Dual Balancing: Is it really the solution to all Tata's battery problems?

Source: Circuit Digest

Tata is being in the EV business since 2020. They are now having whole portfolio of EVs ranging from Tiago.ev to Sierra.ev. With to many EV's in the portfolio, TATA is also famous for the SOC drop issue in their EVs since 2020. Main question arises as to why they still can't solve this famous SOC drop issue? Recently they have released a update for the BMS to solve this issue lying in majority of their car batteries. We did several studies in numerous cars facing SOC drop. Well its time to share the results of our study.

1. Owner with soc drop from 28%.

This owner of Punch.ev had soc drop in his battery from 28%. He used to get cell min voltage to be 3.33V when charged to 100%. So his cell minimum was never hitting higher soc when charged to 100%. 3.33V can be anywhere between 70-90%. So this minimum cell was acting as the limiting factor for whole battery to shutdown when battery percentage was hitting 28%. After update and doing many slow charging cycles with 6 amps now for the first time he is observing cell min to hit 3.341V which is a massive improvement. So now his soc drop might be lying somewhere close to 15 to 20% (not checked in reality).

Below images shows the voltage profile for set of cells in his battery pack.

2. Now let us take another example where Dual balancing can't help at all (even if it intends to do help). Below is the cell min and cell max voltages of a punch ev car at 25% soc in car.

It can be clearly seen that battery pack is very nicely bottom balanced but cell min and max are so low for 25% that it will soon face soc drop issue. This is not an imbalance problem but cell degradation problem. In this case dual balancing can't be of any help even if it intends to do the help. Below is one more example of tiago.ev with battery cell degradation. At 100% cell min is 3.34V which is around 15% imbalance. Bout he experiences drop from 32% and his cells start to deviate as soon as he starts driving..

3. Now this is the 3rd example where after every cycle total voltage of the pack is rising for 100% resting battery pack. We will be discussing these in sequence.

First case of 100% where cell min is 3.33 but total voltage of the pack is 341.9.

Second case f charging to 100% rest where cell min is still 3.33 but total voltage rose by 343.9.

Third case of charging to 100% where cell min is still 3.33 but total voltage is rising, showing that cells which were lagging behind are now catching up to be charged to 100%.

Conclusion

Dual balancing is working but these kind of updates should be given right from the start. Also what about the batteries which are having cell degradation issue? This update will do no benefit for them. This question still remains unanswered