Equalisation charge - NMC battery

[MarkHoward]

Two questions please:

1. Has it been clarified yet whether a full equalisation charge is achieved with an NMC battery at 80%? (or will that only happen at 100%?)

2. Is it possible to achieve a full equalisation charge (at either 80% or 100%) using scheduled charging - or will the charge terminate as soon as the charge gets to the specified charge level, without maintaining the charge connection so as to achieve the balancing? (Reason for this is to balance during off-peak power - in the middle of the night when I'm not able to watch the charge rate)

 

[GaryMG4]

My 2022MY Trophy (64kW NMC) will balance the individual cells when it reaches the specified %age. This is evident from the gradually reducing/fluctuating current draw one the desired %age has been reached, down as low as 80w. The range and social do not change during this process and eventually the car will terminate the charge, if I use the Zappi it reports EV Full, even if I've only requested 80%. I don't use the cars schedule charging unless I am using the 3 pin evse, If I use the Zappi I'm either on eco+ to just keep adding solar until the car terminates the charge or using Octopus smart charge I ask for nn% to be added certain time, in which I usually ask for 10% more than there's space for to ensure a balance charge.

 

[Martinonline]

Unfortunately this is a topic riven with false statements and often dogmatic positions.

@MarkHoward, you have introduced the adjective 'full' in front of 'equalisation' and this can lead to some saying that a 'full equalisation' can only be done at 100% battery level.

The MG4 manual shows the information, from different parts, as below.







From the first extract it could seem that an equalisation charge means charging to 100%. However the emphasis of the extract, to me, is the achievement of the 'optimal equilibrium state' through slow charging with an equalisation phase.

The second abstract provides a definition of equalisation charging, that it is charging continuing after a 'normal' charging process has completed. It does not define a 'normal charging process'. For MG4s with an LFP battery this must be 100% since there is no charge level adjustment. However, those MG4s with NMC batteries can alter the charge level and 80% is consider the 'Health' level. So it can be considered that equalisation charging can occur after normal (80%) charging has occurred. By analogy other set levels can be substituted. But not going to 100% does not create an 'optimal equilibrium state'.

I have not seen a post by anyone stating they have seen the 'Please Slow-charge the Vehicle's message.

The third extract requires you to do a regular slow equalisation charge.

 

[Martinonline]

To continue the above, I believe my MG4 Trophy with its NMC battery equalises the battery cells after completing its charge to the set level.

Whilst it is doing the equalisation the info screen shows that it is doing 'Balancing', which can be considered the same activity. See image below (not mine):

 

[MarkHoward]

Thanks for your thorough responses.
So, the MG4 manual is not crystal clear about exactly what is required. Seems this has been caused by them trying to keep the manual (too) simple when covering both LFP and NMC batteries.
I think the only way for me to be sure is to sit in the car and watch what happens to the charge rate at the end of a charge to 80%, when I'm doing a scheduled charge. (just using a granny charger)
The car is getting its 1 year service today and I've asked if they can tell me how well balanced the cells are. Not sure if their software will tell them this. How many cells are there, BTW?

 

[wattmatters]

How many cells are there, BTW?

The 51 and 64 kWh models each have 104 cells in series. Difference is 51 is LFP and 64 is NMC.

In my 64 at the end of a charge to 80% the charge rate drops down and fluctuates between ~50-100 W indicating a period of balancing. Usually takes about 5 minutes give or take. If it's been a long time though since the last balance and/or with several DC fast charge sessions then it can take an hour or so.

It will do the same at pretty much any max SOC charge setting.

LFP however should only balance when at full charge. It's to do with the voltage - SOC relationship, it is quite different between the two chemistries meaning balancing logic needs to be different.

 

[MarkHoward]

So (for the avoidance of doubt) even using scheduled charging, with an NMC battery, the balancing will occur after the charge reaches 80% and before the car disconnects the scheduled charging session? That would settle my mind greatly, if that is the case.

 

[ReintjeWA]

Another way to see the car balancing the cells is when the app states you’ve reached your set % (so 60%, 70%, and 90% should also balance) but hasn’t stopped charging yet. The finish time will keep updating to the current time.

You could sit in the car and wait for the balance screen, but I would just trust that the car balances.

 

[Jamesrr]

Yep - mine "balances" when I hit the 80% mark I use for battery health "week to week" charging.
I can see the slow charging at the end happening via the Tesla app with my Tesla Wall Charger.

 

[wattmatters]

So (for the avoidance of doubt) even using scheduled charging, with an NMC battery, the balancing will occur after the charge reaches 80% and before the car disconnects the scheduled charging session? That would settle my mind greatly, if that is the case.

Scheduled charging implies a time limit.

If the scheduled charge ends before the car reaches its max SOC setting then no, it won't have performed a balance (equalisation).

If the car reaches the max SOC target before the end of the schedule, then it will commence balance (equalisation) charging.

Whether it gets to complete that equalisation process depends on whether or not the time remaining in the charge schedule is long enough. It may only need a few minutes, however it might need a couple of hours. It depends on how out of balance the pack is.

 

[MarkHoward]

Great. I understand. Thanks very much.

 

[Kithmo]

Mine balanced at 70% a couple of months ago as I charged it during a period of free electricity I had, during the day, from EDF. I didn't want to go over the free time or change the Pod point schedule so I set it to manual and 70% in the car, I knew it would reach 70% before the end of the time.
FWIW I get about 10% per hour on my charger, so it's easy to work out roughly when it will reach the required SOC.

 

[Joningy]

Unfortunately this is a topic riven with false statements and often dogmatic positions.

@MarkHoward, you have introduced the adjective 'full' in front of 'equalisation' and this can lead to some saying that a 'full equalisation' can only be done at 100% battery level.

