LFP Car Battery Not Balancing - Cause for Worry?

[PeterWA]

I know this is a bit off-topic, but I believe this forum is where I find the people who have the most experience with LFP batteries.

I noticed that my EV does not seem to balance the battery cells. At the end of charging the voltage difference between highest and lowest cells is quite large, around 270mV. The highest cell reaches 3.65V, the lowest cell is only about 3.38V. At that point charging just stops, no balancing takes place. See first screenshot. Please note the current shown at the bottom is the AC current (@230V). DC current would be about 1/2 that (@~400V DC).

I only charge to 100% about once per month (the users manual says that is all that is required).

I went back in time to look at previous 100% charge cycles and found virtually the same voltage difference every time. The oldest information I have is in the second screenshot, almost 2 years ago. In a way that is somewhat reassuring: at least the issue is not getting bigger over time.

Being a car, the whole system is pretty much a black box. I can't influence when the battery balances. I can only see some selected measurements, like some cell voltages and of course charge current.

The main question to the group: is this level of cell imbalance enough cause for concern that I should get the manufacturer to investigate? Car dealerships here in Australia, and their workshops, are very much clueless when it comes to EVs, but talking to them would be the only way to get in contact with the manufacturer.

There are other problems with my car's software. For example: the 12V battery does not automatically get topped up from the high voltage battery when the car is left unused for a while and the 12V battery gets slowly drained. The same page in the users manual that talks about cell balancing (they call it equalising), also mentions that the 12V battery is supposed to get topped up when needed. The dealership failed in fixing this. So I don't have much hope they can help with anything software or battery related...

 

[wpns]

That’s bizarre, what car? If the mfr didn’t incorporate any balancing into the BMS then I don’t know what you can do other than make sure of your warranty terms. I guess the good news is it’s not getting worse.

Can you run it all the way down and try a bottom balance?

 

[kommando]

Its a problem, get it booked into a dealer, even if they are clueless it needs to be registered before worse symptoms appear. You paid for a warranty so use it.

 

[PeterWA]

If the mfr didn’t incorporate any balancing into the BMS then I don’t know what you can do other than make sure of your warranty terms. I guess the good news is it’s not getting worse.

Can you run it all the way down and try a bottom balance?

The BMS is supposed to balance. See screenshot from the users manual. In a different section it mentions that the battery needs to be full for that to happen. So top balance only.

The problem isn't what it should do, but what it does. It is entirely possible that my car has hit a software bug. It probably wouldn't be the only car affected, but possibly only one of a few.

An example: if balancing is only activated once cells reach a threshold of say 3.45V, and they look at the minimum cell voltage, or even the average cell voltage that would be a problem. Because the highest cell voltage is so much higher than most others the BMS will trigger the over-voltage protection and stop charging before any balancing takes place.

The above scenario isn't exactly far fetched either: this is what happened in an old software version of the JK BMS, the BMS I use for my house battery. Fortunately that BMS manufacturer did release an update...

No reason the car manufacturer can't release an update too, it's just the communication isn't straightforward. They don't talk to end customers, only to dealerships, and dealerships are clueless about EVs. I would only go through the hassle if a majority of experienced LFP battery users tell me that the imbalance I observe is a serious issue.

 

[wpns]

Yeah, sounds like a design or implementation of software issue, can you force a 'slow charge'?

What car?

 

[740GLE]

So much information provided in the first post, yet no information about make and model.

Seems kinda fishy.

 

[PeterWA]

So much information provided in the first post, yet no information about make and model.

Seems kinda fishy.

Whatever one does, someone finds something to criticize. I left out the brand because I didn't want the discussion to drift off into the merits of Chinese car brands.

Suffice to say it is one that is quite successful in Europe and here in Australia, and not available in the US due to politics...

They deliver quite good hardware, but software tends to be buggy. Battery cells are from CATL and pack is built in a joint venture with CATL.

 

[PeterWA]

Yeah, sounds like a design or implementation of software issue, can you force a 'slow charge'?

They define "slow" as anything less than 7kW. My home charger, which does 98% of the charging for me, follows surplus solar and will delivery anywhere between 1.6kW and 5kW.

 

[CaliSunHarvester]

How do you access the data shown in the original post? Not through the OBD2 interface, I guess?

 

[740GLE]

Whatever one does, someone finds something to criticize. I left out the brand because I didn't want the discussion to drift off into the merits of Chinese car brands.

Suffice to say it is one that is quite successful in Europe and here in Australia, and not available in the US due to politics...

