Had the Dreaded 12V Battery Drain Overnight

[thepfy]

Re: Lights
I find it is easy to leave the lights on.
I find that it doesn't warn me that are on when getting out of the car or locking.

It does warn me when I unlock the car....

It's also a case of me unlearning what my previous Kia Soul did. What that, all the lights turned off when the car was locked.

 

[DixieTGS]

If the 12V battery is being drained by non essential circuits and will shortly go flat anyway, why doesn't it turn off those non essential circuits when there's still just enough battery to unlock the car and start it I wonder?

 

[Martinonline]

If you power down the car from the infotainment screen the car will then tell you the lights are on when you open the door. This is the same as in my previous ICE car where the car warned me about lights when I switched off the ignition and opened the door.

I can understand the logic; the car shows the lights are on, on the driver's screen. If you decide to leave the car with the lights on that's your decision, the car has warned you of the state. You may want the lights on. However, powering down the car clears the driver's screen so another warning is needed, which you can ignore.

If only MG had kept a simple start/stop button rather than the current convoluted power down procedure.

 

[Tuareg]

When I lock my car, all the lights and infotainment are switched off immediately, nothing left on whatsoever. Left my car for 5 weeks parked up a few months ago, 12V battery had the same voltage on return and drove away like it was used the day before. In June, 2 more weeks unused and same experience, 12V battery fine.

 

[siteguru]

When I lock my car, all the lights and infotainment are switched off immediately, nothing left on whatsoever. Left my car for 5 weeks parked up a few months ago, 12V battery had the same voltage on return and drove away like it was used the day before. In June, 2 more weeks unused and same experience, 12V battery fine.

Pretty much every MG4 is the same ... IF you've left the lights on Auto. The discussion relates to scenarios where the lights have been manually turned on, such as in misty/foggy conditions where the auto sensor doesn't realise and so leaves the lights off, thus you have to manually turn them on.

 

[tsedge]

Where does the >50% needed to charge the 12V battery come from? I think cars with software updates charge the 12V battery at much lower states of charge but I can't find anything that confirms it either way.

With such a small capacity 12V battery, having the lights left on is actually a much bigger proportional drain than in an ICE car, so not surprising that it goes dead flat overnight if it is not being topped up.

Modern 12V car batteries can be low quality, so worth thinking about changing them every few years anyway to avoid problems.

 

[ReintjeWA]

Where does the >50% needed to charge the 12V battery come from? I think cars with software updates charge the 12V battery at much lower states of charge but I can't find anything that confirms it either way.

I think the >50% sort of came together from anecdotal "evidence" spread out over several sources, as far as I know it has never been confirmed by MG or any manual.

 

[Kithmo]

With such a small capacity 12V battery, having the lights left on is actually a much bigger proportional drain than in an ICE car, so not surprising that it goes dead flat overnight if it is not being topped up.
.

But the only lights left on are the parking lights, tail lights and the front DRLs, which are all LED. The headlights go off when you turn the car off and even if they didn't, they are LED anyway so much less of a drain than halogen headlights.
The only time the headlights don't go off is if you don't switch the car off (leaving it unlocked) and it automatically switches itself off after 30 minutes and the headlights go off just leaving the parking lights on.
IMO 30 minutes with the headlights on then overnight with the parking lights on shouldn't flatten a healthy 12v battery.
TBH the MG4 is one of the best cars I've had for 12v battery health/conditioning. When it's working properly, it charges the 12v as soon as you sit in the drivers seat, it doesn't have to be in Ready mode, it boosts the 12v when switched off if the HV SOC is above 50% and it charges the 12v as you charge the HV on AC.
This makes it difficult to flatten the 12v unless the 12v is duff or has bad connections or there is a fault in the 12v charging system.

 

[T1 Terry]

Tonight, the wife sat in the MG4 with the dog, while I dropped something off to a friend in hospital. Because the car was still on to keep the heater running, it wanted to keep the headlights on, because it was dark by that stage, so I manually switched to park lights ......

We travelled on to the son's place for dinner, pulled up and the meter said 34% .... we were late, I left the wife to lock the car and headed inside to make our apologies.
Came out 2 hrs later, to walk the dog, and the park lights were still on .... bugga, turned them off and hoped for the best. Another 2 hrs later, in the car to head off and the battery was 35% So, not only didn't the 12v battery go flat, but the LFP battery gained 1% somehow

T1 Terry

 

[tsedge]

But the only lights left on are the parking lights, tail lights and the front DRLs, which are all LED. The headlights go off when you turn the car off and even if they didn't, they are LED anyway so much less of a drain than halogen headlights.

My parking and tail lights are not LED, they are incandescent bulbs. I don't know if new MG4s now come with all LEDs or not.

The only time the headlights don't go off is if you don't switch the car off (leaving it unlocked) and it automatically switches itself off after 30 minutes and the headlights go off just leaving the parking lights on.

