Auxiliary Battery flat

[girl on the hill]

I have searched but can’t find a simple easy to understand explanation! I have had my MG4 since November 2025. Last week the car would not open. Called the AA who said the auxiliary battery was flat. They started it and I could get in.
1/should that have happened so soon?
2/ what would drain it? - my dash cam?
3/ I have found what I think is the battery info by the tyre pressure gauge it is reading 14.1 - 14.4 what should it be and how do I charge it? Just driven 50 miles today and it hasn’t moved up.
4/ can I buy a charger in case it happens again?
Thanks everyone

 

[Biker67]

Hi,

If you have access to a 3 pin waterproof outdoor socket (if the car isn't garaged?)
Purchase a 12v trickle charger like an Optimate 4.
They come with a lead that permanently attaches to the car/battery. So attaching the charger is a simple 2 second plug & play exercise.
Connecting your car too it for 4 days per month should cure the occasional flat battery issue?

PS...
14v when the engine is running is fine.

 

[MickeySw]

2/ what would drain it? - my dash cam?

Most likely

4/ can I buy a charger in case it happens again?

Buy a car battery jump starter and make sure you charge it every couple of

 

[Kithmo]

The 14.1V reading on the dash is the charging voltage when the car is on. You need to check the resting voltage in the app when the car is switched off, it's usually around 12.4V.
The dash cam is the most likely suspect, to flatten the battery, especially if the car was not used for a few days.
The 12v is charged whenever the car is on (it doesn't have to be in ready mode) and also when charging on AC, but not on a DC rapid charger.

 

[EvTek23]

Keeping the battery topped up using a maintenance charger is a short term solution to why the auxiliary battery is going flat.
Is your dash cam on a permanent 12v feed wired into the car? If it is, and has no internal battery in the dash cam then it will consume some power from the battery, and over a period of time say 2 weeks may drain the battery enough to cause you problems.
You say you got the car in Nov 25, is that new or pre-owned, if so what age is car? Is the car used daily or left standing all week and just used at weekends or some pattern of infrequent use.
It might just be a battery issue, they have been known to fail quite quickly sometimes due to manufacturing defects.
MG only guarantee the auxiliary battery for 1 year from new.
It maybe worth getting a dealer to check the car for any drain on the battery when it's all shut down.
If you have the app then you can check and monitor some things yourself.
If you click on the shield icon on the main page, and then refresh on the top right of the screen and scroll down the voltage of the auxiliary battery is near the bottom of the list and should be at around 12v when the car is locked up and you check it the next day after its been used. Around 14v is the figure you should get if you carry out the same procedure when it's on charge.
You say its 14v to 14.4 when driving which means it should be charging.

 

[MickeySw]

but not on a DC rapid charger.

How come?

 

[ReintjeWA]

How come?

The auxiliary battery is charged (and normally also maintained) from the High Voltage pack. But the HV pack gets “disconnected” from the car during DC charging. This is why after you finish a DC charge the car will say there is a fault with the 12v, which clears when you put the car in Ready mode (and it re-engages the HV pack).

I think.

2/ what would drain it? - my dash cam?

4/ can I buy a charger in case it happens again?

2/ does your dashcam have an option to cut off when it detects a low voltage? This would prevent it from draining the 12v battery too much.

4/ another option is a small booster pack, something a few MG4 owners keep in their car for exactly this reason. Then if you find the car unresponsive you can use the hidden blade inside your key fob to open the door, open the bonnet, and supply the car with enough electricity to engage the HV pack which saves you waiting on the AA.

Something like this;

https://amzn.eu/d/02RdQoA3

Some of these also come with a build in compressor to inflate your tyres. There is some anecdotal evidence the supplied compressor isn’t very reliable.

Of course this would be more of an immediate problem solver, not a long term solution. The batteries that the MG4 shipped with aren’t the best, I think a 3 year life span is to be expected and it might be worth just replacing it with a good brand instead of trying to hold on to a mediocre battery that you don’t trust.

 

[MickeySw]

But the HV pack gets “disconnected” from the car during DC charging.

Last time I checked, the DC/DC showed 14V during a rapid charge which suggests the CCU is getting power from the HV pack?

 

[ReintjeWA]

Maybe the CCU powers low voltage systems directly while DC rapid charging, but it may not be supplying the 12V battery with any charge. Does your MG4 show any warnings about the 12V battery after concluding a DC rapid charge?

