Low voltage battery discharging

[Nickel]

Car is a ZS Trophy LR, just under a year old.

We have started seeing messages that the "Low voltage battery needs charging, do not leave car more than 3 days unused." Or words to that effect.

Yesterday evening we parked up with the battery registering 13.8V. Later in the evening it was down to 13.0V, measured via the MG Smart app.

Now (lunchtime), it's down to 12.2V.

As far as we can tell, nothing we control is on. And the car hasn't been used since we parked.

Can anyone shed any light?

Nick

 

[Gadget Geek]

I would get it to the dealer and get the 12 volt battery changed before the 12 month warranty runs out, because it sounds like it isn't holding its charge.

 

[Nickel]

Is the battery only covered for a year, then? I didn't know that.

That's what it feels like to me. Fortunately I have a service booked before the12 month date, but I'll phone them on Monday anyway. We're going away in a week...

In further funniness, after I posted above, my wife went and put the car in Ready mode - and the battery immediately came up at 13.7V...

 

[Eclectic]

In further funniness, after I posted above, my wife went and put the car in Ready mode - and the battery immediately came up at 13.7V...

Yes, that's as it should be, the traction HV battery charges the 12v battery in ready mode.

 

[Alb]

When in ready mode, you are measuring the charging voltage, not the battery voltage.
Once off charge, you still need to allow some time for the battery to stabilise before you get an accurate reading.

 

[TassieDi]

It sounds like the 12 V battery is failing, as @Gadget Geek said.

 

[Nickel]

Thanks for everyone's input. I'm worrying that the 12V battery is failing, obviously. But I've also realised that teh different ways of measuring teh voltage are all measuring something different. In Ready mode, the HV battery is charging the LV, and something is supplying current for the car systems. (I see 13.7-14.6V that way with no obvious cause for the differences.) In Standby, the voltage is lower, but of course the 12V battery is now definitely powering the car, as the HV battery is disconnected. I've taken to looking at the MG Smart app with car off, as that is closest to an off load state. But even then there must be enough powered up to run the central locking/key detection system and the internet connectivity, plus anything else I don't know about.

That has now declined to 12.1V in 24 hours, and now seems stable. Also the car is not complaining about a low state of charge any more. So I don't know what to think.

Would anyone do me a favour and check the voltage on their battery via the app after the car has been standing a while? It would be good to have another data point before a conversation with my dealer. Thanks in advance.

 

[Daver]

Car charged overnight Saturday after long trip on Friday and unused since. App shows 12 volt battery at 13v a few minutes ago. Hope this helps.
By the way the app often shows the 12 volt battery at rather less than 13v but never as low as 12v - yet! I was a little worried about the state of the 12 volt battery when the car was delivered in early June and gave it a number of trickle charge sessions in case it had been left at a low state of charge during transit. It never seemed to get to a fully charged state, based upon the fact that it was still drawing a current from the charger but it has never let me down so perhaps I was worrying unduly.

 

[Nickel]

Thanks. That's interesting.

By "overnight Saturday", do you mean Friday Night/Saturday morning or Saturday night/Sunday morning?

I've paid little attention to the app voltage before now as the car has been fine. It's receiving the error messages (now gone for the time being) that has worried me. Especially since in the week/ten days before receiving the first message we did two 8 hour driving days. Admittedly with the HVAC system going full chat...

 

[Daver]

Sorry for the confusion.
Long trip on Friday and charged car on my Octopus Go tariff from 00.30 to 04.30 Saturday. I will need another session to get it back to 80% but pleased to see that Octopus have just reduced the off peak rate to 9p/kW now so another little step in the right direction.

 

[Lovemyev]

Here is what our car is displaying on the App at 6,20pm ( 17.09.2023 ).
It has been sitting on the drive for about 30 hours now.
This is about the usual voltage I regularly witness when running a system check on the App.
We have a Trophy LR about 18 months old and covered roughly 13,500 miles.
Charge via home wall box to 80% SOC.
Remember the 12 volt battery will be receiving a charge when the car is in ready mode and NOT standby mode and when charging of course.

 

[TassieDi]

12.8V via the app.

 

[evcog]

I had to leave my car for 7 days as I was on holiday. Came back and the car started fine. Have left parked over this weekend and the app shows 12.8v. I am sure with normal use next week it will return to over 13v displayed in the app.

Interestingly the app reset while away and I couldn't reconnect until I returned. It is also worth checking if you have something slow draining the battery. I have a bluetooth android auto that does take a small amount of power. I would disconnect that if leaving the car idle for longer than a week in the future.

Hopefully your battery will be sorted.

 

[Nicko]

Mine is showing 12.8v on the app.

 

[evcog]

I had to leave my car for 7 days as I was on holiday. Came back and the car started fine. Have left parked over this weekend and the app shows 12.8v. I am sure with normal use next week it will return to over 13v displayed in the app.

Interestingly the app reset while away and I couldn't reconnect until I returned. It is also worth checking if you have something slow draining the battery. I have a bluetooth android auto that does take a small amount of power. I would disconnect that if leaving the car idle for longer than a week in the future.

Hopefully your battery will be sorted.

After a shortish journey for a coffee app now shows 13.2v

 

[Lovemyev]

Update !.
After sitting almost another 20 hours, here is the current status reported by the App on our car.
It is reports a slightly lower reading than yesterday, but I am not concerned at all.
This maybe due to the fact that I always carry a battery jumper pack under the drivers seat .
I will be using the car later this morning and will post back with a new 12 volt battery status report later.

 

[Daver]

Update
Car unused since Friday, charged to 67% on Saturday morning (cheap night time rate). Yesterday the app showed 13v, now fallen to 12.8v.

 

[Nicko]

Still at 12.8v

 

[Lovemyev]

Another quick update after a short 20 miles trip this morning.
The battery status has now increased ( as you would expect ) the weather is terrible here this morning, a lot of surface water etc and a lot electrical items running, including the HVAC.

 

[GameZone]

Had this message on my M4. Same issue even after charging the 12v battery to 100%. Dealer couldn't find anything wrong. 2 days later the problem was gone and hasn't come back.