They deliver quite good hardware, but software tends to be buggy. Battery cells are from CATL and pack is built in a joint venture with CATL.

Understandable.

So I take it there is no manufacture branded dealer or service centers for warranty support? Is there even a warranty?

But unless balancing is managed, the vehicle will start to lose range.

Have you noticed a drop in estimated range since you bought it?

Maybe it balances on a calendar cycle not each charge 100% cycle?

 

[Brucey]

I only charge to 100% about once per month (the users manual says that is all that is required).

I would try leaving it plugged in for a week and see what happens. Charging up once a month is never going to give it balancing time no matter what the manual says.

 

[meetyg]

Let me guess... You have a BYD, right?
If so, then I have also notice virtually no balancing at the top and the same cell voltage delta when at 100%.
I had a BYD Dolphin (60kwh LFP Blade battery), for almost a year. Unfortunately it was damaged in an accident, deemed total loss (long story, but I wasn't in the car when it happened, so all is good).

Anyways...
At first I was worried about this, but afterwards I really didn't mind it. I charged to 100% almost weekly, but I don't think it changed anything.
You see 3.35v or above for LFP is practical a full cell's resting voltage. So those cells are somewhat full, just not balanced.

I have seen some BYD owners mention a message that come up on the dash, that recommends they charge to 100%. I guess there is a larger threshold that triggers this message. Or it just may be a timed trigger since the last charge to 100%. IDK...

As for the 12v battery, in the Dolphin they put a little 13Ah 12v LFP battery as standard, which works great. But in the Atto3 they use a Lead-acid which causes many problems and fails prematurely.
Also notice, that from what I understand, in both models, if the HV battery is under 10%, the onboard DC-DC charger stops charging the 12v battery (probably in order not to run down the HV battery deeming it dead). So many BYD owners have complained that their car simply died on them at 10%. They had to jump start it. I myself didn't get to this situation, even though I ran my battery down even under 10%. I'm still not sure this speculation is true, and have yet to prove it. But it could possibly be that these owners had a large imbalance in the cells, and the low SOC triggered LVD (even at 10%, we all know that BMSs aren't always accurate at measuring SOC, especially with LFP). This LVD (low voltage disconnect) would then stop providing power to all of the car's systems, including the onboard DC-DC converter, causing the 12v battery to drain quickly.

In general, you would expect better battery management from a large battery manufacturer like BYD. But either they have some special reasoning as to why they manage the batteries so poorly (no balancing), or they simply suck at it...
This was my first EV with LFP. My previous one was an Opel Corsa-e (45kwh NMC battery). It had other issues, but at least the battery was well balanced.

Having said all of that, I'm still somewhat of a BYD EV fan.
I'm currently waiting for my new Atto3 to arrive (unfortunately the Dolphin wasn't in stock at the time of ordering, sonI went for the Atto3).

If indeed you do have a BYD, I suggest that you verify that your software is updated, as I know that at least for the Atto3, they have tried to address the 12v charging issue with software updates. From what I hear from other BYD owners this hasn't fully solved the problem, but decreases it's frequency.

As for the cell voltage diff: I suggest that you try to slow charge it with a granny-charger (or if you your charger is capable, lower the charge rate to as low as possible).
This might help, but again I'm not sure when exactly cell balancing happens, if at all.
I really don't understand why EV manufacturers don't incorporate active balancers in thier battery packs...
Passive balancing is so slow and is rarely enough to overcome a larger cell voltage difference.

 

[Brucey]

I think it's an MG

 

[PeterWA]

How do you access the data shown in the original post? Not through the OBD2 interface, I guess?

Yes, OBD2. Via an open source piece of hardware called OVMS that stays connected permanently. It feeds data into Home Assistant.

The battery cell data makes sense with typical charging behavior. Around 2mV cell deviation in the flat part of the curve (anything below 3.3V). Deviation growing rapidly when the battery approaches 100%.

I wouldn't care if that was happening when all cells are over 3.45V, or if some balancing was apparent. But seeing this happen when most cells are still below 3.4V and having the charge current drop to zero is worth looking into more closely.

 

[PeterWA]

Let me guess... You have a BYD, right?

@Brucey guessed right, it's an MG. CATL would have been a give-away. BYD make everything in-house with their blade batteries.

Nevertheless thanks so much for the detailed reply. Looks like there are many more similarities under the hood between those brands than I expected.

Good to know that you have seen similar cell voltage deltas. Maybe there is nothing to worry about.

