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The BMS really doesn't seem involved in the actual charging in the Model S and battery. The Model S charger itself, as visible in some photos others have posted, just feeds the battery high voltage to the main connector. The same with supercharging. So, sanely applying a charge voltage to the pack should charge it fine.
My discharge test was at 500mA, which would be closer to a 12.7kW average draw for a full pack, much lighter load I'd guess.
The fact that both my test and the car's dash yield a number around 75kWh is definitely interesting, though.
If I get the time I can try to do a more car-like test, but probably won't be soon.
Suffice it to say, I'm very curious where the 85kWh number actually comes from now.
The BMS has limited capability to balance the pack using the bleed resistors of which there are 4 in parallel per cell block. On the picture R20,R21 & R22 are examples. They cannot dissipate much power say 2W total at a guess so may need to operate for longish periods.
I did a quick 3d modeling of the tesla pack based on the wheel base dimensions and patent drawings....and measured the pack volume to be ~ 365L which gives the pack --> 85 kWh/365L = 234 Wh/L ... Also if you assume a module to be 300x680x80 mm --> 16.32L --> 5.31 kWh/16.32 --> ~ 325 Wh/L. Can anyone confirm? Thanks!
So, finished my first 500mA discharge curve for a single cell from a Model S module. Charged to 4.2V @ 1A constant-current, then constant-voltage until current dropped to 120mA, 5 minute delay, then discharge at 500mA until voltage read 2.85V. Test was done using FMA Powerlab 8 and the custom settings above. After charging the cell settled to a resting voltage of about 4.16V.
Was able to draw 2,963mAh. Using 10 second voltage averages this came out to 10.605 Wh. That'd be about 4.7kWh for a module, or about 75kWh for a full 85kWh pack using this method. I think there is room for improvement on the extreme ends, however.
My 120mA charge current during CV stage could probably be dropped to squeeze some more juice into the cell, and I could probably discharge it lower than 2.85V. There was ~100mV voltage sag at the begining of the discharge, also, and this seemed to increase quite a bit during the cycle as evident by a resting voltage of ~3.1V after removal of the 500mA load. I may try soldering heavier gauge wire to my 18650 cell holder later and retesting to see if that improves the voltage sag.
I think wk057 is off the forum until further notice.
I think wk057 is off the forum until further notice.
@wk057 - Any idea how well these modules buffer temperature changes with coolant present in the lines? I'm curious how well a standing module does while it's sitting out in my driveway. Could be a neat experiment to try to model the environment of the cell resting in the main pack and have two temp sensors - one on an individual cell and another ambient and make a plot of the two.
Actually, I'm keeping an eye on my project threads and technical threads like this one. Just staying out of the nonsense.
I know this is OT for the thread, but as someone who's dug perhaps more than anyone else into the nuts and bolts of the Tesla pack, any thoughts on Porsche's proposal to use an 800V pack? Does this say anything particularly interesting about the technology they're planning to use?
Porsche annnounces BEV version of 911 -