Mystery of the Missing 5 kWh

[WhiteKnight]

I spend way too much time on this website tracking all that has been said about the Model S but I have not heard any discussion yet about the "missing 5 kWh."

When adding 20 kWh, by moving up from the 40 kWh pack to the 60 kWh pack, range jumps by 70 miles from 160 to 230 miles. This is despite a weight increase of approximately 400 pounds as the battery pack moves from roughly 6,000 cells to roughly 9,000 cells.

But when adding 25 kWh, by moving up from the 60 kWh pack to the 85 kWh pack, range jumps the same 70 miles from 230 to 300 miles. And this occurs without an increase in weight but is instead achieved by using a better, more advanced, battery cell chemistry.

Why does that extra 5 kWh not produce an extra 17.5 miles in added range especially since weight is not increasing?

All these range estimates are calculated at a constant 55 mph. Are all kWh not created equal?

 

[GSP]

Good question. We will need to wait for the EPA range numbers to know the true difference. Right now we just have early marketing/sales numbers.

One possibility is that the chemistry in the "300 mile" cells needs more of the SOC range to be avoided to get adequate life. So, yes, all kWh are not created equal, at least when discussing battery "nameplate" ratings.

GSP

 

[richkae]

Where did you get the weight and cell count data?

 

[qwk]

We don't know what type of cells Tesla is using for each pack. If they stick to what they said initially, the 300 mile pack will be the heaviest because the 3.1mah cells weigh about .5 grams more.

 

[Lloyd]

That makes only about 20 lbs.

 

[NielsChr]

You may see this tread
+100 Kwh battery
Where a detailed review of diffrent cells are discussed in detalis - also how the life of the cells are expected to be - you need to to go 1:15 hour into the video...

It is likely that Tesla have forseen that the larges (new) types of cells losse some capacity the first year of use, and therefore added the 5 Kw extra to compensate this
Or it is likely that Tesla have take into consideration that these cells does not keep the 80% capacity for as long as others cell types - but these cells does how-ever keep 70% capasity for longer time - haveing 5% extra to start will compensate this in long term.

- this means that your 300 miles version might run a litle longer the first couble of months.


This is all gestimates, and no one really knows yet beside Tesla.

 

[stopcrazypp]

Tesla has stated there will be about 5500 cells for the 160 mile pack and 7800 cells for the 60kWh and 85kWh packs.

For people curious about the difference in degradation of the cells, I have previously posted two fairly instructive graphs (charting Model S range vs cycles) here:
http://www.teslamotorsclub.com/show...ells-for-Model-S?p=61317&viewfull=1#post61317

Note that the 3100mah cells drop to about 3000mah in about 5 cycles. That may be the reason for the lower increase in range.

 

[ahimberg]

seems like some of the initial degradation should be included in the 85kWh number then if it is so fast? You buy a "85kWh" pack and its an 82kWh pack a few weeks later?

or will the battery warranty % degradation before warranty-covered replacement be at different %'s per pack size to account for the different cell types?

 

[richkae]

Tesla has stated there will be about 5500 cells for the 160 mile pack and 7800 cells for the 60kWh and 85kWh packs.

For people curious about the difference in degradation of the cells, I have previously posted two fairly instructive graphs (charting Model S range vs cycles) here:
http://www.teslamotorsclub.com/show...ells-for-Model-S?p=61317&viewfull=1#post61317

Note that the 3100mah cells drop to about 3000mah in about 5 cycles. That may be the reason for the lower increase in range.

It is important to remember that those graphs show extreme charge/discharge cycles ( full charging to 4.2volts and discharge voltage is different for each 3.0volts for one and 2.5volts for the other )
There is significant reason to believe that the performance with be different under the cycle range that Tesla will allow,and what will be typical.
Somewhere there was a document that describes the voltage ranges Tesla uses in the Roadster ( something like 4.1volts for range mode, 4.05 volts for standard mode, and doesn't discharge below 3.X volts ) but I can't find it ... how about you TEG? TEG, how long will it take you to find it...

 

[stopcrazypp]

seems like some of the initial degradation should be included in the 85kWh number then if it is so fast? You buy a "85kWh" pack and its an 82kWh pack a few weeks later?

or will the battery warranty % degradation before warranty-covered replacement be at different %'s per pack size to account for the different cell types?

