Curious unexpected SOC performance

[MJCTHR]

Hi All

As a relative newbie who bought my MG5 Trophy (72 plate) in November last year, I'm hoping to tap some of your great knowledge on something that baffled me on Monday.

I had a house guest who I took to Heathrow Airport from Surrey. I had used my Intelligent Octopus charging regime to get to 80% the night before. I drove on local roads avoiding the M25 but it was still a 19 mile journey. During the trip, the SOC was showing the full 80% for a very long time, and it only ticked down to 79% as I entered the tunnel at the airport to get to Terminal 3. I thought there must be something wrong with it. Driving back from Heathrow it went from 79% to 72% which was still better than I expected.

For context, I had driven from London to Leicester and back using motorways on the 10th/11th August (used a granny charger at their house to 100% before return) but had picked my friend up from Heathrow on the 13th with what I had left in the battery.

Can anyone explain why I got so far on 80% on the outward journey to Heathrow as it's a mystery to me. There must be a rational explanation, I'm just not knowledgeable enough to understand it .

Interested in any thoughts.

 

[MickeySw]

There must be a rational explanation

Interested in any thoughts.

Interesting indeed

When you start the car it calculates/updates its current capacity depending on various factors. When you drive, that value is adjusted using coulomb counting method, i.e battery current is measured and integrated over time. The higher the current (speed) the quicker SoC reduces. Breaking using regen works the opposite, (capacity is added)

While it’s possible to keep your SoC constant for a long period of time going downhill, I don’t think this is what’s happening here although the return journey ‘uphill’ shows higher consumption.

It’s either one of those MG5 ‘quirks’ or if it’s still happening, a fault within the SoC calculation circuit.
If it’s the latter, a hard reset (12V aux disconnect) or a calibration charge might prevent a trip to the dealer.

 

[MJCTHR]

Thanks! It seems to be reducing as expected since, but I'm intending to do a calibration charge next time anyway.

 

[crispyjay]

I’ve had similar. Driven 10miles before % started to drop after immediately disconnecting from 7kW charger.

 

[Pebblefeline]

I have had this a couple of times.
My guess is that the actual soc is higher than indicated and it takes a while to work out exactly where it should be.

Every now and then on short journeys my consumption looks amazing

Happens occasionally but always seems to work itself out

 

[5teep]

Happens to me also, I think pebblefeline above is right, it's just a mismatch between actual soc and that displayed.

 

[philby123]

I’ve had similar. Driven 10miles before % started to drop after immediately disconnecting from 7kW charger.

Yup me too and only recently after disconnecting from my Ohme 7kW charger. I typically charge to 85% but very recently it's taken around 10 miles of driving before it drops by 1%, very odd, do you have an Ohme charger? Also, what is a calibration charge and how to do it and how frequently?

 

[MJCTHR]

No, mine is a Myenergi Zappi. A calibration charge, as I understand it, is to run the battery to less than 10% then charge fully to 100% in one session. It is supposed to help ensure the SoC reading is accurate as it tells the car the limits of the battery. I don't do much mileage at all so only do this once every three months but if you drive regularly, it is recommended more frequently.

 

[crispyjay]

Yup me too and only recently after disconnecting from my Ohme 7kW charger. I typically charge to 85% but very recently it's taken around 10 miles of driving before it drops by 1%, very odd, do you have an Ohme charger? Also, what is a calibration charge and how to do it and how frequently?

My 7kW charger is a post in the car park, nothing special. I try to do a sub 10% to full once a month on the same charger. Managed to catch it on the app doing 0.2kW at 100% for a while.

 

[Geoffers]

This has been happening to my pfl5 for as long as I can remember. I only use a granny charger and I probably only put on about 12% during my cheap overnight charging session yet I always get somewhere between 8 and 10 miles before it starts to drop. Except on occasions I charge to 100% - then it drops within a couple of miles. Which begs the question - would I get a longer range by charging to 99% (not that I'm going to get up in the middle of the night to stop the charge just to test this!)

 

[Verdigris]

The 100% charge balances the cells. With a partial charge I suspect the GOM is responding to the highest cell voltages which perhaps don't drop to closer to the majority of cells until a few kWh have been discharged.

 

[MickeySw]

The 100% charge balances the cells. With a partial charge I suspect the GOM is responding to the highest cell voltages which perhaps don't drop to closer to the majority of cells until a few kWh have been discharged.

I have just experienced something similar but after a rapid charging session, (slower than usual reduction of SoC) however trip consumption didn’t show excellent figures ?

Determining the exact SoC isn’t actually straightforward, e.g. cell voltage based calculations are only accurate if the battery has been idle for a couple of minutes.

It’s therefore quite plausible that SoC after a charging session is calculated on the low side; if one takes off immediately afterwards it will take a time to correct?

Balancing is done with a very low current, hence better SoC accuracy?

Looks like @Verdigris has a point, SoC determination after a charging session could be voltage based initially and then corrected using coulomb counting method.

It’s probably by design that SoC doesn’t increase and therefore remains ‘constant’ until the values match again?