SOC vs HV/LV delta and balancing….

[siteguru]

all BYD are LFP for all current production cars

That'll likely explain why the BYD battery pack SOH is seemingly much lower ... the SAIC/CATL LFP pack in my SE SR (51kWh) has about 92% SOH after 2.5 years. It seems that LFP packs report SOH degradation as being much higher initially and then it flattens out, whereas the NMC packs seem to retain a higher SOH initially. (Probably because of a bigger buffer vs usable capacity).

 

[mg4owner2024essence64]

That'll likely explain why the BYD battery pack SOH is seemingly much lower ... the SAIC/CATL LFP pack in my SE SR (51kWh) has about 92% SOH after 2.5 years. It seems that LFP packs report SOH degradation as being much higher initially and then it flattens out, whereas the NMC packs seem to retain a higher SOH initially. (Probably because of a bigger buffer vs usable capacity).

Noted. My MG4 is 2 years . Will be interesting to watch. I suspect these modern NMC , especially by CATL, will hold up really well.

Probably why MG/CATL are happy to support 10 year battery warranty now

 

[hojnikb]

That'll likely explain why the BYD battery pack SOH is seemingly much lower ... the SAIC/CATL LFP pack in my SE SR (51kWh) has about 92% SOH after 2.5 years. It seems that LFP packs report SOH degradation as being much higher initially and then it flattens out, whereas the NMC packs seem to retain a higher SOH initially. (Probably because of a bigger buffer vs usable capacity).

MG4 SR counts down SOH at 0.01%/day, regardless of what the actual degradation is.

Maybe someone can confirm, if 0-100% cycling resets this counter though.

 

[Everest]

MG4 SR counts down SOH at 0.01%/day, regardless of what the actual degradation is.

Oh really? Do you have a reference for that?

 

[Romeo77]

That'll likely explain why the BYD battery pack SOH is seemingly much lower ... the SAIC/CATL LFP pack in my SE SR (51kWh) has about 92% SOH after 2.5 years. It seems that LFP packs report SOH degradation as being much higher initially and then it flattens out, whereas the NMC packs seem to retain a higher SOH initially. (Probably because of a bigger buffer vs usable capacity).

Had 93% SOH too, before i understood how it balances cells !!!
Today SOH 96% but third party app that have access to car data shows me 54.5 kWh remaining when 7 kWh already used.
That mean it should show 61.5 kWh when battery full and if calculating done by using nominal voltage that gives almost 100% SOH
One year ago that app showed from 58.2 to 60.4 kWh when pack fully charged with high delta.
But what i also found is when i drop pack down to 2.5 volts (no i not recommend that !!!) on weakest cell and then cycles several times with low power EVSE 0-10% the weak cell get more energy, and delta falls at 5% SOC every cycle.
That way i got 97% SOC in Car Scanner once.
BYD SOH is a bad habits of charge and balancing strategy - once high delta build up crucial not to give car work more then 4 hours between charging cycles otherwise this “balancing” will be never ending story -
In 17 hours balancing circuit can discharge up to 2.5 ah.
Now i keep delta not too high (130-160) so balancing not discharging cells and i can use car as i want, and not forced to charge battery every day.

 

[hojnikb]

Oh really? Do you have a reference for that?

Look at this forum; there's a few references to that.

 

[Romeo77]

This from repair manual for my car about 90mV at 50% , wish to everyone not to reach such situation !!!
Meanwhile my battery balancing cells better every cycle, who that idiot that did such dumb balancing procedure in BYD
Now after i found how it works i see 123 mV at moment of finish changing.
Cost me buying Launch X-431 diagnostic….
Month before delta fluctuated 360 mV to 250 mV

@EVentropy
Just out of curiosity
How much time your MG4 spend powered on between charging to 100%
Why i ask?
Because when i got access to charger at work i started charging every day without enough time of car powered on (driving only 15 minutes to work and back to home)
On Atto 3 balancing work mostly when we drive car or sitting in it when it powered on.
Then also during charging (not at the end of charging) But balancing time limited for some reason - only works 17 hours from time of full charge.
Also stopping balancing if we charge by using DC rapid station.

@Romeo77 @EVentropy @wattmatters

Since 2023 I had BYD Atto3 Extended Range and now have MG4 Essence 64 (NMC)

Both same age (2023 build), same 25,000km driven, much the same amount of minimal DC fast charging.

BYD SOH = 93%

MG4 SOH = 99.7 %

I can never get my BYD min cell voltage to be 3.6V - it is always 3.4V when reaching 100% and as Romeo says, this is when BYD stops charging completely.

With the MG4 the delta is <10 mV !

Today's balancing took 4 hours ! The car display actually says "battery balancing"

I can hear the relays clicking between HV battery modules.

I think SAIC/CATL have managed battery cell balancing better than BYD ?

I guess if I can balance my cells better in BYD then the SOH might increase higher than 93% ?

For my MG4 Car Scanner below, I haven't had time to optimise some sensors - some things are from my BYD layout

MG4:
View attachment 36929

BYD:
View attachment 36930

Yes it will increase, but after doing calibration.
Do what i did, fully charge then when use car count power on time, remember that A/C purge cycle adds every time 2 hours of power on time.
Don’t let it work more than 4-7 hours, if higher delta let it work more hours if lower delta less hours, balancing starts if delta 180-185 and stops if less than this value.
Probably balancing work also during charging with lower but still high delta measured , but that not registered as equalisation time.
The problem is that only time when delta high and balancers can work enough time with it is at lower part of SOC - less than 5%, at top of SOC voltage of cells increase too fast because cells already full and they soon will stop charging so balancing there inefficient (constant power profile used).

 

[mg4owner2024essence64]

Yes it will increase, but after doing calibration.
Do what i did, fully charge then when use car count power on time, remember that A/C purge cycle adds every time 2 hours of power on time.
Don’t let it work more than 4-7 hours, if higher delta let it work more hours if lower delta less hours, balancing starts if delta 180-185 and stops if less than this value.
Probably balancing work also during charging with lower but still high delta measured , but that not registered as equalisation time.
The problem is that only time when delta high and balancers can work enough time with it is at lower part of SOC - less than 5%, at top of SOC voltage of cells increase too fast because cells already full and they soon will stop charging so balancing there inefficient (constant power profile used).

I wonder if BYD improved their cell balancing in BMS, from 2022 to 2025 Atto3 for example

Or if it is the same and only next generation will improve

 

[mg4owner2024essence64]

Huge improvement by world's biggest battery maker. I think in 5 years we will have 1000km real world range in affordable cars AU $40k

https://insideevs.com/news/761074/catl-lithium-metal-battery-breakthrough/

 

[Romeo77]

I wonder if BYD improved their cell balancing in BMS, from 2022 to 2025 Atto3 for example

Or if it is the same and only next generation will improve

Actually not, i scanned absolutely new car model 2025, only 180 km on road, they from factory in awful condition, bottom soc delta too high, and also at top after first full charge 340 mV

 

[mg4owner2024essence64]

Actually not, i scanned absolutely new car model 2025, only 180 km on road, they from factory in awful condition, bottom soc delta too high, and also at top after first full charge 340 mV

very disappointing. We will have to see at 10 years in 2032 but it would appear MG handles cell balancing much better.

Time will tell of course