rapid charging battery percentage

[Fudge]

i have had a few rapid charges now and i have noticed that then ive had a quick few kwh's top up, that the battery percentage and GOM seams to drop alot faster than it should.

so when i am doing a trip i stop at chobham services and normally top up from 70% to 85% as this would mean i have enough charge for the hole trip.
but the increase seams to drop like a stone when i get back onto the road, then settles down a few miles in.

has anyone else noticed this?

but if i do a rapid charge from below 50%, to about 80ish it seams to be ok and drop at the normal rate, so could it be that the computer doesnt like a small rapid charge?

 

[5teep]

Possibly, it is a chinesium computer after all, putting the guess into guessometer?

 

[Stuart Wright]

Possibly, it is a chinesium computer after all, putting the guess into guessometer?

The word you're meaning is Chinese, it may not be a Chinese computer (could be sourced from a third party unless you know different) and what's wrong with Chinese computers?
Think carefully before answering.

 

[5teep]

Thank you Stuart, the word chinesium applies to much of the manufacturing that comes out of China. It is not a derogatory term applied to the Chinese people but a comment on the all too often questionable quality control on Chinese made products. I think its a fair comment but will refrain from using it for the sake of forum harmony.

 

[Stuart Wright]

Thank you Stuart, the word chinesium applies to much of the manufacturing that comes out of China. It is not a derogatory term applied to the Chinese people but a comment on the all too often questionable quality control on Chinese made products. I think its a fair comment but will refrain from using it for the sake of forum harmony.

We’ve discussed this topic on the podcast. Consumers can’t expect lower cost products to be the same quality as higher cost products, so they shouldn’t be surprised when there are obvious cost cutting measures or make sweeping statements about the output of a whole country.
Having said that, economies of scale means that products can be produced which are less expensive than you would expect but still compete favourably with the competition and MG EVs are a perfect example of this.
Look at DJI for an example of exceptional quality products from China. Incredible, class leading consumer drones. So the old stereotype of Chinese products = poor quality should be abandoned.

 

[Petriix]

I've not noticed what you are talking about. It's possible that the extra heat from charging is causing the battery cooling to run flat out for a while which might add a bit to the consumption. But I have seen some anomalies with other aspects of the MG software so I wouldn't rule out that being responsible for any inconsistencies. If you check the voltage and the background power draw when you finish charging, that might give you some insight.

70-85% does seem like a strange part of the battery to be charging. It would charge faster if you charged from lower. The charge rate tapers off to protect the battery so the implication is that rapid charging causes more damage at higher states of charge. Obviously if it's more convenient then do whatever works, but I wouldn't normally initiate a rapid charge with more than 50% SOC.

 

[Fudge]

i do agree it is a little strange, but the extra little bit was all i needed to make the 200 mile trip fully loaded yesterday.

 

[n8sail]

I did my 2nd fast DC charge the other day, typical 31C ambient temp, and noticed the same thing... charged it from ~25% up to ~85% in around 30 minutes. On the trip immediately following the charge, which normally uses about 10% of the battery, it had already used more than 15%.

Also noticed that the HV battery was going to a much lower voltage than normal when hitting the accelerator hard...

After charging on granny charger the next day, everything was back to normal.

Suspect that as mentioned above the cooling circuit may have been working harder than normal, but it didn't appear to be pulling more than the usual amperage at stop lights for me. (on lowest setting mine seems to pull about 1-2 amps. 6 amps on max AC settings).

 

[Paulie68]

I too have noticed this phenomena, I think a feasible reason for this may be the following. during rapid, high current charging, the battery voltage is pushed up significantly, if the software doesn't adequately allow for this, it will report a higher state of charge than is actually present. Therefore when you unplug and immediately drive from very high current charging, the car is initially still reporting the incorrect, higher, state of charge. This is rapidly scrubbed away under load back to the real world figure. This is what we see here.

This wouldn't happen if the car was allowed to run to 100%, with the usual current taper to a much lower amperage, or after sitting for some time, when the battery would settle back to it's actual voltage. It's specific to stopping whilst still high current charging and immediately driving away.

