of the driver's foot!).

Tesla may not pick cells with the highest storage capacity because testing could show that some of the lower energy density cells can handle abusive discharge better in the long run (e.g.: after 100,000 miles of 'spirited' driving).

Which is why I support using a small cap bank(20-50lbs) to minimize this. I know GM tried to use Safts LiNiAlCO2 cells, but discarded them due to thermal problems at high discharge rates. However, the Tesla's battery pack has a much lower dischare rate than any Hybrid or Phev, so I think they'd work(just as well as the cobalt cells they now have) fine for the Tesla. Plus right now the max regen is 27Kw, which leaves alot on the table in terms of energy recapture and would be solved by a cap bank. The other nice thing about a cap bank is that it decouples motor performance from the battery to a large extent. Which means that Tesla can go about increasing the power of the PEM and motor without regard to high C discharge on the battery and the resulting Peukert losses and thermal management issues provided it resizes the cap bank accordingly.

But we probably won't see any of this until the next iteration of the Roadster. Did anyone else notice that the quoted energy of the ESS per Road and Track pdf is now up to 59kwh? That is higher than I've seen quoted in the past. Maybe they've already upgraded the cells.

 

[TEG]

Which is why I support using a small cap bank(20-50lbs) to minimize this...

Yeah, I offered that up as a suggestion long ago. I think the response was that it is a lot harder to deal with the larger voltage swings you get with caps compared to batteries, but who knows. It sounds like a good idea to buffer the batteries with ultracaps... Not just because of the high current draw but you could potentially use the caps for quick regen/accel/regen and take advantage of their high cycle life without cycling the batteries so much.

 

[Tim]

Its more engineering work to be sure, but its not a particularly difficult challenge. Boost buck converters aren't exactly new science. All AC drives use caps exactly this way and the converters for V/HZ control. As for cost, I looked at the Maxwell website and online prices, and it looks like the requisite number of ultracaps would cost about $2-3K per car(for full accel buffering, decel buffering would be less). Expensive, but given the benefits, not overly so, especially when considering the benefits. I'm pretty sure JB is aware of all this, and it was simply a cost/design issue, more akin to the mantra that the perfect is the enemy of the good. Still, this is a pretty low hanging fruit that would make for a MUCH better track car.

 

[shark2k]

Yes 2200mAh to 2900mAh is a 30% increase. Does that mean they could knock out 3 of the 11 "sheets" in the ESS?

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]

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?

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]

Yes 2200mAh to 2900mAh is a 30% increase. Does that mean they could knock out 3 of the 11 "sheets" in the ESS?

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]

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

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 :tongue:.

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.

 

[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]

#2: Smaller ESS with less cells (but each is higher capacity), same range, lower cost, better performance.

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

 

[Tim]

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

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