Unusual charging behaviour at the end of an AC charge.

[Rolfe]

Since I got my car (SR, LFP battery) I have noticed consistent behaviour on AC charging. The app will give an estimated finish time, and round about that time the charging power would fall from about 1.88 kw (on the granny charger) or about 6.5 kw (on a type 2) to negligible, usually about 20 watts. It would go on drawing that small amount of power (can't be seen now on the new app which only gives one digit after the decimal point) for about 30-35 minutes, and during that time the estimated finish time displayed would be the current time. Eventually, when the balancing was complete, the app would show no power being drawn and no estimated finish time, and that was that.

The last couple of times (when I've looked, obviously I often don't look so this could have happened more often) it's been different. Once the car is at about 99% charge the estimated finish time jumps to about half an hour later than it has been throughout the charge, and the charging power starts jumping up and down, anywhere from zero to 1.9 kw whenever I refresh the screen. Once the new "finish time" has been reached this gradually stops and the car settles down to its usual balance charge act for another 30-35 minutes.

So it seems as if the car has added a strange half-hour phase at the top of the charge where it fluctuates for half an hour before starting the half-hour balance. Anyone else noticing this?

 

[Ian Key]

You've probably just not noticed it before. I've noticed mine doing something similar when I've been watching the charging on the Zappi app.

 

[Rolfe]

The thing is, I've watched it a lot in the past and never seen this, and now it's done it twice in a row. I very much doubt that it's a problem, but I'm intrigued.

What is particularly striking is the sudden extension of the estimated finish time by about half an hour, which still doesn't include the half-hour balance time, because that happens as usual after the extended finish time has been reached.

 

[tsedge]

You've probably just not noticed it before. I've noticed mine doing something similar when I've been watching the charging on the Zappi app.

But the NMC battery is known to behave very differently with balancing. Not sure we can extrapolate for Rolfe's case.

 

[Ian Key]

Hmmm ? I guess Rolfe is a special case ?

 

[richlove]

I've noticed similar behaviour with mine

 

[TerryN]

Since I got my car (SR, LFP battery) I have noticed consistent behaviour on AC charging. The app will give an estimated finish time, and round about that time the charging power would fall from about 1.88 kw (on the granny charger) or about 6.5 kw (on a type 2) to negligible, usually about 20 watts. It would go on drawing that small amount of power (can't be seen now on the new app which only gives one digit after the decimal point) for about 30-35 minutes, and during that time the estimated finish time displayed would be the current time. Eventually, when the balancing was complete, the app would show no power being drawn and no estimated finish time, and that was that.

The last couple of times (when I've looked, obviously I often don't look so this could have happened more often) it's been different. Once the car is at about 99% charge the estimated finish time jumps to about half an hour later than it has been throughout the charge, and the charging power starts jumping up and down, anywhere from zero to 1.9 kw whenever I refresh the screen. Once the new "finish time" has been reached this gradually stops and the car settles down to its usual balance charge act for another 30-35 minutes.

So it seems as if the car has added a strange half-hour phase at the top of the charge where it fluctuates for half an hour before starting the half-hour balance. Anyone else noticing this?

Yes I saw exactly that behavior for the first time when I charged on Sunday. I thought it unusual so I took a screenshot. The charge actually finished at 1535 (I heard the relay clunk!)

 

[Rolfe]

Interesting. I know the NMC battery will balance for very variable times - I've heard everything from ten minutes to two hours - but the LFP has always been very predictable at around half an hour, every time. It's this extra half-hour of jumping up and down once it has reached 99% and before the balance charge starts that seems new.

I don't remember when I saw a 100% range of over 200 miles last! Warm with you, is it?

 

[TerryN]

Indeed it has been quite mild considering the time of year.
It took around half an hour to balance on my first charge but every time since it's only been about 5-10 minutes. So far I've always charged to 100% (on my 7kW home charger).

 

[Rolfe]

Mine always takes half an hour, either on the granny lead or on whatever type 2 I happen to be on.

 

[Tight Git]

Mine did exactly this when I was balance charging on the granny last week after a couple of months using the Zappi. I hadn't seen it before either!

 

[fnegroni]

when charging in freezing temps, the battery heater will kick in on the last few percent points. That’s what slowing down the charge.

 

[Rolfe]

Ah! I wouldn't say it was freezing but it was maybe close. In that case it's probably done it other times and I just didn't notice.

 

[Kithmo]

My ZS used to do that. I've not noticed it on the 4 though.
My 4 just drops and shows 0.0W on the app battery screen when the charge finishing time is reached and carries on for anything between 10 and 30 minutes balancing, drawing about 200W gradually decreasing, on my smart meter display. I also watch the Pod Point app which shows me the balancing has finished as the charger goes to "charged" status on the screen whereas it says "charging" whilst it's balancing.

 

[fnegroni]

Ah! I wouldn't say it was freezing but it was maybe close. In that case it's probably done it other times and I just didn't notice.

I’d add to that my consideration: if charging a cold battery with a granny charger, you are better off stopping at 90%.
Depending on how cold the battery pack is, on the last few percent points it will kick off the battery heating but on a granny charger that will just take forever and won’t be as effective since the granny charger only supplies 2kW while the battery heater needs 3kW. It might be ok if it’s on the last percent point but not worth it imho.
If you really need the battery to 100%, disconnect the charger at 90%, pre heat the battery, once that’s done you can charge to 100% as by that time the battery will stay warm for a couple of hours

 

[Rolfe]

It didn't cause me any trouble, and I was on the granny charger. I was just curious as to what it was all about.

The car charged normally to 99%, then added about 30 minutes to its estimated finish time and faffed around for a bit, then finally went into balance charge mode as normal. So all it did was to extend the charging time by half an hour, which is neither here nor there if you're on a granny charger and not champing at the bit to be away. It probably did it again last night but I was asleep at the time.