Balancing the SR (LFP) battery pack

[SteveBirchall]

With the standard range SE the batteries can take 100% every time and so we don’t have the option of the automatic stop at 80% and surely we should have that option. Even though they can take 100% each charge sometimes you might just want 80%?

 

[siteguru]

For about 2 weeks after I got my SE SR the app included an option to set a charge limit. However that soon changed - without any app update. I think this option is controlled on the server side, so the default is to allow the limit setting but once your account/car is reconciled this option is removed. (I don't recall ever being able to set a limit in the car itself).

 

[FiferMG4]

Ok thanks. Glad they could correct it.

But with the standard range SE the batteries can take 100 % every time and so we don’t have the option of the automatic stop at 80% and surely we should have that option. Even though they can take 100% each charge sometimes you might just want 80%?

I suppose with the SR, you’d have to stop it manually if you were wishing to have a lower percentage of charge. While I had the software issue, I just calculated how long it would take the car to charge to 80% and then stopped it manually.

 

[SteveBirchall]

For about 2 weeks after I got my SE SR the app included an option to set a charge limit. However that soon changed - without any app update. I think this option is controlled on the server side, so the default is to allow the limit setting but once your account/car is reconciled this option is removed. (I don't recall ever being able to set a limit in the car itself).

I wonder if there is a factory setting that tells the system whether it is a long range or short range and sometimes they set it in error and then correct it.

 

[Rolfe]

I could be wrong, but my feeling is that they want to discourage SR owners from stopping below 100% (and so preventing the pack from balancing) as much as they can. It's always possible to stop it manually in the app, but you really shouldn't do it as routine.

Last time I did my <10% to 100% charge I had calculated how much charge I needed overnight so that I'd have enough for the following day's trip, but only just enough, so that I'd be down to <10% by the time I got home. I set the alarm on my phone for 4 am, when it woke me I went into iSmart and stopped the charge, then went back to sleep without even putting on the light.

A bit of a hassle, but I reckon they really don't want you stopping the charge before 100% as a general rule.

 

[SteveBirchall]

You don’t need to use balancing on the SE standard range. The battery is 51 kWh with a usable of 50.8 kWh. So there is no spare capacity to use for balancing.
They are different batteries to the long range. The standard range batteries are LFP and do not need balancing and can safely be charged to 100% capacity without having any significant damage.

 

[Rolfe]

They do need balancing. At least every week according to the manual. My car balances for 30-35 minutes every time it reaches 100%.

 

[SteveBirchall]

They do need balancing. At least every week according to the manual. My car balances for 30-35 minutes every time it reaches 100%.

Balancing occurs when you charge to 100% and so they are balanced every time you charge a standard range SE (unless you stop the charge.
There is 0.2 percent spare in the 51kWh LFP batteries so there is virtually zero spare to provide balancing.
Honestly for the LFP standard range batteries you do not need to do anything special, you just charge to 100% every time.

Please Google LFP battery charging and you will see that.

 

[siteguru]

I think you're both saying the same thing in different ways - the end result is the same: just charge to 100% and don't worry about it.

 

[SteveBirchall]

I think you're both saying the same thing in different ways - the end result is the same: just charge to 100% and don't worry about it.

Agreed

 

[Rolfe]

Yes, exactly. My original point was that I think MG has removed the facility to stop the SR's charge before 100% so that it does in fact balance every time. It's now a bit of a faff to stop it early, so people will only do it if they have a particular reason.

 

[fshuk1]

I am not sure about this.
When I charge to 100% for another 30mins the charger needs to be connected to balance the battery. It starts to use less and less power until it stops. If I take it off when it reaches 100% it doesn't do the balancing. So my advice for what it is worth is, if you charger does not stop automatically at 100% then you are ok just leave it connected until it stops taking power. If you charger automatically stops when it get to 100% (mine does, OHME home pro) then every now and then get the granny cable and connect your car when it has 100%. You will see it takes in power and slowly reduces it to zero in 30-45mins. I first heard it from Rolfe and I do this now.

 

[SteveBirchall]

I am not sure about this.
When I charge to 100% for another 30mins the charger needs to be connected to balance the battery. It starts to use less and less power until it stops. If I take it off when it reaches 100% it doesn't do the balancing. So my advice for what it is worth is, if you charger does not stop automatically at 100% then you are ok just leave it connected until it stops taking power. If you charger automatically stops when it get to 100% (mine does, OHME home pro) then every now and then get the granny cable and connect your car when it has 100%. You will see it takes in power and slowly reduces it to zero in 30-45mins. I first heard it from Rolfe and I do this now.

One way I suppose, but charging an EV for the general public shouldn’t be so complicated. Just plug it in and let it charge. Limiting it to 80.% mostly (if you have the option) and occasionally to 100%.

But Joe/Joan public should not worry about a complex process.
After all, independent studies have proven that a set of EV batteries should handle 1,000 to 2,000 full charging cycles and that equates to 200,000 to 400,000 miles.
So probably the life of the car. Without being nursemaided.