The headlights stay on if you manually turn the lights on by pushing right on the lights switch (to disable automatic lights) and then push left to select what you want (manual lights). Whatever you select stays on until manually turned off, whether you lock the car or not.

When it's working properly, it charges the 12v as soon as you sit in the drivers seat, it doesn't have to be in Ready mode, it boosts the 12v when switched off if the HV SOC is above 50% and it charges the 12v as you charge the HV on AC.

Everyone keeps taking about this 50% threshold but I don't see where this comes from. I believe it charges the 12V battery below this figure.

 

[decrep]

I guess it shouldn't be too hard to test, remind me next time I'm below 50%

 

[Kithmo]

My parking and tail lights are not LED, they are incandescent bulbs. I don't know if new MG4s now come with all LEDs or not.

The headlights stay on if you manually turn the lights on by pushing right on the lights switch (to disable automatic lights) and then push left to select what you want (manual lights). Whatever you select stays on until manually turned off, whether you lock the car or not.

Everyone keeps taking about this 50% threshold but I don't see where this comes from. I believe it charges the 12V battery below this figure.

Ok I'm guessing you have the SE if you have incandescent bulbs, my Trophy is all LED.
The headlights shouldn't stay on when you lock the car, effectively switching it off, even when the switch is set to manual headlights, unless that is another SE quirk. Are you sure the headlamps are lit and not just the front parking lights or DRLs ?
IIRC the 50% figure was banded about on here somewhere.

 

[Martinonline]

IIRC the 50% figure was banded about on here somewhere.

The manual says that maintaining the 12V battery will stop if the HV battery is at 'a low state of charge'. I would have thought that a 50% level was not 'low'.

 

[decrep]

Here's all I can find in my excite 51 manual.

"
Note: It is recommended to set the power mode to
READY for more than half an hour every week to
help extend the service life of the battery. When the
vehicle will not be used for an extended period (more
than 1 month), it is recommended to disconnect the
battery negative terminal clamping pile head. Make
sure that the vehicle power system has been turned
off before connecting or disconnecting the negative
battery cable."

So that infers that the vehicle needs to be in ready to charge the battery.
And the -ve lead needs to be undone if not used for more than a month.

So much conflicting info here.
Is this due to different models in different countries?

 

[Martinonline]

Could well be but the UK versions will charge the 12V battery once the car is powered up by sitting in the car, it does need to be in Ready Mode (brake pedal pressed). To see if this is true for you sit in the car and then look at the tyre pressure screen which shows the 12V battery level.

 

[ardsar]

Just woke up to a dead car! Drove the car yesterday with no issues. Voltage on the 12V battery is down to 6V. HV battery is 75% charged!

Reading this thread, is there an issue with the logic to top up the 12V battery or do i have a dud 12V battery. Ther terminals were tight, so no issues there.

What should i expect when i reconnect the battery - I’m aware that a load of errors will appear - is it a case of just driving to clear them and turning the steering full lock each way?

EDIT - I put the car in the garage yesterday and left the lights on. So clearly the car does not top up the 12v battery! Why is there no audible signal that you have left the lights on.??? I have been caught out with this before - there is an audible warning when you get back in the car - but that’s too late!!!!

 

[Martinonline]

You are probably correct that there is an issue with maintaining the 12V battery level. Some cars have a problem, others don't. My non-charging of the low voltage battery issue occurred when the car was detected when the car was in Ready Mode and could not be corrected through power cycling or trying to charge the HV battery (reported elsewhere). In that case I just connected my booster to the battery without disconnecting it and this triggered the charging circuit to work again. (I assumed the circuit had reverse current protection.)

As regards an audible warning, please my post at #23.

 

[ardsar]

You are probably correct that there is an issue with maintaining the 12V battery level. Some cars have a problem, others don't. My non-charging of the low voltage battery issue occurred when the car was detected when the car was in Ready Mode and could not be corrected through power cycling or trying to charge the HV battery (reported elsewhere). In that case I just connected my booster to the battery without disconnecting it and this triggered the charging circuit to work again. (I assumed the circuit had reverse current protection.)

As regards an audible warning, please my post at #23.

Understand your point with post 23, but unfortunately most people just get out the car and lock it rather than going through the menus to select power down, I can’t see why there is no audible warning when the drivers door is open even if not powered down.

 

[pjmmk]

I can’t see why there is no audible warning when the drivers door is open even if not powered down.

It does seem strange. Cars have had a lights-on warning for decades.

 

[tsedge]

Ok I'm guessing you have the SE if you have incandescent bulbs, my Trophy is all LED.
The headlights shouldn't stay on when you lock the car, effectively switching it off, even when the switch is set to manual headlights, unless that is another SE quirk. Are you sure the headlamps are lit and not just the front parking lights or DRLs ?
IIRC the 50% figure was banded about on here somewhere.

I have the Trophy and they are not all LED. Headlights are designed to stay on if you set them to be manually on (as I described).