Balance Charging - a few questions

[2Sheds]

A few EV noob questions

I have had my phase 2 Trophy for about 2.5 months now, and as it is owned not leased, I am keen to keep the battery in as good a state of health as I can. With that in mind I have been cycling it between around 20-80% as per recommended practice. The thing is that we aren't doing a lot of mileage, particularly after the first couple of weeks of getting the car, and I've been waiting for a long while now for a charge cycle to fall just before a long run to do a balance charge, so the car doesn't sit around at 100%. Just isn't happening though! As a consequence I haven't yet done a balance charge . (It was at 100% when we picked it up though). So far the car has done about 1400 miles, been exclusively charged at home, averaging about once every ten days or so (using Ohme charger, not granny lead) with the exception of one 25% top up at a 50kW rapid. So...I have a few questions:

1) The recommendation is to perform a balance charge once a month. So is this requirement purely a time based thing, or is that just based on an average usage? In other words, is it ok to do it less often if the car is used infrequently/does light mileage?

2). Does it matter what SoC the battery starts off at when doing a balance charge, ie is it ok to do it from 80%, 60%, whatever, or should it be a from low SoC? (I think I read somewhere that the battery should be occasionally charged 10-100%). I ask this so that if we do a longer trip in-between the normal 20-80% routine, I could balance it then, eg if it was at 60%.

3). Am I overthinking this? I am worrying too much about it?!

Thanks in advance.

 

[MuzGB1970]

From my experience you get a balance charge even if only charging to 80%. I charge at work at around 10kW, when the car reaches 80% it goes into balance mode and draws around 0.3kW for up to 30 minutes. That's the balancing. So if you're regularly charging to 80% (set in the car/app) and leaving your charger connected, then you will be balancing.

I tend to charge to 100% every few weeks, not necessarily for long journeys but just to charge to maximum. Don't leave the car on 100% for days though, usually some small trips and it's back down to 80% in no time anyway.

I think it's normal to overthink or worry about charging, just relax and charge how it works for you. The BMS should take care of things.

 

[Corindikev]

The car will perform an equalisation charge every time it is charged on a slow charger, i.e 11kW or less. Plug the charger in & set the max to 80% & when the car says charging is complete that includes the battery equalisation process.

Equalisation only becomes an issue when using fast and rapid chargers repeatedly. The car will recognise that the batteries require equalisation and will display a message as per the the manual reprinted below.

If an equalisation charge has not been carried out for
sometime, the message centre in the instrument pack will
display 'Please Slow-charge the Vehicle'.

 

[Rolfe]

I think that @MuzGB1970 is right, you don't need to overthink this. Your battery will be fine with what you're doing, you're not shortening its life. Just try to get a 100% balance charge in occasionally.

Bear in mind that you don't need to be doing a really long run when you do a 100% balance charge. You only need to take the battery down below 80% and then it's OK to let it sit at that. That's what, about 50 miles in the Trophy? Surely you have the occasional 50 mile trip?

 

[Martinonline]

I, too, am a low-mileage user with occasional longer runs. I would have bought the SE SR except that I wanted the extra features of the Trophy. There are 2 issues here: charging to 100% and balancing. I believe balancing will be done at any charge level on slow chargers and needs to be done to share the load, make the BMS aware of individual battery cell conditions and thus give a more accurate SOH and mileage at that level. Charging to 100% tests the capabilities of the individual cells to the limit and thus gives the BMS early warning of failures/faults with the best prediction of max range. I will regularly charge between 20% and 80% until I need 100% whatever the interval between.

 

[Rolfe]

I do like the SR. Just plug in, let it go to 100% and balance for half an hour whenever you need to, and drive off with a full tank. No fuss no muss.

But still, I'm convinced that if I'd needed the range of the LR I'd have got on fine. I don't think it's as critical as all that, or that much of a hassle.

 

[PeteC]

I do like the SR. Just plug in, let it go to 100% and balance for half an hour whenever you need to, and drive off with a full tank. No fuss no muss.

