Battery pack balancing techniques

According to this paper it would be switched-resistance passive balancing. Their premise is that balancing is required due to an assumption that cells can become unbalanced--this is based upon one citation from a battery conference in 1997 and a self-reference to one of their own papers.

But how can a parallel string of 74 cells (a 200+ Amp-Hr modular CELL) become unbalanced? They can't, the voltage across each cell is equal.

The Tesla BMB/BMS is primarily a monitoring device (volt meter) to provide real-time voltage of the 96 modules in the 85kWh pack. The 0.1 Amp switched resistance balancing circuit could be used to make very slight adjustments at the end-of-charging current, or could make bulk pack adjustments over ~30 hours if a single cell were to fail open or blow a fuse.

The more important device in my mind is all the electronics on the large current shunt resistor that measures pack current. There is no current monitoring circuitry on the BMBs.

If the quality control is good (the cells are matched), they will go out of balance very slowly. A bit of controlled shunting is all that's necessary to keep them drifting out of balance over a long period of time. The gains to be had from rerouting currents is going to be quite small. I expect the amount of power in question would be pretty small compared to any that would be used to heat or cool the cells. Simplicity wins here, without much tradeoff.

Based on switched resistor charge balancing of the battery pack, some questions come to mind:
1. What is the threshold that triggers balancing?
2. What is the rate of the discharge on the cell?
3. How long does it take to achieve balance? Is it done in one cycle or completed in multiple cycles?

Soolim said:

What is the threshold that triggers balancing?

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This is a value you set in the battery Main control unit firmware. A good guess would be when any of cells deviate more than 5% from the average state of charge. When i say cells I mean the whole parallel block of 18650 cells.

Soolim said:

What is the rate of the discharge on the cell?

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This something which is dependent on the chemistry and the cells safe operating region. An 18650 cell generally has a safe nominal discharge rate of 0.5C. Given the cell capacity is ~3Ah. You could discharge each cell at 1.5A nominal. considering 74 parallel cells, you could discharge 74*1.5 = 37A.

Soolim said:

How long does it take to achieve balance? Is it done in one cycle or completed in multiple cycles?

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The time taken again depends on the balance current and the ability to bleed charge. More imbalance more time. More balance current lesser balance time. It is done in multiple cycles. Its like this, the BMS balances the cells a bit, checks for deviated cells. balances again checks again. This process goes on till all cells are under allowed state of charge deviation.

Thanks for the info. I have one question though. Shouldn't BMS look into every cell instead of a parallel group? Because those 74 cells within a parallel group can also have an imbalance, or a cell might die blowing the fuse. Wouldn't not checking each and every cell in that 74p group be risky? I don't quite understand why every parallel group is always in balance.

One of the issues causing imbalances is manufacturing capacity variance. In a parallel group those variances are still there. One cell could have 3280mAh capacity while another has 3340mAh and so fort. Or is your group as strong as the weakest cell? So if a cell loses capacity all the way down to 2800mAh, would that make the whole group go 2,8 * 74 = 207Ah capacity? I'm guessing no beacuse if that were the case a cell's failure would cause a whole group to die, instead of just losing ~3Ah capacity.

You can't look into every cell in the paralleled group unless you provide a way to disconnect every cell from the paralleled group. This would add a lot of cost and a rather large increase in failure modes with no advantages. Fortunately you don't need to. If a cell shorts and blows the fuse then you will quickly see that paralleled cell groups loss of capacity. There are 74 cells paralleled and ~ 3.2AH per cell for a total of 237 AH. 100*(3.2/237) = 1.4% loss of capacity. The BMS will pretty easily see this cell group is out of balance with the others and prevent you from over discharging. A reduction in range of 1.4% will be the end result. You would probably see a revised range estimate once you did a 100% charge including the long balance that would take place followed by running down to the bottom. For me this would be going from the 263 indicated full charge miles down to 261 miles. A single blown cell fuse might just be passed off as normal loss of range due to ageing. But the car would know.

Sanity check time. 237 AH * 3.7 volts nominal * 6 series cell in a module * 16 modules = 84.182 kWh. That is pretty close to 85 kWh and could be either the 3.7 volts being a tiny bit low or the 237 AH being a tiny bit low. Sanity check complete. The 90 kWh packs are probably closer to 3.4 AH per cell. The 100's have re-engineered modules and may have more cells in parallel. I've not seen a breakdown but Elon did say that the same cells are used so I would assume more cells of the 3.4 AH capacity in parallel. This would be going from 74 to 83 cells in parallel. But I am now off topic.

Doug Ingraham
2013 Model S 85 Multi-coat Red "The Woman in the Red Dress"