 

[Petriix]

I too have noticed this phenomena, I think a feasible reason for this may be the following. during rapid, high current charging, the battery voltage is pushed up significantly, if the software doesn't adequately allow for this, it will report a higher state of charge than is actually present. Therefore when you unplug and immediately drive from very high current charging, the car is initially still reporting the incorrect, higher, state of charge. This is rapidly scrubbed away under load back to the real world figure. This is what we see here.

This wouldn't happen if the car was allowed to run to 100%, with the usual current taper to a much lower amperage, or after sitting for some time, when the battery would settle back to it's actual voltage. It's specific to stopping whilst still high current charging and immediately driving away.

The battery voltage increases whenever it is charged and decreases as it's discharged - that's just a function of how the energy is stored.

It's possible that rapid charging causes a significant imbalance in the voltage of the different cells in the battery which would limit the amount of the stored energy available until balanced again, but I'd be surprised if a single rapid charge would cause a tangible effect.

 

[Paulie68]

The battery voltage increases whenever it is charged and decreases as it's discharged - that's just a function of how the energy is stored.

It's possible that rapid charging causes a significant imbalance in the voltage of the different cells in the battery which would limit the amount of the stored energy available until balanced again, but I'd be surprised if a single rapid charge would cause a tangible effect.

Yes, of course, but you misunderstand my point. Whilst high current charging, the voltage pushes up much higher than the actual normal voltage at a given state of charge voltage. This is known as "surface charge" and dissipates over time, once charging stops. If the software establishing SOC doesn't allow for this, it will give a false, higher than correct reading for a little while, after stopping a high current charge.

Given how minimally viable the MG software development seems to be, I'm pretty certain this is the cause.

 

[Petriix]

Yes, of course, but you misunderstand my point. Whilst high current charging, the voltage pushes up much higher than the actual normal voltage at a given state of charge voltage. This is known as "surface charge" and dissipates over time, once charging stops. If the software establishing SOC doesn't allow for this, it will give a false, higher than correct reading for a little while, after stopping a high current charge.

Given how minimally viable the MG software development seems to be, I'm pretty certain this is the cause.

Interesting concept. Do you have any links about this phenomenon? I'd like to read more but can't find anything obviously relevant to EV rapid charging.

 

[Paulie68]

Interesting concept. Do you have any links about this phenomenon? I'd like to read more but can't find anything obviously relevant to EV rapid charging.

No links, but I'm sure you can search charging behavior of various cell chemistry types. My own experience is an EE background and some experience with lithium battery behavior from my own EV conversion build, I've observed the phenomenon, it happens to a lesser degree even at lower charging currents. But it's observable with pretty much any battery chemistry.

It should be allowed for and taken into account by the cars software, but I suspect it's an oversight by SAIC/MG as this seems a little, shall we say, less sophisticated than it could be.

 

[mj224]

I can only add that with lead acid batteries, you need to wait 30 minutes after chargeing for the actual voltage to settle down. As said, the surface charge is matching the input from the charger, and is not the actual "proper" battery voltage.

 

[iwantatr8]

Interesting concept. Do you have any links about this phenomenon? I'd like to read more but can't find anything obviously relevant to EV rapid charging.

Um are you sure... these are not for the faint of heart.....

Anyway this is clearly a rapidly evolving field where the current state of the art is changing all the time but as a for instance here's [1] a paper on the state of charge and surface charge of Lithium ion batteries, and this one [2] looking at improving the modelling process issued in December 2020. the overall process of charging and discharging batteries is a non linear process that the BMS has to convert to a user intuative linear percentage / range estimation.

Given the improvements that have been made in range estimation over say the original Leaf, I don't think we can complain too much. I just think that using the range estimation post fast charging you need to take the readings with a pinch of salt. It's just not as simple as a correctly damped level guage in a calibrated petrol tank anymore...

[1] Lithium-Ion Battery State of Charge and Critical Surface Charge Estimation Using an Electrochemical Model-Based Extended Kalman Filter | Request PDF

[2] (PDF) Electrode-Level State Estimation in Lithium-Ion Batteries via Kalman Decomposition