But still, I'm convinced that if I'd needed the range of the LR I'd have got on fine. I don't think it's as critical as all that, or that much of a hassle.

I'd agree. The LR is most times no more hassle. Our Trophy is set to 80% so it's just as easy plugging in and letting it charge. The only time it causes more effort (i.e. you have to change the charge setting!) is when doing an equalisation charge, but that's not hard to do and not that often. I can usually fit it in to coincide with journeys that use up some of the charge. I don't worry if it's a bit before or a bit after the month is up since the previous balance.

 

[2Sheds]

Thanks for all the replies, very useful and reassuring. Maybe I can get a good night's sleep now!

Good to know that balancing is done at lower SoCs as well, as that happens almost every time with our Ohme Home Pro charger on OVO Charge Anytime. I usually set the car to stop at 80%, then request the Ohme to add the required %age to get there by a certain time. The thing is that there being no API integration, the charger estimates what is required, but it seems it slightly overestimates, so it ends up trying to keep charging for a good while after the car has reached 80%. Which is a good thing, as it allows the balancing to take place. (Makes me wonder what would happen if it did have API integration - probably have to change this method?).

Bear in mind that you don't need to be doing a really long run when you do a 100% balance charge. You only need to take the battery down below 80% and then it's OK to let it sit at that. That's what, about 50 miles in the Trophy? Surely you have the occasional 50 mile trip?

Normally, yes I suppose, but just in the last few weeks it doesn't seem to have been the case somehow.

Anyway, I'll relax about it, try to get the occasional 100% in from whatever SoC when I know I'm going to use 20% in the next day or two.

 

[Rolfe]

I don't think you have anything to worry about.

 

[MGFour]

Agree, don't worry about it. I don't think whatever you do would drastically affect the battery long term, if you are keeping the car for 10yrs and well over 100k miles you might notice a few percent difference but the BMS will keep an eye on your pack and adjust charges etc if needed.

 

[ukslim]

3) Yes.

 

[wattmatters]

One wold hope the onboard BMS will manage cell balancing just fine.

NMC battery chemistry and LiFePO4 battery chemistry however should have quite different cell balance protocols because of the nature of the voltage-SOC relationship is different.

LiFePO4 has a very flat charge (and discharge) curve, meaning between 10%-90% SOC the voltage remains quite constant. Cell voltage differences tend to only become apparent at very high SOC (95%-100%). Balancing for LiFePO4 should only be done at very high SOC levels, and I would expect this is why the recommendation for LiFePO4 models is to charge to 100%.

https://www.researchgate.net/figure/charge-curve-for-LiFePO4-battery-chemistry-10_fig2_342880715

NMC has a far more linear SOC-voltage relationship between 50% and 100% SOC so cell voltage imbalances will show up at much lower SOC and is why performing some balancing at, e.g. 80% is possible.

https://www.researchgate.net/figure/Charge-and-discharge-voltages-at-025-C-in-dependence-of-SOC-for-the-NMC1-battery_fig1_323154990

 

[2Sheds]

Interesting stuff. Although I was an electrical engineer in a previous life, I don't know much about battery technology - learning fast though!

3) Yes.

That was the easy question!

 

[pushdword]

I'm interested in asking you when you get the car (0 km), the BMS ignores the charging limit?
I'm just concerned that this could be a bug and I'm charging to 100% or this is not a bug and it's normal in the first cycles...
Yesterday I did 700km with one 50kW DC charging for half an hour and another charge of 80kW. This could unbalance the cells... I really don't know.

I own a MG4 luxury model (Europe) which I, so far I know, it has NMC811 cells with 64kWh total capacity.

 

[Corindikev]

It will take a few more days of fast charging before the cars internal equalisation software will decide it needs to be done. Then you will get a message on the instrument screen asking you to slow charge the battery. You do not have to be concerned about it. Just take notice of what the car tells you.

