What happens to the 12v Battery when left for two days....

[fx860e]

I know I’ve mentioned this in a previous post about the 12v battery having a repetitive 9-10A draw on it every few minutes even though the car is off, locked and just sitting on the drive. I’ve also been puzzled at the constant warnings for ‘Low 12v Battery’ I get when I’ve left the car standing for a day.

So, I purchased a Bluetooth 12v battery monitor and left it connected while being locked in the car for 48 hours.

I’ve included pictures of the results which confirms that even though the car is disconnected from Wi-Fi, and the keys are placed in a Faraday case, the car 5 or 6 times an hour draws about 10A for a minute or so, and then slowly tapers down to about 250 to 500 mA, before doing it repeatedly again for no apparent reason.
This draw, even after a few hours, makes the car sleep, with the effect that it disconnects from the MG iSMART app which our Scottish Power need to communicate with the car for overnight charging.

To explain the pictures.

Picture 1 Noon to Midnight.
I connected the 12v battery to a battery charger for an hour (11-12pm) and disconnected it at Noon. Immediately you can see the draw and recovery 5-6 times an hour, for seemingly no reason.


Picture 2 Midnight to Midnight.



Picture 3 Midnight to Noon.
At 11.85volts the draw stops, flat lines. I kept it connected for a few hours more and nothing further happened.


Picture 4 at 14.45pm
I was relived to find that I could still open the door with the key fob. After putting a clamp multimeter around the 12v+ wire I turned the car on. (PHEW! everything worked.) I saw the car immediately throw a 60A current into the 12v battery which you can see in the picture.


Picture 5. (Wasn’t expecting this.)

After a few minutes I turned off the car and connected the wall charger and was presently surprised to find that the 12v battery gets charged from the wall charger, so no need for a 12v battery charger. I turned it off and on again to check but at 14.50pm the car main battery reached 100% and both of the battery's stopped charging .



I hope some of you may find any of this helpful and that it may explain what’s happening, if not why…

 

[Totti1001]

I’ve written quite a bit about this before, and I’ve also had issues with a flat 12V battery. In my case, it was the sensor next to the front camera in the windscreen. However, there are also vehicles where the main display doesn’t switch off properly and remains powered the whole time – you only notice this in the dark.
But in your case, this isn’t normal. Normally, a working battery should still be around 12.4V even after a few days. And I’ve only ever encountered this constant fluctuation in one situation. I had the SAIC Integration for Home Assistant running on my system for a while. It’s similar to the MG iSmart app, but for Home Assistant, and displays more information than iSmart. However, this meant the vehicle was constantly being queried (see screenshot 12 March). You don’t happen to have something similar running on your end, do you?
Also, the vehicle’s availability has nothing to do with the charge level of the 12V battery. No matter how full it is, the vehicle eventually goes into deep sleep. I’ve had that happen after just 2–3 hours in some cases. But by the next morning at the latest, when you might want to preheat the car in winter, the vehicle is no longer accessible.
Based on your pictures, I’d definitely have your vehicle checked out, as this isn’t normal. I hope you have a good garage.
My problem with the sensor only started after an update was carried out at the garage.
You can see what happens when the vehicle has been stationary for several days in the screenshots from 9 to 13 April. The voltage remains fairly constant at 12.5V.
Your 11.85V is clearly too low.
When the vehicle is checked using the Ismart app, the voltage drops slightly for a moment (screenshot 9 April, red circle) but then goes back up again.

 

[fx860e]

I’ve written quite a bit about this before, and I’ve also had issues with a flat 12V battery. In my case, it was the sensor next to the front camera in the windscreen. However, there are also vehicles where the main display doesn’t switch off properly and remains powered the whole time – you only notice this in the dark.
But in your case, this isn’t normal. Normally, a working battery should still be around 12.4V even after a few days. And I’ve only ever encountered this constant fluctuation in one situation. I had the SAIC Integration for Home Assistant running on my system for a while. It’s similar to the MG iSmart app, but for Home Assistant, and displays more information than iSmart. However, this meant the vehicle was constantly being queried (see screenshot 12 March). You don’t happen to have something similar running on your end, do you?
Also, the vehicle’s availability has nothing to do with the charge level of the 12V battery. No matter how full it is, the vehicle eventually goes into deep sleep. I’ve had that happen after just 2–3 hours in some cases. But by the next morning at the latest, when you might want to preheat the car in winter, the vehicle is no longer accessible.
Based on your pictures, I’d definitely have your vehicle checked out, as this isn’t normal. I hope you have a good garage.
My problem with the sensor only started after an update was carried out at the garage.
You can see what happens when the vehicle has been stationary for several days in the screenshots from 9 to 13 April. The voltage remains fairly constant at 12.5V.
Your 11.85V is clearly too low.
When the vehicle is checked using the Ismart app, the voltage drops slightly for a moment (screenshot 9 April, red circle) but then goes back up again.

Thanks for your reply. I have noted that sometimes the cars screen will come on immediately as soon as if pushed the button twice, sometimes the drivers screen comes on first and the I have to wait maybe 30 seconds for the second to boot.
Also, I’ve only had nighttime charging when I’ve left a battery charger connected.
I can’t see the screen being on in the dark, and I’ve tried disconnecting WiFi and everything else I can think of, even putting the keys in a faraday case.
I surprised that you say that’s it’s not normal because when I’ve mentioned it to the garage they seem to say that it’s normal and at least I haven’t been locked out and the car starts.
They have though booked the car in for a check in a couple of months but I’m not sure how qualified they are to find it.

Can I ask if yourself, and anyone else, gets ‘Low 12v Battery’ warnings after 12-24 hours of not using the car? I’ve had this since I got the car in January, and the garage seem to give me the impression that it’s as quirky as the front parking sensors only coming on after being in reverse.

 

[Totti1001]

I’ve had that 12V battery warning once before. It happened after an update at the garage; the sensor next to the rain/light sensor got so hot that it even melted a 10cm-thick layer of snow. Within two days, the battery was completely flat (below 9V). Something was probably done wrong during the update, because after the car spent another 7 days in the workshop and the software for a control unit was supposedly re-installed, the fault was gone. But my workshop apparently doesn’t know much about the MG and has to keep asking MG for advice.

Take a look at my screenshot dated 9 to 13 March; the voltage remains stable throughout that period. I only had this zigzag pattern with the SAIC integration for Home Assistant.
Something is activating in your car every minute or second. And what your garage is telling you – that it’s just a quirk of the vehicle – is nonsense. Apparently, your garage is just as useless as mi
ne.

 

[fx860e]

Thanks, I have a try to convince them. I may post a question to everyone here and see how common it is. Thanks for your help.