Warned too late

[SteveRW]

This morning the car is sounding the horn which I can’t stop. The panels occasionally flicker into life telling me to restart the car immediately but nothing happens on switching and another warning tells me the 12v battery is flat.
Weirdly although the wing mirrors are folded as in the car locked position I can enter the doors, there’s a suspicion that one of the rear doors was not properly shut but I got no alarm if so and as mentioned the wing mirrors have folded in as if locked? Annoyingly I’m at holiday accommodation at the moment and the alarm is disturbing my neighbour (have apologised) - so there’s power to sound this alarm continually and no way I can turn it off. The other annoyance was the boot stayed locked so trying to access the granny charger was fun from the back seat. Didn’t quite have the length to reach a socket anyway so currently waiting for the AA MG assistance. The real mystery is why the 12v battery has let me down.

 

[Alb]

Do you have anything plugged into the OBD port?

 

[MG steve]

Nightmare, I hope they come out quickly!

The 12v battery is a weak point on all EV's it seems as my Dad's Kia went flat a few times. I bought a jump start kit and stuck it under the passenger seat.

 

[boira]

This is strange, as you said that you have locked the car, it should automatically cut energy power for the USB ports. And if any light was on/or any doors were open, it should have warned you with a honk the moment that you locked the car.

Although I wish that I never have to deal with this situation, I have bought a portable 12v battery starter kit and let in the car, just in case.

 

[Tim Green]

But something must have gone wrong or failed. I have left my Mk1 unused for several weeks with no issues.

 

[SteveRW]

The AA came pretty quickly and used their power pack to charge the 12v battery. I had to cling onto the bonnet as we were facing into 60mph winds straight off the sea and I was worried it would be torn off. All systems seem OK although after restarting a few hours later I did get the message that the battery was low on charge. This made no sense as the dash is showing 14v. Ignored this and everything seems in order. Nothing had been plugged into any ports to answer your question. It seems it must have drained overnight and just as it was flat and beyond any chance of retrieval it began to honk loudly at 8am as if to say I’m dead and there’s nothing you can do about it. Only glad it wasn’t through the night.
The horn sounding was the worst thing. Had to endure that for around two hours. I suppose if I’d had a spanner I could have disconnected the battery but I had visions of HAL warning me not to try anything or it would lock me out permanently.
Just to be on the safe side I’ve parked within reach of a window that has a nearby socket. Think I’ll be investing in an emergency battery booster and monitor of some sort.

 

[Martinonline]

If you power up the car the main battery will start charging the 12v battery, hence the 14v. You must check using the app or a voltmeter with car powered down.

 

[Lovemyev]

not to try anything or it would lock me out permanently.

The alarm sounding the cars horn is common when the 12volt battery status is running really low.
Inducing power back into the battery will rectify this situation of cause, by connecting either a jumper pack or charging / replacing the battery all together of course.
The guy from the A.A. ( MG Assistance ) will have only connected the jumper pack for enough time in order that the car will boot up into READY mode, due to time factor.
If you are unable to place the car on charge, then if safe to do so, boot up the car into READY mode and it will charge the 12 volt battery from the traction battery.
Don't worry about getting locked out of the car, when the battery is flat.
You can still gain entry by using the emergency manual key that can be found in key fob.
On the underside of the small section of external drivers door handle, look under the small section and you will see a small oblong slot cut into the plastic cover.
Put the blade of the emergency key into that slot and you can prize off the plastic cover.
This will display the manual locking barrel, now insert your emergency blade and turn it, to release the lock and you can open the door.
Add a 10mm spanner to your shopping list of the booster pack and battery monitor .
If you have an EV - you should be investing in a battery booster pack.
They are more important on an electric car than on an ICE car.
You can't bump start an EV .

 

[Coulomb]

a few hours later I did get the message that the battery was low on charge. This made no sense as the dash is showing 14v.

When a battery is ruined, it has high internal resistance. What this means is, it has little opposition to voltage change; it's weak as a kitten. So when the DC-DC converter tries to push it up to 14 V, which usually should be met with many tens of amps of charge current and a gradual increase of voltage, it finds that the battery shoots up to 14 V straight away and only draws a few amps. The DC-DC, or the controller monitoring the battery, could well notice this, and pronounce the battery as dead, even though it is at 14 V. The problem with a ruined battery is that instead of falling to about 12.8 V after charging and holding there, it falls immediately to 10 V or so right away.

The rule of thumb is that higher battery voltage implies higher state of charge (fuller battery), but this doesn't really apply when the battery is ruined.

Because EVs usually have very powerful DC-DC converters (far more powerful than the alternator in an ICE vehicle), you really should not attempt to judge the state of charge from the auxiliary (12V) battery voltage if it's being charged. Any time the car is in ready mode, or the main battery is being charged, the auxiliary battery is being charged.