What happens at a very Low State of Charge

[JodyS21]

Like watching paint dry....I've recorded my car sitting on the driveway with the heater on full running the battery completely flat (I sped it up x8!)

The car was purchased after 15/1/2021 and has had all of the latest updates applied when I collected it.
At full charge and balance, it shows 449Volts (under no load 0Amps)

The GOM had turned to - - - when I started the test, voltage was showing 376 under load (15-20Amps).
@375Volts (under 15-20amp load) Message popped up "Low Charge. Aggressive Driving is Not Recommended"
@369Volts (under 15-20amp load) Message popped up "Air Conditioning is Limited by Low SOC"
@358Volts (under 15-20amp load) Message popped up "Power Limited. Limiting Speed" Tortoise Icon lights up.
@349Volts (would've been under 15-20amp load) Message popped up "HV Battery Shut Off" Battery crossed through Icon lights up.
Only thing now being used was the 12V battery.
After a minute or so of letting the battery "recover", on switching back on Voltage showed 367V (at 0Amps as off)
Plugged in AC granny charger. Charging: 0% SoC / --- mls / 17:00

Latest BMS (15/1):
Max Voltage 449V
Min Voltage 367V
Voltage range available 82Volts.

@Cocijo is doing the same for his car, with the Pre-October buggy BMS (455v).
We will see whether we have the same voltage range available (5 taken from the top and added to the bottom)

(Shame I haven't got OBD2 working yet so see what that showed)

 

[Cricketer007]

This may be a thick question for which I apologise in advance ....

If the car still has 367V left with zero range and no ability to use any functions, what does the car do with those 367V?

 

[JodyS21]

This may be a thick question for which I apologise in advance ....

If the car still has 367V left with zero range and no ability to use any functions, what does the car do with those 367V?

I get why you ask the question, it sounds like there's a load of "volts" left!

From playing around with electronics over the years and batteries, you'll probably know if you stick a multimeter on a "flat" AA battery it'll show a voltage albeit a bit low, but will be useless.

Realistically, although it can output X volts, the actual current (amps) it can now supply is basically next to nothing. As it's all about "power", X volts (even if 367) multiplied by practically zero = practically zero power can be got out of it. If that makes any sense!

 

[Cocijo]

Like watching paint dry....I've recorded my car sitting on the driveway with the heater on full running the battery completely flat (I sped it up x8!)

The car was purchased after 15/1/2021 and has had all of the latest updates applied when I collected it.
At full charge and balance, it shows 449Volts (under no load 0Amps)

The GOM had turned to - - - when I started the test, voltage was showing 376 under load (15-20Amps).
@375Volts (under 15-20amp load) Message popped up "Low Charge. Aggressive Driving is Not Recommended"
@369Volts (under 15-20amp load) Message popped up "Air Conditioning is Limited by Low SOC"
@358Volts (under 15-20amp load) Message popped up "Power Limited. Limiting Speed" Tortoise Icon lights up.
@349Volts (would've been under 15-20amp load) Message popped up "HV Battery Shut Off" Battery crossed through Icon lights up.
Only thing now being used was the 12V battery.
After a minute or so of letting the battery "recover", on switching back on Voltage showed 367V (at 0Amps as off)
Plugged in AC granny charger. Charging: 0% SoC / --- mls / 17:00

Latest BMS (15/1):
Max Voltage 449V
Min Voltage 367V
Voltage range available 82Volts.

@Cocijo is doing the same for his car, with the Pre-October buggy BMS (455v).
We will see whether we have the same voltage range available (5 taken from the top and added to the bottom)

(Shame I haven't got OBD2 working yet so see what that showed)

Excellent work!

 

[ARealHuman]

I always though of volts as the force by which amps arrive, so you could have loads of volts, but if there's nothing to "push" it doesnt matter. I might be miles off the mark though!

 

[Mgmedd]

This is a good question. Power = volts * current. The 367 volts is the voltage with no current flowing and is related to the chemistry and structure of the battery. At this point there is almost no energy stored in the battery and what little energy there is at this point goes to supporting this voltage with no current flowing. As soon as you start to drawing even a small amount of current the voltage would drop dramatically or collapse completely and actually start to damage the battery chemistry. Hence the reason the BMS doesn’t allow this.

 

[JodyS21]

Low State of Charge / Run Flat Test - with OBD2 Data (8/3/21).

The car was purchased after 15/1/2021 and has had all of the latest updates applied when I collected it.


At full charge and balance, this time it showed 448.5Volts 92.0%SoC (under no load 0Amps) (It had been left for ~16hours since fully charged and unplugged) (Previously showed 93.2%)

(Accumulated Trip reset so that GOM calculation is reset, Mode Normal Regen 3.)
I’ve recorded the following on video of dash and screen shots from obd2 app.

