[dougingraham]

The Panasonic 18650 laptop batteries that the Tesla Roadster uses are severely current limited. Not even 2C. I believe they are restricted to 4A. If you want to pull 500 amps that means you need to parallel 125 cells.

It is astounding to me that they went to the effort to build a pack like this with all the problems of using the little cells. You need to keep them within a certain temperature range and you need to be careful not to overcharge or over discharge them.

I haven't found a decent place to buy them either. I guess if you buy them in millions quantities you can get a good price.

An OEM can deal with all the problems/limitations of these cells. Save yourself a lot of effort and go with 100AH LiFePo4 prismatic cells.

[cruisin]

Originally Posted by dougingraham

The Panasonic 18650 laptop batteries that the Tesla Roadster uses are severely current limited. Not even 2C. I believe they are restricted to 4A. If you want to pull 500 amps that means you need to parallel 125 cells.

It is astounding to me that they went to the effort to build a pack like this with all the problems of using the little cells. You need to keep them within a certain temperature range and you need to be careful not to overcharge or over discharge them.

I haven't found a decent place to buy them either. I guess if you buy them in millions quantities you can get a good price.

An OEM can deal with all the problems/limitations of these cells. Save yourself a lot of effort and go with 100AH LiFePo4 prismatic cells.

Your information regarding the 18650 cells is inaccrate. Also, if you look in classifieds you will find the 18650 2.7ah cells for $2 with tabs. You cant beat that price anywhere.

[icec0o1]

Originally Posted by frodus

I about spit my coffee out.....

great analogy.

Funny, but incorrect analogy. The more accurate one would be a harness for 8000 tiny camels which go slow (relatively, since the model S has 0-60 in 4.4s) but for a great distance or a carriage with 8 horses that can gallop for a bit.

[frodus]

Originally Posted by icec0o1

Funny, but incorrect analogy. The more accurate one would be a harness for 8000 tiny camels which go slow (relatively, since the model S has 0-60 in 4.4s) but for a great distance or a carriage with 8 horses that can gallop for a bit.

His analogy was funny, and correct. Tesla designed around what they had at the time (small cells), not what could be (larger higher power cells). But now, we can either design how they did at the time, or design around current technology.

Yours was neither funny nor was it a great analogy. Try again.

[icec0o1]

Originally Posted by frodus

His analogy was funny, and correct. Tesla designed around what they had at the time (small cells), not what could be (larger higher power cells). But now, we can either design how they did at the time, or design around current technology.

Yours was neither funny nor was it a great analogy. Try again.

There is no more current technology than the Panasonic 3.4Ah 18650. Are you really arguing that if there was a better techonology/battery out there, Tesla wouldn't use it?

[dougingraham]

Originally Posted by cruisin

Your information regarding the 18650 cells is inaccrate. Also, if you look in classifieds you will find the 18650 2.7ah cells for $2 with tabs. You cant beat that price anywhere.

Thanks for pointing that out. I was going from memory. I poured over the datasheets and it looks like 2C is the max recommended. And on some of the models they seem a little strained at that current. In the case of the 2.75AH (NCR18650 is typical 2.9AH cell) cell this is 5.5 amps.

They point out that each cell incorporates a PTC device but dont show its characteristics. I assume it goes to high resistance in the event the cell temp exceeds some threshold.

The Tesla Roadster appears to have a 215kw drive system which draws its power from a 99S69P pack of cells. This gives a full power current of around 600 amps. 600A/69Cells = 8.7A per cell which is right at 3C on a 2.9AH cell.

$2 is a good price.

[dougingraham]

Originally Posted by icec0o1

There is no more current technology than the Panasonic 3.4Ah 18650. Are you really arguing that if there was a better techonology/battery out there, Tesla wouldn't use it?

You use what you know. They might still use them even if there was something better. At the moment it looks like the complete package is still superior in wh/kg to other solutions. However it requires a large pack to get the performance they wanted for the roadster. The side effect of that large pack was decent range. You can build a lot smaller pack that would provide the voltage and current used in the roadster with Headway's or A123 20AH pouches. And smaller still using the Haiyin cells. You could replace the Roadster pack with a 99S2P pack of the Haiyin cells and have a pack where the cells weigh something near 33.66kg (74 lbs). With all the interconnects and box to hold it together it would probably end up at 42kg (92 lbs). This would be a savings of 408 kg (897 lbs) although at a considerable reduction in range (56kwh down to 4.3kwh). I would estimate the range would be reduced to around 17 miles. But I bet the 0-60 time would go down considerably as would the 1/4 mile time.

[frodus]

Originally Posted by icec0o1

There is no more current technology than the Panasonic 3.4Ah 18650. Are you really arguing that if there was a better techonology/battery out there, Tesla wouldn't use it?

Why Tesla uses these 3.4's is for economy of scale. The more you buy of a commodity item like a laptop battery, the better price you get it for and the more profit..... it's not because it's the best battery out there. These Panasonics are far from the most dense and powerful battery.

BACK TO THE OP's question, which is what this is really about.

Building a pack of 18650's is time consuming, just ask Al Bullock (cruisin). He hand soldered a ton of them. Made a great pack, but it took a while. There are considerations with a pack like that, that's all we're saying. There are other high power batteries that would make pack building much easier..... but I agree with Al, if you get 18650's, get one's with tabs soldered.

[icec0o1]

Originally Posted by dougingraham

You use what you know. They might still use them even if there was something better. At the moment it looks like the complete package is still superior in wh/kg to other solutions. However it requires a large pack to get the performance they wanted for the roadster. The side effect of that large pack was decent range. You can build a lot smaller pack that would provide the voltage and current used in the roadster with Headway's or A123 20AH pouches. And smaller still using the Haiyin cells. You could replace the Roadster pack with a 99S2P pack of the Haiyin cells and have a pack where the cells weigh something near 33.66kg (74 lbs). With all the interconnects and box to hold it together it would probably end up at 42kg (92 lbs). This would be a savings of 408 kg (897 lbs) although at a considerable reduction in range (56kwh down to 4.3kwh). I would estimate the range would be reduced to around 17 miles. But I bet the 0-60 time would go down considerably as would the 1/4 mile time.

Speed was always of secondary concern to Tesla. And if it's your primary (17 mile car is unusable), you might as well go with LiPoly 45C cells.

If you look at the 9000 or so reservations for the Model S, 60-70% of them are for the 300 mile range model, even though it's $20k more.

Originally Posted by frodus

These Panasonics are far from the most dense and powerful battery.

Which batteries are more energy dense? Maybe I missed something.

P.S.
I'm not arguing for the OP to build his pack from these. They could potentially be unsafe if you don't know what you're doing (as can any battery) and they are more difficult to make into a pack. I just didn't like that analogy which compared Tesla to companies like GM or Chrysler, forever stuck in the past. If a more dense battery came about, they would throw away the 18650's faster than you could say wh/kg. Elon didn't replace NASA by getting stuck with economies of scale.

[frodus]

Can we get back to the OP's question