Battery Tech Part 2

This is a continuation of my rambling about battery tech, this time focused more on Tesla’s batteries.

 

Tesla currently (as of 2016) uses a Li-ion cell called the 18650. The number just refers to the dimensions of the cell. It’s 18mm in diameter (a little less than 3/4 inch), and 65 mm long (about 2.5 inches). This is a standard cell size used in laptop batteries and was available with no retooling by a battery maker when Tesla set out to make electric cars. They look like a AA alkaline battery, but they are a bit bigger.

 

The Model S and X have had two pack types from the beginning, a small pack and a large pack. Tesla arranges the packs in modules. The small pack has 14 modules and the large pack has 16. Each pack is arranged to have 22.2 V per module by stringing 6 cells together in series (voltages add up), then arrange the pack into sets of these 6 cell packets in parallel (same voltage for each packet, but the current capacities add up). In the 1st generation, the small pack had 64 X 6 cell pieces in parallel and the large pack had 74. From the math, it appears the second generation pack has the same arrangement, but the 3rd generation pack has more cells per module (Elon Musk confirmed this for the 100 KWh pack at the announcement). In the smaller pack the unused cell positions are filled with dummy cells.

 

So far the battery cells have been through two generations, and the packs have been through three capacity versions (with many sub-versions of each). Both versions of the battery cells have been the 18650 format. The first version had all graphite anode and the second generation added a little silicon which boosted the current capacity of each cell.

 

We know a lot about the first generation cells and packs because Tesla both talked about them quite a bit when the Model S was launched and a user named Jason Hughes took apart some 1st gen packs for a home solar storage project and documented everything he found. What’s in the 2nd and 3rd generation packs is still open to some debate. Most people assume the 2nd generation packs use the same number of cells as the 1st generation pack, but I can’t find any data where anyone has opened up a 70 or 90 pack to see.

 

The first 3rd generation packs were the new 75 KWh packs introduced in early 2016 and I originally thought they might be a 3rd generation battery cell, but at the P100D launch, Elon Musk said the 75 and 100 packs use the same type of cell as the 2nd generation pack, it just uses more of them.

 

The exact numbers on the cells and hence the exact capacity of the battery packs is open to a bit of debate. Panasonic makes several different versions of the 18650 cell format, each with a little different chemistry. As far as I can figure out, the cells used in the 1st generation pack were NCR18650Bs.

 

The cells inside the Tesla packs are wholesale cells which have some Data Matrix codes on them (2D bar codes), but don’t have any other markings. I saw a post on Reddit where someone compared the codes stamped on the batteries from the disassembled 1st generation pack and the codes on a commercially obtained NCR18650B and the Data Matrix codes are the same. Using the values for the NCR18650B does come up with pack sizes that work for the 1st generation of Model S.

 

https://www.reddit.com/r/teslamotors/comments/454vsz/teslas_18650_battery_identified

 

The same post speculated that the batteries in the 70 and 90 KWh packs were the NCR18650G which was popularized by Panasonic for a while, but then pretty much disappeared. Using the numbers for the NCR18650G does come up with plausible real pack sizes for the 70 KWh and 90 KWh 2nd generation packs.

 

For the 3rd generation packs, we know there are more cells in there because Elon Musk has said so, but nobody outside of Tesla knows how many more. Because the basic building black of Tesla packs is the 6 cell packet, we can assume the 3rd generation packs have some multiple of 6 cells, but beyond that it’s all educated guesswork. In the table below I generated numbers based on two different possible sets of cells. One set of numbers comes up with actual capacities that are a little under the advertised levels and one set goes a little over. I would guess the actual cell count is probably a little under rather than a little over.

 

The exact voltage and current capacities to use when calculating the pack capacity is also a bit of speculation. Panasonic does not publish the data sheets for the NCR 18650B or G on their website (at least I couldn’t find it, though I did find sheets for several other 18650 versions). I did find a supplier in China that had spec sheets on a wide variety of batteries from many makers.

 

http://www.keeppower.com.cn/products_class2.php?bid=17&sid=23

 

The NCA type chemistry these cells use produce a max voltage of 4.2V, but any calculation is going to use the nominal voltage which is rated at 3.6 or 3.7V depending on the source. I used 3.6V which is the more often used number. You want to use the nominal voltage because a battery Is only going to have it’s max voltage when it is at 100% charge and it will drop to the nominal voltage fairly quickly as it discharges.

 

The reported current capacities vary all over the map and they can change depending on conditions. Temperature can have an effect on all batteries. In places that have very cold winters, cars don’t want to start on cold mornings because the lead acid batteries can’t provide enough power to fire the starter. In the oil fields in the north of Alaska, nobody ever shuts off a car in the winter. If they did, there is a good chance they won’t get it started again.

 

Li-ion batteries are also affected by temperature and you get different ratings for the cells at different test temperatures. Using the ratings for the cells measured at 25C (about 77 F) gets values that fall into the ballpark of Tesla’s advertised battery pack capacities. My numbers are in the table below.

 

The cell format that will be built at the GigaFactory is a larger size than the 18650 used now. The 21700 is 21 mm wide and 70mm long and talk is that the larger cell will allow for an improvement in energy density from the size change alone, and there are rumors Tesla will also be using a chemistry that improves energy density even more.

 

The speculation about how much energy density gain is all over the map. I have seen estimates from 10% up to 33%. I think it’s likely going to be 20-25%. A 10% increase would make the small pack 82.5 KWh and the large pack 110 KWh. A 20% increase would mean a small pack at 90 KWh and the large pack up to 120 KWh.

 

We’ll probably see the new packs in Model Ss and Xs in early 2017.

 

Even if the initial GigaFactory built packs are only a modest increase, I expect Tesla will come out with something for the S and X that is much larger by the time of the Model 3 release. They need to do something to keep the Model S distinguished from the Model 3 and the only feature the S has over the Model 3 right now is the hatch. If the Model S and X had much greater battery capacities than you can get on the Model 3, that would leave a market niche for the more expensive cars.