18:15 @375Volts (under 15-20amp load) Message popped up "Low Charge. Aggressive Driving is Not Recommended"
18:37 @370Volts (under 15-20amp load) Message popped up "Air Conditioning is Limited by Low SOC"
18:50 @362Volts (under 15-20amp load) Message popped up "Power Limited. Limiting Speed" Tortoise Icon lights up.
18:54.20 @356Volts (would've been under 15-20amp load) Message popped up "HV Battery Shut Off" Battery crossed through Icon lights up.
Only thing now being used was the 12V battery.
After 15 minutes of letting the battery "recover", the Voltage showed 371V (at 0Amps as HV battery off)
19:13 Plugged in AC granny charger. Charging: 0% SoC / --- mls / 17:00

App Data
(Battery Temp 19.0c)

BMS (15/1):
Maximum Charge 449.75V 93.2% (taken from previous occasion)
Minimum Charge 371V 0.5%
Voltage range available 78.75Volts.
SoC range available 92.7%.

 

[Kithmo]

I see the cell balance improves as it get lower in charge so could it mean that it's quicker to balance the cells by running it right down before charging up and balancing ?

 

[JodyS21]

I see the cell balance improves as it get lower in charge so could it mean that it's quicker to balance the cells by running it right down before charging up and balancing ?

Hmm, I really don't know.

The lower the value, the same percentage difference, obviously means a lower value difference.
I'm assuming there's no way of managing the current drawn from the different cells when pulling power normally, in which case it just does whatever it does - usually reasonably uniformly because of the way these things happen.

I'm not certain how the balancing really does actually work in this car/battery.
It's obviously got access to each cell pack.
So does it actually just give those cell packs that are a bit low a bit of juice, or can it only access each cell pack to drain from - so it really does drain the high cells whilst applying low charge to all of them to try to level them - does not seem like at all a good way of doing it without there being a good technical reason for it.

 

[Dave S]

I take my hat off to you guys - this is excellent work and really useful research.

Brilliant!!!! Thanks.

 

[JodyS21]

Ran battery right down until it shutdown Was 5.0% on the App and 375 total volts. My max is 97.0% at 456-7Volts. Will take screenshot when charged fully. Usable 92%.

I doubt MG have reduced lower buffer from 5% to 2%

... I think someone (Jody) had 3% & 94% (93.8%) giving usable 91%. So I suspect 1% is to hide the first year degradation.

Instavolt charger recorded it at 1% (it stays at 10% for a bit until it starts to charge then drops to 1%). i put 10% rapid charge in (was running at 49KW on a 50kW pump) then took to AC charger.
Was 1% when looked on dash after that.
View attachment 2229View attachment 2230

 

[AbuG]

What was lowest you got to 371V & SoC 0.5%
How were they measured?
Was it the same android app?

 

[JodyS21]

So my Low State of Charge was:
BMS (15/1):
Maximum Charge 449.75V 93.2%SoC
Minimum Charge 371V 0.5%SoC
Voltage range available 78.75Volts.
SoC range available 92.7%.

AbuG's in same format:
BMS (7/2020)
Maximum Charge 457V 97.0%Soc
Minimum Charge 375.0V 5.0%SoC
Voltage range available 82.0Volts.
SoC range available 92.0%.

 

[JodyS21]

What was lowest you got to 371V & SoC 0.5%
How were they measured?
Was it the same android app?

Yes, all measured on the same app. Videod it so I could make sure everything tallied up at the exact right times etc.

So my SoC went down to 0.5% just before it shutdown the HV Battery and, stayed at 0.5%SoC.
The Voltage went down as low as 356.5V as it was still running under load, but this is misleading as it was under load. I left it to recover/settle for 15minutes and the Voltage settled at 371.0V.
(My Max is usually 93.2%Soc and 449.75/450.0V)

- When I then plugged it into AC granny charger, the dash said: Charging: 0% SoC / --- mls / 17:00

 

[AbuG]

I think MG messed with the percentages (as it's relative) to... I think voltage (as more absolute) is what we need to go by

 

[JodyS21]

I think MG messed with the percentages (as it's relative) to... I think voltage (as more absolute) is what we need to go by

I meant to say, how long did you let the battery voltage recover/settle after it switched off before you got your 375V reading from the app?
Mine took about 15mins before it stopped gradually increasing until it settled.
(After 5mins mine said 368.75v)

 

[JodyS21]

I see the cell balance improves as it get lower in charge so could it mean that it's quicker to balance the cells by running it right down before charging up and balancing ?

I don't know.
I could go through the video and get the max/min cell for all the previous data points..

 

[AbuG]

I meant to say, how long did you let the battery voltage recover/settle after it switched off before you got your 371V reading from the app?
Mine took about 15mins before it stopped gradually increasing until it settled.
(After 5mins mine said 368.75v)

I switched everything off within a few seconds and checked. Didn't want the 12V to go flat too. Car was powered down not in Acc mode.

My wife needed the car this morning before balancing was complete. here is SS while still balancing.

 

[JodyS21]

I switched everything off within a few seconds and checked. Didn't want the 12V to go flat too. Car was powered down not in Acc mode.

My wife needed the car this morning before balancing was complete. here is SS while still balancing.
View attachment 2236

That's pretty well balanced anyway (0.013diff).

I highly suspect then that your voltage would have recovered to a higher settled figure than the 375V reading.