 

[Rolfe]

It will take a few more days of fast charging before the cars internal equalisation software will decide it needs to be done. Then you will get a message on the instrument screen asking you to slow charge the battery. You do not have to be concerned about it. Just take notice of what the car tells you.

I never knew the NMC batteries did that. I don't know if the LFC does it or not, because I always take mine to 100% and balance whenever I charge, unless I'm on a long trip. Even when I went down to Brighton, I was able to leave the car on a destination charger long enough to balance while I was there, ready for the trip home.

I do know that it never passes up on the chance to balance. Every single time it hits 100% it faffs around for half an hour at 20 watts or so.

 

[wattmatters]

This could unbalance the cells... I really don't know.

Not by much. Imbalance is really only going to be of concern if you only ever use DC fast chargers. That would be unusual for most I'd have thought.

Pretty much any time you use an AC power supply for charging, the car's on board systems will look after it for you.

 

[Tig170]

Interesting : I have an SE LFP, but normally only charge to what ever I can get overnight, as only have a 5 hrs window at 4.5p / Kwh - so the most I can get is 63% charge (around 35KW / night -I have an ohme pro - and just plug in when ever I'm below around 30%) - sort of twice a week. I dont like when im over 95% charge as the regen-brakes work slightly differently. I dont know if its just bedding in but in the 1st 6 months, i lost 2.5% SOH in the battery, but in the last 6 months, when ive just charged to less nearer 80% max., i've only lost an extra 0.8% SOH - Im told "It should also be noted that a LiFePO4 cell stores very little additional energy above 3.45V" - albeit on my 15kw home energy storage system (where I have much more data on the BMS - it is cleverer than us!) - the key thing here is the 3.45v on LFP's / cell, which ties into post 12 nicely.

 

[wattmatters]

Im told "It should also be noted that a LiFePO4 cell stores very little additional energy above 3.45V" - albeit on my 15kw home energy storage system (where I have much more data on the BMS - it is cleverer than us!) - the key thing here is the 3.45v on LFP's / cell, which ties into post 12 nicely.

3.45 V is about the LiFePO4 cell voltage below which one should not attempt to do cell balancing and yep that will be a pretty high SOC, although charging voltage and resting/open circuit voltage are different things.

I think if I had the LiFePO4 version of the MG4 I would be charging to 100% every so often to allow it to do a top balance. The NMC versions would benefit from the occasional full charge as well.

I too have 15 kWh of LiFePO4 home storage, in a hybrid set up with 20 kWh of sealed lead acid used as our backup reserve. They actually work really well together.

 

[Rolfe]

Interesting : I have an SE LFP, but normally only charge to what ever I can get overnight, as only have a 5 hrs window at 4.5p / Kwh - so the most I can get is 63% charge (around 35KW / night -I have an ohme pro - and just plug in when ever I'm below around 30%) - sort of twice a week. I dont like when im over 95% charge as the regen-brakes work slightly differently. I dont know if its just bedding in but in the 1st 6 months, i lost 2.5% SOH in the battery, but in the last 6 months, when ive just charged to less nearer 80% max., i've only lost an extra 0.8% SOH - Im told "It should also be noted that a LiFePO4 cell stores very little additional energy above 3.45V" - albeit on my 15kw home energy storage system (where I have much more data on the BMS - it is cleverer than us!) - the key thing here is the 3.45v on LFP's / cell, which ties into post 12 nicely.

The recommendation is that you take the LFP to 100% and let it balance once a week. I get the impression this is important for two reasons. One is that if you don't do it your GOM may start lying to you, and if you suddenly discover this on a motorway some distance from the next exit, you may regret it. The other one is that you will appear to lose range, which will recover after a decent balance charge or two.

I just take mine to 100% and let it balance every time I charge, and it seems to like that. I can cope with having little to no regenerative braking for the first few miles after I've done it. But if you're cutting the charge short because of your electricity tariff, you should try to organise these balance charges when you can.