Take a simple hypothetical scenario for example:
Simple 2-cell pack in series that you know the current SOC and capacity of. Cell 1 has a capacity of 1 Ah and is at 50% SOC (500 mAh remaining), Cell 2 has a capacity of 2 Ah but is at 20% SOC (400 mAh remaining). Here you'd like to bleed off about 100 mAh from Cell 1. The BMS knows the value of the balance resistor and voltage of the cell, so it can calculate how long it needs to keep the balance resistor on to bleed off 100 mAh from Cell 1.
Well, it is a very strange example but you would not like to have different SOC between the cells In that case. Any balancing would try to even the voltage = even out the SOC to the same level.
Battery cell number 1 if bleeded 100mah ( = to 40% SOC ) would have much higher cell voltage than cell number 2 at 20% SOC. Remember that SOC = a defined OCV voltage.
When loading the battery, the smaller cell with higher SOC would deliver the same current but a higher voltage, thus having a higher C load and delivering a higher power. Higher C load means higher cyclic aging, so in the long term that cell will break long before the other.
(I have a lot of example of exactly this situation, which rendered a bunch of batteries unusable).
Heres a schematic why top balancing is prefered in many cases: Different cells has different capacity and as we have a bottom buffer which we should not plan to use, the difference can safely be put in the buffer zone, and then we can use the maximum capacity.
(you can not drive the whole buffer as the car will stop/shut down when the first battery reach the low voltage limit. But you get a higher usable capacity above the buffer, which is the goal.).
In the picture below the cell capacity is on the horisontal axis and the voltage is on the vertical axis.
The upper example, with the cells in perfect balance at a specific SOC (it could be *any* SOC, but not 100%).
If we let the car balance the cells at for example 70%, ans many use 70% as the daily SOC target…and the delta is 10mV (not impossible) and the target for balancing is to stop at 4mV delta then it would cost 6mV on the highest cells and maybe 3mV in average to reach a balanced pack. 3/1000 = 0.3%, or 0.25kWh or so. Next charge, a full charge would cost more as the spread is higher at the ends due to the voltage curves being steep at the ends. I guess we could assume 0.5kWh or more. Then if the car is later left with 40% there will be a slight imbalance and if the car spontaniously would balance, there would be a quite big balancing as the last balancing was done at full SOC. Each balancing act with different SOC will cost unnessesary energy as we earlier have displaced the voltage curve to match at another SOC.
To only two reasons to balance a Tesla is top balancing to:
- “Fill it up”
- Correct a balance that is going off limits, which is going to cause a problem if not corrected.
The second reason would not happen often at all. When a series of battery cells in a pack that are not in bad shape is having a resonable load and normal charging sessions they strive to go close in SOC, like self balancing. If one cell get slightly higher in SOC it will deliver more energy and this even out the SOC for normal “nice” use. Using 20-90% SOC with normal charging and driving most likely will not cause a diverting imbalance as they sort of self level.
Hard use with high loads and/or high charge current could cause the imbalance to increase, and the need for balancing could occur. Old battery packs that have been used without proper balancing probably have destroyed some cells which cause imbalance and even more destructiuon of the bad cells.
For a Tesla that probably give the batteries a nice life and with a proper balancing system that keeps the cells in balance when needed i I would think that the cells age well togheter and keep the characteristics so the need for balancing other than at high SOC would be rare.
Balancing should not cause a significant mount of energy consumption - if this were the case you would have a serious issue with the pack quickly going out of balance and more likely, the BMS would not be able to keep up due to the high value of resistors used to bleed energy off.
The energy consumption would come from many balance sessions. Specially if the car would balance at different SOC, that would induce a spread that would be needed to balance back.
For example, charging to 90% and getting the cells perfectly in balance you can discharge to low SOC (and see a higher spread) and then charge back to 90% and the spread recudes again (imbalance would be low) as the battery was in balance at 90%. The need for balancing will be low, with a low energy use.
Charging to 90% + balancing, and discharge to 30% and then balance there would cause a bigger need as the difference in capacity cause the cells to spread a little. If then charging back to 90% and balancing that session also need more balancing at we actually induce imbalance at 90% when we do the 30% balancing.
A lot of balancing at different SOC’s would induce more need for balancing = a bad circle causing enegy loss.
If we have the cells in decent balance at the charging target for example 70% and we accept to have a slight imbalance, lets say 5 or 10mV we do not need to balance there, after discharge and new charge to 70% the imbalance probably ends up about the same. No balancing and no energy loss.