From my home battery I know that 3.35V is in the bend in the LFP voltage curve. I charge to 3.45V (bulk, then absorption), then drop to 3.35V for float. That drop drains 2% of the battery capacity, but that float voltage is reached when the battery is essentially resting. The car has most cells at 3.37 to 3.38V when charging just stops, not at rest. I'll have to have a closer look where the cell voltages settle to after that. Best estimate right now: around 3.33V for the bulk of the cells. That would be at least 5% below full capacity, perhaps significantly more than 5%.

One thing we definitely can agree on: the balancing circuitry (and programmed logic) is very much lacking. I can't understand how car manufacturers can be so careless, seeing what long warranties they give. MG now 10 years.

I have a trip coming up in a month that last year used up 90% of the battery by the time I get to the only charger on the way. I hope I'll still make it this year. The scenario 10% down to 0 in an instant is exactly what I fear might happen with a large imbalance of the HV battery.

 

[SeaGal]

If you haven't stumbled across them already, there are many good threads on the MG forum about balancing the SE's LFP's - see here...

MG4 51kWh LFP Battery Health

Hi everyone! I'm second owner. It had 10500 (6524 mile) kilometers on it when I bought it and SOH was 93.64% . Now my car is 17500(10873 mile) km and SOH is 93.15%. There is a difference -0.49% in 7000 kilometers. I just wanted to hear your ideas and suggestions. First of all is it normal...

www.mgevs.com

and

SOC vs HV/LV delta and balancing….

The following observation I should have done when car was newer. Or better, new, with periodic data points as it aged. Alas, I was occupied with other things. Car was charged to 100% and after a ~4hr rest: HV= 349.xx SOC =100% info 103.xx% car scanner HV/LV delta = 170mV. Car scanner (...

www.mgevs.com

for example.

Also I'll tag @wattmatters and @pollenface as they have MG's in Oz, so maybe they might chip in?

 

[PeterWA]

If you haven't stumbled across them already, there are many good threads on the MG forum about balancing the SE's LFP's - see here...

Thanks! I hadn't seen those. I reckon the MG4 is similar enough the the ZS EV, so probably should have looked in those forums too

The first of your links is mostly about state of health, SOH. I've had some insights from my car that might help out in that thread.

The second link is very interesting. Good to see that people use professional equipment to look into these issues. I'll read in more detail later.

For now I've just plugged in the portable granny charger that came with the car for the very first time, just in case it does something different to the wall mounted one I usually use. Let's see if I can at some point get the car to start balancing...

 

[meetyg]

@Brucey guessed right, it's an MG. CATL would have been a give-away. BYD make everything in-house with their blade batteries.

Nevertheless thanks so much for the detailed reply. Looks like there are many more similarities under the hood between those brands than I expected.

Good to know that you have seen similar cell voltage deltas. Maybe there is nothing to worry about.

From my home battery I know that 3.35V is in the bend in the LFP voltage curve. I charge to 3.45V (bulk, then absorption), then drop to 3.35V for float. That drop drains 2% of the battery capacity, but that float voltage is reached when the battery is essentially resting. The car has most cells at 3.37 to 3.38V when charging just stops, not at rest. I'll have to have a closer look where the cell voltages settle to after that. Best estimate right now: around 3.33V for the bulk of the cells. That would be at least 5% below full capacity, perhaps significantly more than 5%.

One thing we definitely can agree on: the balancing circuitry (and programmed logic) is very much lacking. I can't understand how car manufacturers can be so careless, seeing what long warranties they give. MG now 10 years.

I have a trip coming up in a month that last year used up 90% of the battery by the time I get to the only charger on the way. I hope I'll still make it this year. The scenario 10% down to 0 in an instant is exactly what I fear might happen with a large imbalance of the HV battery.

Sorry, missed the fact that you mentioned CATL...
So I thought it was only a BYD issue. Interesting...

 

[wattmatters]

For now I've just plugged in the portable granny charger that came with the car for the very first time, just in case it does something different to the wall mounted one I usually use. Let's see if I can at some point get the car to start balancing...

I suspect this might be your issue.

While balancing the car will typically continue to draw 50-150 W from the EVSE (it's as much about the power demand for equalisation as it is for keeping the rest of the car's systems operating, plus a trickle charge for the 12 V auxiliary battery).

Is it possible your Open EVSE wall charge station is switching off before cell equalisation is performed? Perhaps it thinks charging is finished when current drops down very low.

The LFP models of MG4 will do a balance at the end of a full AC charge, so I'd expect the MG ZS to be similar. If they are regularly charged to 100% then it only takes a handful of minutes but if less regularly like you mentioned then it will take longer. It can take a couple of hours if they have not been balanced for a while.

