Balancing the SR (LFP) battery pack

[Rolfe]

I think if we drill down on this, the SR is perfectly simple, just let it go to 100% and do its balancing thing whenever you have it on an AC charger. The problem would appear to be that this Ohme Pro thing cuts the charge once the car is at 100% and doesn't let it balance. What to do about that is an issue for people with that make of charger.

 

[Buster72]

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 51kw 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.

Correct.

 

[siteguru]

(Partially) Incorrect ... the buffer has nothing to do with balancing. (The rest is correct).

 

[Martinonline]

(Set hares running.) The LR batteries will also balance at less than 100%.

 

[MGFour]

I think if we drill down on this, the SR is perfectly simple, just let it go to 100% and do its balancing thing whenever you have it on an AC charger. The problem would appear to be that this Ohme Pro thing cuts the charge once the car is at 100% and doesn't let it balance. What to do about that is an issue for people with that make of charger.

How do we know the ohme isn't balancing?

I don't have the pro but the older version Ohme. They seem very similar in how they work and assumed it was balancing.
I select more charge than needed (i.e. 50% or more when I only need 40%) to fill to 100%, I'm presuming it charges to 100% and then continue until balancing complete. I've not sat and watched what rate its pulling and when it actually stops but cant see why it would stop supplying power if car wants some juice to balance?

 

[Rolfe]

How do we know the ohme isn't balancing?

I don't have the pro but the older version Ohme. They seem very similar in how they work and assumed it was balancing.
I select more charge than needed (i.e. 50% or more when I only need 40%) to fill to 100%, I'm presuming it charges to 100% and then continue until balancing complete. I've not sat and watched what rate its pulling and when it actually stops but cant see why it would stop supplying power if car wants some juice to balance?

This is the message I was relying on for that information. I have no personal experience of it.

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.

 

[tsedge]

This discussion is forgetting calibration. The battery pack also needs to go to 100% to calibrate the curve.

Maybe this will make it clearer... this is my current understanding, open to corrections.

Voltage curve: this is how the voltage of the pack varies with state of charge. NMC has quite a strong curve so a full battery will output a considerably higher voltage than an empty one. LFP has an almost flat curve by comparison (except at the extremes).

Balancing: balancing the individual cells in the battery to equalise their voltage. This is important so that individual cells are not over or under charged as the whole pack is charged and discharged. Since NMC has a big voltage change with state of curve and big buffers it is easy for the BMS to keep track and balancing is not needed regularly, monthly is sufficient. LFP needs this more often because the voltage changes are much smaller and harder to keep track of, so weekly is typical. Balancing can occur at any state of charge but is easiest at max charge due to the curves.

Buffers: there are top and bottom buffers which are spare battery capacity to avoid overcharging and overdischarging which can damage the battery. NMC makes good use of these (eg MG4 LR has 64kWh but only 61kWh usable - 0% to 100%). LFP has an almost flat charge curve by comparison and has a very small buffer (low risk of damage). This is why LFP balancing only takes a few watts, but NMC much more.

Calibration: as the battery pack ages the cells slowly degrade and can hold less, the BMS needs to see a wide range of charge states in order to calibrate the battery as it ages. This is why we have the 10%-100% every few months recommendation.

Advising people to charge LFP to 100% at least weekly achieves calibration and balancing, so is an easy thing to promote.

With NMC, again due to the curve and chemistry, staying at high states of charge will lead to some battery aging, hence it is worth asking people to charge to 80% normally and 100% monthly to maximise their battery life.

However, unless you keep your car for years and years, you may as well just use it as you'd like to.

 

[Buster72]

I have read these posts with interest as I have a SE SR.
Just out of interest, apart from a couple of times on a public charger, I have charged it at home using the granny charger. And the dealer service result for SOH was 99.5.

 

[gwatuk]

I thought some of you might be interested in what happens during LFP battery balancing.
(This is what my car did this morning, measured with a Shelly EM.)

 

[Rolfe]

Mine seems to keep going for more like 30-35 min, and most of the time (when we had the extra significant figure on the app) I was seeing 20 watts, with the occasional spike as you illustrate.

 

[Morleym]

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%?

Mine has the option to stop charging at 80%

 

[SteveBirchall]

Mine has the option to stop charging at 80%

Yes. This appears to be an error in the software. It seems like the car thinks it is the long range version.

 

[Rolfe]

Odd, for other SR owners, that option vanished over a year ago.

 

[unbornidentity]

Since the Ohme has no API integration, the easiest way to make sure it doesn’t cut the power a 100% is to tell you want to add more than you need. I just add an extra 10%.