Engineers: Are groups depleted one-by-one?

[agray]

Wiki states:

The 85 kWh battery pack weighs 1,200 lb (540 kg)[102][better source needed] and contains 7,104 lithium-ion battery cells in 16 modules[103] wired in series (14 in the flat section and two stacked on the front). Each module contains 6 groups of 74 cells[105] wired in parallel; the 6 groups are then wired in series within the module
​

Being Lithium Ion each cell is going to be about 3.6 volts... each module has 6 groups so 6 x 3.6v and then there are 16 modules in series so the total would be 16 x 6 x 3.6 = 345v. Can anyone confirm this?

My question is: since SOC is so significant to charge times does the car somehow manage to deplete each of the group that are wired in parallel before then starting to use another? If it did this then a battery with say 50% charge would actually consist of 47 groups at 100% and 47 groups at 0%.

Can anyone tell me if this is indeed what happens, or if not why not. I suspect that the answer is going to be "no"...but I can't immediately see why this approach of depleting one group at a time would not work (and it would clearly improve charge times if it could be done).
Thanks
Andrew

 

[Veritas1980]

Because constantly running some cells from 100-0% and maintaining others at 100% more or less all the time would quickly kill the battery I suspect?

And I guess you won’t get enough power output in you only use 50% of the cells as well.

 

[HankLloydRight]

You're only thinking volts.

You need all the modules in series to generate the max current/power (amps/watts) needed for acceleration.

So all cells drain fairly evenly across the entire pack.

p.s. I am not an engineer, so this might be a simplified explanation.

 

[Ande]

So all cells drain fairly evenly across the entire pack.

Absolutely correct.

 

[hacer]

Wiki states:

The 85 kWh battery pack weighs 1,200 lb (540 kg)[102][better source needed] and contains 7,104 lithium-ion battery cells in 16 modules[103] wired in series (14 in the flat section and two stacked on the front). Each module contains 6 groups of 74 cells[105] wired in parallel; the 6 groups are then wired in series within the module
​

Being Lithium Ion each cell is going to be about 3.6 volts... each module has 6 groups so 6 x 3.6v and then there are 16 modules in series so the total would be 16 x 6 x 3.6 = 345v. Can anyone confirm this?

My question is: since SOC is so significant to charge times does the car somehow manage to deplete each of the group that are wired in parallel before then starting to use another? If it did this then a battery with say 50% charge would actually consist of 47 groups at 100% and 47 groups at 0%.

Can anyone tell me if this is indeed what happens, or if not why not. I suspect that the answer is going to be "no"...but I can't immediately see why this approach of depleting one group at a time would not work (and it would clearly improve charge times if it could be done).
Thanks
Andrew

As wikipedia correctly notes, the modules are wired in series, so the charging (and discharging) current flows through all of them at the same time.

Your idea is spectacularly bad for a number reasons. Firstly, if the modules were to be discharged separately then the required batteries currents would be 16 times larger in order to achieve the same power. The losses in the wiring and batteries would go up by 256 times because they are proportional to the square of current. There would also need to be a monster power converter to step up the puny voltage of a single module to overcome the motor back-EMF at all but the slowest speeds. Such a converter would be heavy, expensive, and inefficient as well. There would also need to be a switching circuit that carries enormous currents to switch between the modules. Charging such an arrangement would also be much, much slower as well since the peak rate of cell charging would be the same, except that you'd only be charging one module at a time instead of all of them at once.

To speed up charging, it is better to have more cells in series (higher voltage) and fewer in parallel. However the cost, size, weight and efficiency of the motor controls and charger start to get worse as the voltages gets high because of limitations in semiconductor switch technology.

Tesla knows what they are doing and are pretty much in the sweet spot with their battery voltage. Faster charging is possible at the cost of battery longevity, or you can reduce overall range with different battery chemistry that can handle larger currents. But here again, Tesla has managed the trade-offs quite well. In the future there probably will be new battery technology that will handle larger current densities with comparable life and capacity, but they don't exist today.