The MG4 manual shows the information, from different parts, as below.

View attachment 39551

View attachment 39556

View attachment 39557

From the first extract it could seem that an equalisation charge means charging to 100%. However the emphasis of the extract, to me, is the achievement of the 'optimal equilibrium state' through slow charging with an equalisation phase.

The second abstract provides a definition of equalisation charging, that it is charging continuing after a 'normal' charging process has completed. It does not define a 'normal charging process'. For MG4s with an LFP battery this must be 100% since there is no charge level adjustment. However, those MG4s with NMC batteries can alter the charge level and 80% is consider the 'Health' level. So it can be considered that equalisation charging can occur after normal (80%) charging has occurred. By analogy other set levels can be substituted. But not going to 100% does not create an 'optimal equilibrium state'.

I have not seen a post by anyone stating they have seen the 'Please Slow-charge the Vehicle's message.

The third extract requires you to do a regular slow equalisation charge.

 

[Joningy]

I'm carefully reading the owners MGS5 manual ahead of delivery of mine in just over a week.
See attached screenshot showing both battery sizes and typical charging times, in the lower right hand column

it's pretty clear to me that Carrying out a full slow charge to 100 % is the only way for the high voltage battery to reach the optimal equilibrium state (equalisation charge).
So I'm not sure I've carried out this charge fully on my MG4 Trophy as I charge at night with Octopus Energy from 11.30pm to 5.30am, 6 hours, which isn't enough time carry out the full AC slow charge necessary as I usually charge when we get down to around 30%.
So, a new approach for me might be to charge to say 80% overnight then the next night, assuming the car isn't used much between charges, charge to 100% giving the battery enough time to charge from 80% to 100% and time to carry out the necessary lengthy period of equalisation.
But what's miss so far in the instructions is the frequency of equalisation charging, apparently Tesla recommends weekly equalisation. Still a bit confused!

 

[Roger001]

NMC batteries have a very flat voltage curve for most of their % charge level. As such it is not possible to fully balance them at 80%, but the process can be started, which seems to be what the MG4 does when connected to an AC charger.

The balance process of a BMS to fully balance a battery set needs to take the batteries to their highest possible working voltage, it then keeps raising the voltage of those batteries that have not yet reached the voltage, while making sure that the others do not exceed the voltage. This results in all the cells being at their maximum operational capacity. At which point the BMS can calculate the overall capacity of the battery set correctly.

 

[siteguru]

NMC batteries have a very flat voltage curve for most of their % charge level. As such it is not possible to fully balance them at 80%, but the process can be started, which seems to be what the MG4 does when connected to an AC charger.

That comment describes an LFP battery pack, not an NMC one.

 

[Roger001]

That comment describes an LFP battery pack, not an NMC one.

No, LFP are just even worse/better. It all depends on what you are comparing against. Neither NMC nor LFP batteries allow you to get a full sense of their charge state, compared to say a lead-acid battery cell.

An average NMC battery cells' voltage while being charged may vary from around 3.55V (near zero) to 4.34V (near full). While for an LFP the range is around 3.3V to 3.45V and then will rise for the last few percentage points.

The result is that to get a full equalisation of cells you need to take them to 100% and balance them so that they all report the same target voltage. At this point, you also know that the battery is now at 100% charge.

 

[siteguru]

One looks very flat - the other doesn't.

 

[Data]

No, LFP are just even worse/better. It all depends on what you are comparing against. Neither NMC nor LFP batteries allow you to get a full sense of their charge state, compared to say a lead-acid battery cell.

An average NMC battery cells' voltage while being charged may vary from around 3.55V (near zero) to 4.34V (near full). While for an LFP the range is around 3.3V to 3.45V and then will rise for the last few percentage points.

The result is that to get a full equalisation of cells you need to take them to 100% and balance them so that they all report the same target voltage. At this point, you also know that the battery is now at 100% charge.

Yes, you are looking at this from a technical engineers perspective, so I do understand where you are coming from Roger. Strictly speaking you are correct when compared to some other types of battery. And your description of the equalisation event is again perfectly correct.

The main issue for me is that so many folks here & in the ZS forum give differing information/accounts about the equalisation process. Some say they don't have to give a full charge from the 'low battery alarm' yet others say they do have to wait for that low battery alarm before the car will equalise & it must go to 100% soc for it to work. I've had so many differing stories about this. Yet, my dealer & MG customer support seem quite clear. At least for my ZS EV with NMC battery. It HAS to have that low battery warning & then have a 'FULL CHARGE' for the BMS to take full control & properly equalise the battery. Indeed, that's the only way my car will equalise. And I have tried it all ways. But it seems according to some, their earlier cars will equalise without the low battery warning. This seems to be for ZS & some MG4's.

My take on this is that equalisation is variable from car to car irrespective of the model. That is what my research has found. This is probably due to software differences & of course differing battery types. So when someone says "do it this way" they may or may not be right!

 

[wattmatters]

NMC batteries have a very flat voltage curve for most of their % charge level.

Unlike LFP, the NMC charge voltage curve it's not so flat that it prevents the ability to do effective equalisation at a partial state of charge. There is a sufficient voltage gradient and cell voltage differential.

But also importantly the deviation from a near linear voltage-SOC relationship at high states of charge (at a steady charge rate) is pretty small.

In contrast LFP voltage is VERY flat for most of the SOC range, and then rises very sharply as it reaches a very high state of charge.

This lack of voltage gradient and very small cell voltages differentials renders LFP effectively impossible to do any sort of balancing at anything other than very high states of charge. Indeed attempting to do is just as likely to cause further imbalance.

I agree that a proper equalisation for either chemistry does ultimately require a full charge but an NMC pack does not require a full charge quite as often for this purpose since it can get most of the job done more often.