Does anyone here have a new 75kwH MS?

[Swampgator]

If so, please post the voltage reading when the pack is 100% full.
This will tell us how much higher Tesla has been able to increase the per cell voltage from 4.2v

We already know the 60kwH (software locked) pack charges each cell to 4.2v.

 

[TheJMan]

The per cell voltage should not go above 4.2V on any pack. 4.2V is the max for any li-ion cell I thought.
Its possible that the pack cuts off earlier, so instead of a discharge cut off of 3.8V in the 75kwH the 60 could be cutting off at 3.9 or whatever and thus the lower capacity.

 

[TheJMan]

Dont understand why the software lock would still allow the 60 pack to charge to 4.2V each cell. With the S40 back then the pack was locked in the top part. So the pack just never charged fully.

 

[Swampgator]

I think you are misunderstanding the physics here.
The only practical way for Tesla to software limit a 75 kwH pack to 60 kwH is via voltage regulation.
Since we already have confirmation that the 60kwH pack reaches 353 kw we know that 353 / 84 cells = 4,2 v
So the 75 kwH pack must have achieved a per cell charge that is higher than 4.2 v

I'm just trying to help Randy Carlson do the math and see what we have here....and what M3 may be working with next year.

 

[wilheldp]

I think you are misunderstanding the physics here.
The only practical way for Tesla to software limit a 75 kwH pack to 60 kwH is via voltage regulation.
Since we already have confirmation that the 60kwH pack reaches 353 kw we know that 353 / 84 cells = 4,2 v
So the 75 kwH pack must have achieved a per cell charge that is higher than 4.2 v.

I can't tell if you're joking or not, but if you're not, it is you that is misunderstanding the "physics" (more accurately, Ohm's Law).

kW (kilowatts) are thousands of VAs (volts times amperes). kW is also an instantaneous measurement. So if you have 84 cells at a voltage of 4.2V, then you have a pack voltage of ~353V. If you have a pack output power of 353 kW, then each cell is delivering 1 ampere.

Adding energy to Li-ion cells (which is what Telsa is trying to do by making larger cells, and Panasonic is trying to do with better chemistry), means increasing the kWh rating of the cells. kWh is a measurement of energy, not power. It is the average of the amount of power that can be delivered by a source over a period of time. The best way to measure kWh is to chart the voltage of a cell over time under a constant current load. The area under that curve times the current is the energy (in kWh).

In order to calculate the energy capacity of a battery, the "nominal voltage" of the cell is used. The nominal voltage is the average cell voltage over its useable range, which is typically 4.2V to 3V on a Li-ion cell. The nominal voltage of a typical 18650 is 3.6V or 3.7V depending on whether the cell is designed to be used to 3V or 3.2V. It is the chemical reaction used in a Li-ion battery that limits the top voltage of the cell, not really any technological improvement to the cell.

The technological improvements lead to higher current output (either instantaneous or average). For instance, if you can increase the amount of time it takes for the cell to fully deplete at a given current, or if you can retain the discharge duration at a higher current, you have increased the energy density of the cell without increasing the voltage.

 

[Mknac]

If so, please post the voltage reading when the pack is 100% full.
This will tell us how much higher Tesla has been able to increase the per cell voltage from 4.2v

We already know the 60kwH (software locked) pack charges each cell to 4.2v.

247 at 100% 237 at 95% 201 at 80%

 

[Saghost]

247 at 100% 237 at 95% 201 at 80%

That can't be the Voltage - maybe that's your rated range in miles?

 

[Mknac]

yes, rated range, sorry

 

[Swampgator]

Wilheldp,

I'm sorry you are correct. I meant to say 353v not kwh

 

[Swampgator]

MKnac,

Next time you supercharge can you note the pack voltage at 100% SOC?

 

[wilheldp]

Wilheldp,

I'm sorry you are correct. I meant to say 353v not kwh

Noted...but I think you are still on a wild goose chase. The cell voltage will not exceed 4.2V. If the pack voltage is different, that means the number of cells is different. If the battery pack has a higher capacity with the same number of cells, it will be kWh of each cell that increases...NOT VOLTAGE.

 

[Swampgator]

They how do you think Tesla is software limiting the 75 kwh pack to 50 kwh if not by SOC?

 

[Swampgator]

To elaborate:
The cells in a battery pack store energy and have an operating range with a nominal, minimum, and maximum state of charge which is measured by the voltage per cell.
The BMS must keep all cells in a group at the same SOC at all times. One cell can NOT be charged to 3.5 volts for example while another in the module is 4.2v. Some cells cannot safelt be "turned off" to activate later either.
So the only practical way for Tesla to software limit a battery pack is to stop it from charging to it's maximum voltage.
Also, as a battery nears it's maximum voltage the BMS and charging system must slow down the rate of charge to protect the battery from overheating.
As we are seeing, the 60 kwh packs that are software limited are charging quickly right up to the 60kwh limit on a supercharger. This makes sense if 4.2v is no longer the safe limit for Tesla 18650 cells. They have improved the chemistry and then introduced this first into the 60/75 packs and now I contend the P100D packs as well. That also explains why the P100D upgrade did not add the expected weight increase to Model S.

We can verify the new limit by getting the pack voltage for a fully charge 75 kwh pack.

 

[Ingineer]

They are probably limiting on the bottom end. This way they can maintain a top balance capability. Next time I get my hands on one, I'll find out. I seriously doubt they are charging over 4.2v/cell or limiting on the top end, as that would prevent a top balance.

