Power draw when doors are locked and low 12V warning. Updated.

[fx860e]

A couple of days ago I found that when I turned the ignition on, I received a message on the driver’s screen informing me that the 12V battery was low and some things might not work as expected. I needed to go out at the time so I couldn’t check the voltage, but everything seemed to work as it should anyway.

Yesterday I put my clamp multi-meter around the positive lead of the 12V battery and monitored the amperage to see what was going on.
On turning off the Heating and disconnecting the Wi-Fi signal I pressed the start button once and locked the doors.

UPDATE.. I received the message again so i took my multi-meter and found that the voltage was 11.95 volts.....Hardly low, i was expecting 10 or below.

I initially saw a charge going from the main battery to the 12V battery at around 60A.
When the voltage reached 14.4V it tapered off and got to around 0.5A, then it settled at 14.20V.
I then switched off the ignition, locked the doors and put the key fob indoors as I noticed that when it was near the car it caused it to wake up and draw around 10A.
After locking the doors at a distance, the car drew around 10A for a couple of minutes, and then it would tapper off withing 5 minutes and would settle around 250 mA to 500mA.
It never went below that over the next few hours that I monitored it, but sometimes for no reason it would jump to 10-11A and then start to settle down again withing 5-10 minutes, though it never went below 250 mA to 500 mA.

I know that a quarter to half an amp doesn’t sound much, but over a few days it would build up and may explain why I received the low voltage warning message.
I haven’t had the chance to see what happens when the car enters the sleep mode. I’m guessing it could be why my car sometimes won’t charge overnight, and my Scottish Power app informs me that the car was disconnected and they couldn’t start a charge. (If the cars been standing for quite a few hours, even though the charging cables were attached.) I can only start the charge by sitting in the car and turning the ignition on and off.
I also notice sometimes the infotainment screen and driver's screen comes on immediately as soon as I push the power button, and sometimes it takes a while as it has to boot.
Thought I’d share my findings.

 

[Totti1001]

250 to 500 mA is far too high!
Modern cars should draw a maximum of 70 mA when idle.
There were models where the speedometer display didn’t switch off properly. In the dark, you could see it glowing faintly.
Mine had a problem with the rain and light sensor in December; it got so hot that it melted even 10–15 cm of snow in that spot, at minus 10 degrees. After just under two days, the 12V battery was dead.

 

[Coulomb]

I haven’t had the chance to see what happens when the car enters the sleep mode.

It typically takes EVs (and probably most modern vehicles) 5-10 minutes to properly go to sleep, where they draw minimum current from the 12V battery. So I'd expect that 250-500 mA of 12V current to reduce to under 10mA about 10 minutes after locking. I have no idea what sort of housekeeping it does in that time.

I do suspect that MG cars have a vug or bugs that under rare circumstances drains the 12V battery completely. This "wounds" tge 12V battery, potentially causing weird problems several months later. I had such a drain, and hope that replacement of the 12V battery will fix my Adaptive Cruise Control, speed limit, and other issues.

 

[fx860e]

It typically takes EVs (and probably most modern vehicles) 5-10 minutes to properly go to sleep, where they draw minimum current from the 12V battery. So I'd expect that 250-500 mA of 12V current to reduce to under 10mA about 10 minutes after locking. I have no idea what sort of housekeeping it does in that time.

I do suspect that MG cars have a vug or bugs that under rare circumstances drains the 12V battery completely. This "wounds" tge 12V battery, potentially causing weird problems several months later. I had such a drain, and hope that replacement of the 12V battery will fix my Adaptive Cruise Control, speed limit, and other issues.

Thanks, I did have the clamp multi meter connected for about 4 hours and monitored it via Bluetooth to its app. It also drew a table and graph. At no time did it go under 250 mA.
It does seem high over time, not sure what it’s doing.
Sorry to hear about your cars problems, the only strange thing I’ve experienced with ACC was its ability to read crooked speed signs at passing junctions, and also on a overpass as we traveled underneath slowing us down from 60 mph to 20. Not good for the drivers behind us.
I think we are a long way from driverless cars. First thing would be to tidy up the road signs and keep them tidy. Though I also think it will give a bored teenager something else to play with…….

 

[ronnie216r]

About the road signs, in Europe, you do have some large trucks having speed signs put on them to indicate what is their max. speed. My car sometimes reads them slowing me down from 110 to 90.

 

[fx860e]

About the road signs, in Europe, you do have some large trucks having speed signs put on them to indicate what is their max. speed. My car sometimes reads them slowing me down from 110 to 90.

Yes, that happened to us once except here we use MPH so suddenly our car changed to 100mph (160 KPH,) So with all of that we had to switch that side of it off and haven't trusted it to use since.

 

[Coulomb]

I did have the clamp multi meter connected for about 4 hours and monitored it via Bluetooth to its app. At no time did it go under 250 mA.

Could that be due to Hall Effect sensor drift? Those things are terrible with long term offset drift. That would also help explain why it sometimes measures as much as 500 mA, when I would expect the sleeping current draw to be approximately constant.

It would be best to use the clamp meter to "calibrate" your negative battery lead resistance (usually including an actual shunt resistor), and using the multimeter on its millivolt range, measuring the voltage drop from the negative battery terminal to a solid chassis connection.

That way you avoid the drift/offset problem.

For example, with the headlights on you might measure 8 A draw and say 15 mV drop. So the resistance is just under 2 mΩ, or just over 2 A per mV. Then 250 mA would measure some 0.5 mV across the negative battery cable; hopefully your multimeter can easily distinguish between negligible current (reading of 0 or 0.1 mV) and 250-500 mA (0.5-1.0 mV).

 

[fx860e]

Thanks.
The meter did zero when I took it off, and I had pushed the REL button to zero it before I clamped it round the two leads that are coming from the positive.
I did consider doing it the conventional way but I I’m not sure what it would do to the car if I were to disconnect it from the battery to set it up.
I hope it answers what to mention about the ‘Hall Effect’, you obviously have more knowledge than myself but thanks for your response. It may be the setup, I hope so, but as I mentioned I am having to get used to have a few unexpected events as I get used to the car.
As I've now mentioned above, today i received the same warning and found the battery was at 11.95V which to me isn't low, so maybe it's a bit premature anyway.
Thanks again.

 

[Coulomb]

The meter did zero when I took it off, and I had pushed the REL button to zero it before I clamped it round the two leads that are coming from the positive.

That means that the first few minutes of measurements would have been accurate. But after that, the measurements could drift, especially over hours of measurement.