The granny charger should do the job if left on. Interested to hear if you have more luck with that. If not then I think you may need to raise a technical case with MG Australia.

There are other problems with my car's software. For example: the 12V battery does not automatically get topped up from the high voltage battery when the car is left unused for a while and the 12V battery gets slowly drained. The same page in the users manual that talks about cell balancing (they call it equalising), also mentions that the 12V battery is supposed to get topped up when needed.

The need for a top up would be pretty rare I'd expect as the 12 V auxiliary battery gets charged when the car's drive battery is being charged as well as when it is being driven/powered on.

Another place to ask is the AEVA MG EV forum as there are some knowledgeable ZS owners:

MG EVs - AEVA Forums

My MG4 is an NMC model. It will do equalisation at the end of pretty much whatever charge level you set the car to (LFP models have no settable charge SOC limit), but a complete balance does require an occasional full charge to 100%. Last time I did that was a couple of weeks ago (it had been 96 days since the last time I charged to 100%) and it took 2 hours.

Can see below the AC power draw (as recorded by my Iotawatt) as I had it charging slowing overnight to 100% in prep for a long drive the next day. Drive battery charging ended ~4:15AM but it kept drawing low power until a little after 6:15 AM.



Can note also that drive battery charging continued for ~ 20-minutes after SOC had reached 100.0%. That's because the SOC readings drift/lose accuracy over time. This is why MG also recommend occasionally performing a full change following a deep discharge (to <= ~10%) as it recalibrates the BMS SOC and SOH readings.

 

[pollenface]

I have a MY22 MG ZS EV with the LFP 50.3kwh battery.

I've never noticed it do an equalization at the end of a full charge, it gets charged to 100% maybe once a week, other times just whatever it gets while the sun is shining while the car is not being driven. I've read that aggressive driving and fast DC charging is likely to put the cells out of balance. We drive it normally and slow-AC charge only so cells probably never get out of balance enough.

I don't really worry about it too much, I just charge it and drive it. Almost 50,000kms in 2yrs, so far so good

 

[wattmatters]

I've never noticed it do an equalization at the end of a full charge

Most will never know as they are not monitoring it. Slow charging is not something people tend to hang around and watch.

The BMS will provide a warning if an equalisation is required and advise a long slow charge should be performed. If most charging is already AC slow charging then it's unlikely to ever need to give such advice.

 

[meetyg]

Most will never know as they are not monitoring it. Slow charging is not something people tend to hang around and watch.

The BMS will provide a warning if an equalisation is required and advise a long slow charge should be performed. If most charging is already AC slow charging then it's unlikely to ever need to give such advice.

Sometimes balancing is noticeable: The charging stays on 99% for a long time. I had this on my previous (non LFP) EV. But haven't seen such a behavior on my (R.I.P) BYD EV w/LFP.

 

[pollenface]

. If most charging is already AC slow charging then it's unlikely to ever need to give such advice.

my thoughts also

 

[sunsurfer]

I was just thinking earlier about HV packs and people being all excited about HVDC batteries for solar cycling with their fancy HV HF inverters that can do it all in one box nonsense. The this post popped up.

Then it occurred to me, solar cells are typically lower grade than auto and I wondered if any of the cheaper EVs or EVs with budget replacement batteries have had issues with cell balancing? Its a ton of cells in series and if they are lower grade/not well matched....well sounds like a nightmare and a circus act with the balancer.

I don't know if thats OPs problem but I suspect this will be a common topic with HVDC solar batteries in the future.

Sounds like herding cats. I will take a pass.

 

[wattmatters]

Then it occurred to me, solar cells are typically lower grade than auto and I wondered if any of the cheaper EVs or EVs with budget replacement batteries have had issues with cell balancing? Its a ton of cells in series and if they are lower grade/not well matched....well sounds like a nightmare and a circus act with the balancer.

Balancing isn't a big deal. My car has a 104 cell series pack, and it balances at end of reaching its set charge limit, normally finished balancing in 5-10 minutes. If you do a full charge then it might take longer, how long it takes depends on how long since the last full charge and how much DC fast charging you do. e.g. just a little while ago mine finished its charge to a set 80% SOC. It balanced for 8 minutes at the end:



The change in charging power is due to automations that:
i. ensure I don't over draw the current limit on that phase during the free energy period (12-2 PM)
ii. match excess available solar PV

Right at the end can see the period when the car is performing a cell equalisation, power draw drops to ~100+/-50 W, most of that is just keeping the car's system on/operating during this process and trickle charging the 12 V battery. Once done, power draw is stopped by the car.