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BYD Balancers: Are they really perfect?

BYD Blade Battery. Source: BYD

LFP or LifePo4 batteries are gaining alot of momentum in recent times due to the robustness of its chemistry. This chemistry is capable of sustaining extreme temperatures + other climatic conditions without having much impact on its health. The only thing that can affect LFP battery is the imbalance which exists in the battery pack.

What is Imbalance in LifePo4 batteries?

Battery pack of any electric car is made up of multiple cells. All these cells are supposed to perform together at equal potential for EV to run smoothly. When one or more cells are not at same SOC (State of Charge) as other cells then it is termed as an imbalanced battery pack. This can cause the cells of the battery pack to degrade non-uniformly, causing the rapid reduction of SOH (state of health) of the whole battery pack.

Factors affecting battery to get imbalance

1. Fast charging the battery frequently.

2. Not performing 100% charge regularly on a level 2 charger. Normally car manufacturer with LFP batteries recommends to charge to 100% atleast once a week.

3. Too extreme climatic temperatures affecting the cells of the battery.

4. One of the cell developing higher internal resistance as compared to the rest of the cells in the battery pack.

How BYD balancers work?

In case of BYD blade cell batteries, balancers are working continuously unlike some other manufacturers who only balances the battery at 100% SOC. While in case of other manufacturers where balancing is only performed at 100% SOC, one needs a decent quality of balancer which can rapidly balance all the cells of battery pack when 100% SOC is reached. This will further hinder the charging speed of the battery when it is reaching close to 100% SOC..

As compared to that, BYD doesn't require to balance the cells only at 100% SOC. When one of the cell hits the maximum permissible voltage at 100%, BMS resets the counter to 100%, and also charging will be stopped. Now when car is actually in Ready mode, balancer will be working continiously to balance the cells. These balancers will be balancing cells considering several aspects of imbalance which might be existing in the battery pack. Lets discuss those aspects.

1. BMS will try to balance as many cells as possible when instrument cluster is showing 99% SOC and battery cell has hit 100% voltage i.e. 3.7 V. As a result, you will find car taking little more time to reach 100% from 99% SOC.

2. For example Cell no 2 and 68 has reached the maximum volt of 3.7 (100% reset voltage for BYD Atto 3) then bms resets the calculation + monitors all the cells which are far from 3.7V. It also monitors how much those cells are far from 100%. So during the next discharging cycle, cells with higher potential difference would be discharged at a little higher rate than other cells.

Source: BYD Atto 3 car

From this we can clearly understand that BYD has quite smart BMS which can balance the cells when Car is running. It doesnt require to charge to 100% always to stay balanced. I havent done much research on balancer working of MG cars but i suspect even ZS EV also balances in the same way from the fact that, it doesnt slow down the charging when SOC goes above 95%.

High voltage delta in BYD car at 100%. Source: BYD Atto 3 car.

Why AC Charge?

This question is quite evident that as BYD is balancing the cells while running, then why AC and DC charge balancing is different? Even DC charge till 100% can do the similar thing as Ac charge. This is true too for a large extent for BYD. But if you want to see better results in already unbalanced battery then AC slow charge is working more better (experimented on BYD seal with slowest granny charger). Maybe with slow charger it is able to estimate the actual voltage of the cell more accurately.

But for people having high running and doing ac charge till 100% once a week, doing dc charge till 100% on trips would act in same way as AC. This is mainly because their battery is not too off-balanced. This is particularly important for BYD because of extremely slow balancer. This balancer is so slow that if your battery cells are off by 4-5% then it will take around 3-4 hours to balance them.

Conclusion

The only thing one needs to learn about lfp batteries is that you are not going to face degradation in a very long time. So dont worry much about doing 100% and do it atleast once a week. Also once in 3 months do a deep discharge and charge back to 100% using AC charger. This is the only way you can use maximum capacity of your battery for a very very long time in MG and BYD.

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