 

[wilheldp]

Let me explain it to you this way...

Take two AA-sized batteries, a Duracell long life alkaline and an Energizer Lithium. Both of the batteries are identical size and shape (AA cell), and they have the same maximum output voltage (1.6V). The Duracell has a rated power output of 2.1 ampere-hours (Ah) and the Energizer has a rated power output of 3 Ah. You multiply the Ah times the nominal voltage (which is 1.5V for an AA) to get the energy output (3.15 Wh for the Duracell, 4.5 Wh for the Energizer). Neither the cell size or the maximum cell voltage changes, but you get a higher energy output, simply through better chemistry inside the cell.

Most of the energy density strides that Panasonic is making on the Tesla cells is introducing Silicon as a replacement to Graphite in the anode material. There is a theoretical order of magnitude (10 times) more energy density available without changing any other variables in the cell (same electrolyte, same cathode...just more of it, same size, etc.). Of course, Tesla is making a larger cell at the Gigafactory (21700 v. 18650) in order to get a higher energy density. But neither of those changes increase the maximum or nominal voltage of the cell. The max voltage is still 4.2V, but you can get 100 Wh instead of 10 Wh out of each cell. That is how they are making higher capacity battery packs in the same footprint.

With regards to the software limited 75 kWh battery pack, you have to understand the charging curve of a Lithium battery. There are two phases: constant current and constant voltage. When charging first starts, you put in a constant current at a voltage at or higher than the maximum voltage of the battery (say 1 ampere at 5 volts). The battery voltage will climb during this phase. Once the battery reaches its maximum voltage (4.2 volts, in this case), the battery is not fully charged. This is when you enter constant voltage mode. You hold the voltage steady, and the current consumed by the battery will slowly drop. Charging typically ends when the current drawn by the battery is about 3% of its rated output current.

The constant current phase of charging, when the battery voltage is increasing, can be 1/3 of the charging cycle or less. So having a cell charged to 4.2 volts does not mean that it is fully charged. Therefore, the battery pack voltage of the 60 kWh pack is irrelevant. The voltage of a 100% charged 75 kWh pack will be identical to the 100% charged 60 kWh pack.

 

[CKCland]

I posted this on the main Tesla forum a few days ago- a documentation of charging data from my 3 week old refresh model 60D (software limited 75)

Stopped at the Newark, DE SC this morning to collect a better data sample. Pulled in with 37 miles left (17%). Plugged in at exactly 9am. Detailed the first six minutes (minute by minute) and then in 5 minute increments.
9:01- 192 mi/hr, 296 a, 311v, 38 miles
9:02- 280 mi/hr, 296 a, 315v, 44 miles
9:03- 296 mi/hr, 295 a, 318v, 49 miles
9:04- 302 mi/hr, 233 a, 317v, 56 miles
9:05- 288 mi/hr, 186 a, 315v, 59 miles
9:06- 272 mi/hr, 167 a, 315v, 61 miles
9:10- 241 mi/hr, 191 a, 320v, 75 miles
9:15- 230 mi/hr, 180 a, 324v, 92 miles
9:20- 222 mi/hr, 167 a, 329v, 108 miles
9:25- 216 mi/hr, 171 a, 334v, 126 miles
9:30- 214 mi/hr, 167 a, 338v, 140 miles
9:35- 212 mi/hr, 170 a, 342v, 158 miles
9:40- 210 mi/hr, 164 a, 346v, 175 miles
9:45- 208 mi/hr, 152 a, 351v, 190 miles
9:50- 203 mi/hr, 115 a, 353v, 206 miles *** manually stopped charging here

See the thread here for more good info and analysis - Refresh Model S60D SC data | Tesla Motors

 

[Swampgator]

Still needing the data from a fully charged 75kwH pack in order to either prove or disprove that Tesla is limiting the battery capacity by voltage.
Wilheldp thinks they are simply letting the pack reach max voltage which could = 85% SOC and not letting the battery continue on to full charge capacity.
Randy Carlson thinks that they are limiting SOC by voltage and that Tesla have broken the 4.2 v barrier.
If we get a 75 pack and the pack voltage is 353 v then Wilhelpd is correct, but if the pack voltage is greater than 353 Randy may be on to something.
I am leaning towards WIlhelpd's position but I really respect Randy who is also an engineer and has been spot on with his M3 predictions so far.

 

[wilheldp]

@Swampgator can you post a link to the thread you are talking about? I would like to know Randy's reasoning behind thinking that Tesla is trying to get more than 4.2V out of a Lithium cell. It doesn't make any sense to me. Voltage is dictated by the chemistry of the cell, and there are FAR easier ways to get more power density out of a cell other than changing that chemistry.

 

[Ingineer]

They aren't exceeding 4.2v per cell. They have been SLOWLY adjusting the chemistry, being very cautious. They aren't going to change everything up that dramatically.

I just got access to a refresh 75. It's BMS reports a calculated 243Ah min. (Compared to a 2015 70 with only 226Ah) It has 14 modules and at 60% SOC each cell is about 3.85V. I suspect these will be just like all other Tesla cells at about 4.2 volts fully charged, 25V per module and 350V per pack. I'll be able to get more details from it soon.

On the new 60, my guess is they cap the bottom end. They could be capping the top end, but then it would never be able to top balance. Maybe they use a sliding window? I won't know for sure until I get access to one.

 

[Swampgator]

Wildheldp,

Sorry for not getting back sooner: http://seekingalpha.com/article/4001134-tesla-already