If you like to be thorough and follow a complicated process then fine.
But most people just need to plug in the charger and not worry about balancing etc. Modern batteries don’t need to be treated as if they need to be “micro managed”

 

[Rolfe]

If you have an SR there is absolutely no point whatsoever in limiting it to 80%. There is nothing complex about leaving it to charge to 100% and then letting it balance. The complication here seems to be introduced by a particular charger that is cutting the power when the car reaches 100% and not letting it balance.

It seems to me that is an issue with the charger that needs to be addressed. It's not the fault of the car and it's not the fault of MG, who tell SE owners quite clearly to let the car go to 100% and balance whenever you AC charge and at least once a week (if you're charging that often).

 

[SteveBirchall]

If you have an SR there is absolutely no point whatsoever in limiting it to 80%. There is nothing complex about leaving it to charge to 100% and then letting it balance. The complication here seems to be introduced by a particular charger that is cutting the power when the car reaches 100% and not letting it balance.

It seems to me that is an issue with the charger that needs to be addressed. It's not the fault of the car and it's not the fault of MG, who tell SE owners quite clearly to let the car go to 100% and balance whenever you AC charge and at least once a week (if you're charging that often).

You seem to be missing one vital point. The standard range (LFP) batteries do not have any real balancing capacity. Of the 51kWh the “usable amount” is 50.8 kWh leaving just 0.2 kWh for balancing. So it makes very little difference if you balance or not. Normal usage is all that is needed.
The rest of the mg4 range have a larger balance that does need balancing every now and then.
The 64 kWh has a reserve (for balancing purposes) of 2.3 kWh. I.e only 61.7 kWh available for use. That is why you are advised that charging to 100% every now and then might help a little.

But my basic point is that the cars are designed to be just used by people who want to drive and charge. Let’s not make people have complicated rituals to have to carry out.

 

[Coulomb]

The standard range (LFP) batteries do not have any real balancing capacity. Of the 51kWh the “usable amount” is 50.8 kWh leaving just 0.2 kWh for balancing.

The amount that isn't usable (the "buffer") is for longevity of the cells in the battery, not for balancing. Using the bottom half percent or so of LFP would encourage bad reactions, from poor memory I think it's plating of one of the electrodes. The larger batteries are NMC chemistry, which charge to a higher voltage per cell, so they need a top buffer as well as a bottom buffer. Different bad reactions happen at the top end (high cell voltage). LFP charges to a lower voltage, so it doesn't need a top buffer.

These buffers have nothing to do with balancing. Balancing involves taking some energy from some cells, and either dissipating it or transferring it to other cells, so all the cells become full or very close to full at the same time. Balancing is just as important and effective in LFP batteries as in NMC batteries. Because of the very flat voltage versus SoC curve of LFP, this only makes sense when the battery is near 100% SoC.

 

[SteveBirchall]

The amount that isn't usable (the "buffer") is for longevity of the cells in the battery, not for balancing. Using the bottom half percent or so of LFP would encourage bad reactions, from poor memory I think it's plating of one of the electrodes. The larger batteries are NMC chemistry, which charge to a higher voltage per cell, so they need a top buffer as well as a bottom buffer. Different bad reactions happen at the top end (high cell voltage). LFP charges to a lower voltage, so it doesn't need a top buffer.

These buffers have nothing to do with balancing. Balancing involves taking some energy from some cells, and either dissipating it or transferring it to other cells, so all the cells become full or very close to full at the same time. Balancing is just as important and effective in LFP batteries as in NMC batteries. Because of the very flat voltage versus SoC curve of LFP, this only makes sense when the battery is near 100% SoC.

But that is exactly what balancing does, it varies the cells there are getting the charge so that you are not always charging the same cells each time.

 

[Coulomb]

But that is exactly what balancing does, it varies the cells there are getting the charge so that you are not always charging the same cells each time.

The cells are all in series, so they all get the same charge current, except for the quite small difference that the balancing resistor or diverter circuit can make. If you are charging at 6.6 kW, that's say 17 A into the battery; the bleed resistor or diverter can likely only shunt away much less than one amp, so no cell is getting less than about 16.8 A.

 

[SteveBirchall]

The cells are all in series, so they all get the same charge current, except for the quite small difference that the balancing resistor or diverter circuit can make. If you are charging at 6.6 kW, that's say 17 A into the battery; the bleed resistor or diverter can likely only shunt away much less than one amp, so no cell is getting less than about 16.8 A.

I think that it would be fair to say that the discussions about charging, however informative they may be, have drifted way off topic. Perhaps it might be worth moving this to a new topic, do you think?
Thanks.

Edit moderator: Now moved.

 

[siteguru]

But that is exactly what balancing does, it varies the cells there are getting the charge so that you are not always charging the same cells each time.

Sort of, but modified as per @Coulomb. However the key point is that the buffer plays no role at all in the balancing activity - which was the point you were suggesting to @Rolfe and which @Coulomb corrected.