I guess that would make more sense. The only other technical reason I can think of is maybe Tesla will limit the available capacity on the 40kWh pack more vs the 60kWh/85kWh packs. The non-technical reason may simply be that the numbers are very rough (the EPA numbers may be different).

 

[AndrewBissell]

I also noticed this but assumed that the quoted capacities were rounded, however on further reflection this doesn't work.

If we assumes miles/kWh is constant, and that the 85 kWh number is closest to "true" (I.e. is least rounded) then that one gets 3.53 miles/kWh. Applying that to 160 miles implies a 45 kWh battery; applying it to 230 implies a 65kWh battery.

Looked at the other way up:

40 kWh. 160 miles. = 4.00 mpk
60 kWh. 230 miles. = 3.83 mpk
85 kWh. 300 miles. = 3.53 mpk

Thoughts:

1. It is related to rising mass (but the mass rise on the second step 230 -> 300 is too small)
2. Battery parameters mentioned above mean the 85kWh battery is only that capable for a few drive cycles after which it's really a lower capacity.
3. Tesla could be assumIng different weightings of city:highway drive cycles between models when they rate their range - after all you won't normally drive 200-300 in the city. More battery means more highway miles. So this could be Tesla reducing the chance of dis-satisfied customers.

 

[user497]

Note that the 3100mah cells drop to about 3000mah in about 5 cycles. That may be the reason for the lower increase in range.

I imagine that Tesla would cycle the car the 5 times before delivering it so that the customers wouldn't see a drop in the first few weeks they own the car right?

 

[Doug_G]

I imagine that Tesla would cycle the car the 5 times before delivering it so that the customers wouldn't see a drop in the first few weeks they own the car right?

Not even necessary... they can program the firmware to report what they want to report...

 

[Kipernicus]

If we assumes miles/kWh is constant, and that the 85 kWh number is closest to "true" (I.e. is least rounded) then that one gets 3.53 miles/kWh. Applying that to 160 miles implies a 45 kWh battery; applying it to 230 implies a 65kWh battery.

Another way to look at it, which would be quite concerning to many, is that the 40kWh pack will only provide 141 ideal miles and the 60 goes 212.
We will have to wait for official EPA figures and final announcements on battery chemistry, # of cells, etc.

 

[WhiteKnight]

Another way to look at it, which would be quite concerning to many, is that the 40kWh pack will only provide 141 ideal miles and the 60 goes 212.
We will have to wait for official EPA figures and final announcements on battery chemistry, # of cells, etc.

Rather than the 40 kWh and 60 kWh being worse than advertised I'm guessing that the 85 kWh might be better than advertised (how long that better than advertised range lasts is up for debate).

There was a comment on a different thread that said (paraphrasing) that J.B. Straubel commented that greater than 300 miles of range is possible but that 300 miles is the sweet spot. So whether Tesla says 300 miles of range or 318 miles of range does not really matter much from a marketing standpoint so they choose to be conservative and throw up that round number with a 3 in front.

Since the leap from 60 kWh to 85 kWh was achieved through a different chemistry in the cells maybe it did not make any sense to use slightly less cells and just go with 80 kWh?

 

[Doug_G]

Is it possible that the larger cells have slightly higher internal resistance, so the efficiency is slightly lower?

 

[ahimberg]

I guess another question is, if tesla is now selling/marketing the battery size in kWh's, where in the car can you check your current charge's kWh to know how much of your 85kWh you still have?

 

[VolkerP]

In the Roadster, the remaining SOC is reported as % and as ideal miles left.

 

[Brian H]

seems like some of the initial degradation should be included in the 85kWh number then if it is so fast? You buy a "85kWh" pack and its an 82kWh pack a few weeks later?

or will the battery warranty % degradation before warranty-covered replacement be at different %'s per pack size to account for the different cell types?

Maybe it is. The early numbers I read were for a 90kwh pack; maybe it's been "lowered" to account for that fast drop-off.

 

[Andrew Wolfe]

I imagine that Tesla would cycle the car the 5 times before delivering it so that the customers wouldn't see a drop in the first few weeks they own the car right?

With 425KwH of power! I don't think so. That's almost a month's consumption for an average Californian. http://205.254.135.7/cneaf/electricity/esr/table5.html

Not too good for their environmental cred....