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Why MG and BYD cars battery SOH is dropping faster?

Source: BYD

We all know that all the BYD cars are equipped with LFP batteries nowadays. These battery chemistry has become quite popular in today's time due to its evident advantages over NMC battery cells. We can see lots of other manufacturer's also using lfp battery chemistry nowadays for their Electric vehicles. 

LFP batteries are required to have constantly balanced cells at all times. This will help the battery pack to perform at their optimum levels at all times. For this, companies suggests to charge the car all the way up to 100% on regular basis. Doing this will ensure that the battery of your car is at optimum top balanced level at all times.

What is happening with MG and BYD Batteries?

As we all know, in case of LFP batteries it is very difficult to judge the actual SOC of the battery unless it is charged upto 100% on dropped below 10%, So BMS calculates the SOC based on columb count method. Now when the battery pack has degraded or is imbalanced, it is likely that this total amount of power which can be extracted from the battery will reduce.

Now to ensure that when customers go to lower SOC they don't encounter any issues with their battery pack even when they are imbalanced, companies have incorporated a method of calculating SOH based on time. But this SOH reduction is quite faster than the actual SOH of the battery pack.

To ensure that SOH of the battery pack is not reduced unnessarily, BYD has suggested a technique of discharging below 10% once in 3 to 4 months and then charging back to 100%. So BMS knows the actual SOH and recalibrates accordingly. No such technique is suggested by MG in their ZS EV. So their SOH keeps on reducing based on time.

Even BYD users whose running is low experiences the same issue found in ZS EV. One more issue with their battery pack is extremely slow balancer. So even if you are charging upto 100% on weekly basis, it will take a very long time for the battery pack to be properly balanced. So even when car is charged to 100% its not getting properly balanced due to slow balancer + low cycles every month due to bigger battery pack.

When calibration is performed with this imbalanced battery pack, bms will think battery bottom is at higher SOC. This will lead to bms think that battery has degraded while in reality its not.

To verify whether my theory is correct or not, BYD and ZS EV users can charge their car to 100% and check their imbalance. If they are experiencing reduced SOH, you will likely observe imbalance at 100% SOC.




Imbalance found in BYD batteries even when its near to 100%
ZS EV battery imbalance at 100%


Solution

(Use these methods if you are experiencing rapid SOH reduction in your CAR.)

Based on my interaction with few BYD owners, I have pointed out these suggestions. These suggestions works 100% on BYD cars, but not sure about ZS EV cars, because they have not mentioned anywhere about calibration cycle.

1. Discharge your car even below 0% till you can go.

2. Charge back to 100% using a 7kW charger.

3. Repeat this process 2-3 times to observe actual results.

4. Keep the car ON for few minutes at 100% without letting SOC to drop.


BYD cars will definately see good results as this method is proven on their batteries. ZS EV users can try these method on their own risk.

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