 

[MickeySw]

Maybe the CCU powers low voltage systems directly while DC rapid charging, but it may not be supplying the 12v battery with any charger.

It makes no sense to directly power the DC/DC but then deliberately disconnect the 12V aux?

Afaik at the end of a rapid charge the HV contactors open, therefore turning off the DC/DC which is causing the low voltage circuit error?

 

[ReintjeWA]

It makes no sense to directly power the DC/DC but then deliberately disconnect the 12V aux?

Afaik at the end of a rapid charge the HV contactors open, therefore turning off the DC/DC which is causing the low voltage circuit error?

Sure, you might be right, I wouldn’t know.

Either way, whether the 12V gets charged during DC charging or not, it’s unlikely to be the cause of the OPs issue.

And it wouldn’t be the first time MG does something that doesn’t make sense on the MG4.

 

[Kithmo]

Last time I checked, the DC/DC showed 14V during a rapid charge which suggests the CCU is getting power from the HV pack?

The 12v is charged from the HV battery or the AC to DC charger, if the car is switched on it may well show the charging voltage as it's charging from the HV battery.
I think what @ReintjeWA meant is that the 12v is not connected to the HV DC charger.
Turn the car off when DC charging and check in the app it'll show just over 12v and may even drop below that, this shows it's not charging from the charger.

 

[MickeySw]

Turn the car off when DC charging and check in the app it'll show just over 12v and may even drop below that, this shows it's not charging from the charger.

I have a Bluetooth monitor/datalogger connected to the 12V aux.

Looking back at DC (and AC) charging sessions (with the car switched on or off) I can see no difference, I.e it always shows DC/DC voltage (14V ish)??

 

[ReintjeWA]

I have a Bluetooth monitor/datalogger connected to the 12V aux.

Looking back at DC (and AC) charging sessions (with the car switched on or off) I can see no difference, I.e it always shows DC/DC voltage (14V ish)??

Maybe the MG5 is different from the MG4?

 

[MickeySw]

Maybe the MG5 is different from the MG4?

Afaik any other EV charges the 12V aux whenever the DC/DC is active. I can’t think of any reason why the MG4 should behave differently other than “it wouldn’t be the first time MG does something that doesn’t make sense on the MG4”

 

[Kithmo]

Afaik any other EV charges the 12V aux whenever the DC/DC is active. I can’t think of any reason why the MG4 should behave differently other than “it wouldn’t be the first time MG does something that doesn’t make sense on the MG4”

FWIW, my previous EV a 2020 MG ZS didn't charge the 12v on DC.

I have a Bluetooth monitor/datalogger connected to the 12V aux.

Looking back at DC (and AC) charging sessions (with the car switched on or off) I can see no difference, I.e it always shows DC/DC voltage (14V ish)??

Just as a matter of interest, check it in the MG iSmart app, with the car off, when DC charging next time and let us know. I'm interested to know as we haven't DC charged SWMBO's MG5 as of yet.

 

[MickeySw]

Just as a matter of interest, check it in the MG iSmart app, with the car off, when DC charging next time and let us know.

Only saw your post after I finished today’s rapid charge (13:00-13:30) so I didn’t check iSmart but the voltage diagram below (measured across the battery terminals) does show the aux charging with the MG5 switched off (voltage drops just before DC session starts)

 

[Slipstream]

I have searched but can’t find a simple easy to understand explanation! I have had my MG4 since November 2025. Last week the car would not open. Called the AA who said the auxiliary battery was flat. They started it and I could get in.
1/should that have happened so soon?
2/ what would drain it? - my dash cam?
3/ I have found what I think is the battery info by the tyre pressure gauge it is reading 14.1 - 14.4 what should it be and how do I charge it? Just driven 50 miles today and it hasn’t moved up.
4/ can I buy a charger in case it happens again?
Thanks everyone

Have you checked the battery terminals.
Meny owners have found them to be loose causing these sort of problems.
Apparently they are removed onboard ship and then not tightened. And not checked by the dealer at psi.

 

[MickeySw]

Have you checked the battery terminals.
Meny owners have found them to be loose causing these sort of problems.
Apparently they are removed onboard ship and then not tightened. And not checked by the dealer at psi.

You would think that’s one of the first things the AA guy checked?

 

[Slipstream]

You would think that’s one of the first things the AA guy checked?

I hope so.