Parasitic drain on 12V battery

[emmrecs]

@marcbesty

Welcome to the forums!

If you do a quick search in the MGS5 forum area you will find there are a number of us experiencing exactly what you describe! Mine is currently on a Diagnostic Check with my local dealer, who has, apparently, managed to convince MG Technical that there is some sort of parasitic drain on the 12V battery. They (the dealer and MG) have requested more time to complete further tests, after which I will, hopefully, have a fully-functioning car! "Hope springs eternal" and all that!

 

[Joningy]

Sounds like the same issue as I have @marcbesty .

I was told a similar thing: "MG expect the car to be driven more often" what a load of b******s,
Don't stand for it, keep pushing.

Not sure how much you've read of this was my thread on the same subject.

12V battery drained in 48 hours whilst locked​

But my car is NOT charging overnight as I think it should be, Penton MG tell me there is NO facility for the S5 to do this, which is a serious conflict with what I understand.

Who is right?

This may be useful to copy and print for your local MG service centre to look at, waste of time emailing, they speed read and miss most of the relevant info.

Who is right? my MG garage says NO facility for the high Voltage battery to charge the 12V auxiliary battery whilst the car is locked and switched, the below contradicts that statement.

Yes the MG S5 EV, the high-voltage (traction) battery does maintain the 12 V auxiliary battery even when the car is switched off and locked, but only periodically, not continuously.


How the MG S5 EV handles the 12 V battery when off

DC-DC converter behaviour

• The car uses a DC-DC converter (instead of an alternator) to step down HV battery voltage to ~14 V for the 12 V system.
• When the vehicle is off and locked, the DC-DC converter:
  • Wakes up intermittently
  • Checks 12 V battery voltage
  • Tops it up if needed
  • Then goes back to sleep

This is normal MG EV behaviour and is shared across MG4, MG5 EV, ZS EV, and S5 EV platforms.

​

---

What this means in real terms

What

• The 12 V battery is maintained automatically
• Short periods parked (days or weeks) are usually no problem
• Normal parasitic loads (alarm, telematics, keyless entry) are covered

What

• It is not a constant charge
• If the 12 V battery is:
  • Old
  • Sulphated
  • Internally failing
    …the system may not rescue it

In those cases the car can still end up with a flat 12 V battery despite a full traction battery

​

---

When the car wakes itself up

The S5 EV will briefly energise systems:

• After locking
• At scheduled intervals
• When remote services / telematics check in
• When HV battery SoC is healthy

You may hear:

• A faint click
• Relays engaging
• A brief whirr

That’s the DC-DC converter doing its job.

​

---

Important edge cases to be aware of

HV battery very low

• If the traction battery is critically low, the car may stop maintaining the 12 V
• This is a safety strategy

Long storage (many weeks)

• MG still recommends:
  • Driving periodically or
  • Using a smart 12 V maintainer if the car is parked for a long time

After software updates

• Some MGs temporarily increase standby drain post-update
• This can expose a weak 12 V battery

 

[Big L]

Hi all,

I have purchased a MGS5 in November, had a 12v battery issue on in December and AA said I had to take it to the dealership for them to replace the battery. After a few days, they advised that they couldn't find what was causing the battery to drain from their checks and said the battery was faulty.

On Thursday, same issue. AA came out and boosted the 12v battery. Spoke to the dealership service desk and they said I don't drive the car enough and my journeys aren't long enough. Pretty sure I didn't buy a car to drive it round for fun . One of the main selling points is for people who have my driving habits. Woke up this morning and the battery is dead again .

Waiting for a booster to be delivered and will keep that handy but it's ridiculous that I have to do that.

Hi Marcbesty, I know what you are going through, I've had the same issue and MG have had my car over 3 months, they are fitting a new CCU unit but that might not turn up until 18th Feb. I'm fighting now to get my money back, as the car has 5 issues in total with only 2 fixed so far

 

[Joningy]

An important thing to remember @marcbesty , storing your 12V booster is crucial, NEVER keep it in the boot as its very difficult to access when the 12V is down, I used to keep it under the seat, but it did slip out occasionally, now I keep it on the shelf, under the centre console, after reading other comments here.

 

[Warden]

Sounds like the same issue as I have @marcbesty .

I was told a similar thing: "MG expect the car to be driven more often" what a load of b******s,
Don't stand for it, keep pushing.

Not sure how much you've read of this was my thread on the same subject.

12V battery drained in 48 hours whilst locked​

But my car is NOT charging overnight as I think it should be, Penton MG tell me there is NO facility for the S5 to do this, which is a serious conflict with what I understand.

Who is right?

This may be useful to copy and print for your local MG service centre to look at, waste of time emailing, they speed read and miss most of the relevant info.

Who is right? my MG garage says NO facility for the high Voltage battery to charge the 12v auxiliary battery whilst the car is locked and switched, the below contradicts that statement.

Yes the MG S5 EV, the high-voltage (traction) battery does maintain the 12 V auxiliary battery even when the car is switched off and locked, but only periodically, not continuously.


How the MG S5 EV handles the 12 V battery when off

DC-DC converter behaviour

• The car uses a DC-DC converter (instead of an alternator) to step down HV battery voltage to ~14 V for the 12 V system.
• When the vehicle is off and locked, the DC-DC converter:
  • Wakes up intermittently
  • Checks 12 V battery voltage
  • Tops it up if needed
  • Then goes back to sleep

This is normal MG EV behaviour and is shared across MG4, MG5 EV, ZS EV, and S5 EV platforms.

​

---

What this means in real terms

What

• The 12 V battery is maintained automatically
• Short periods parked (days or weeks) are usually no problem
• Normal parasitic loads (alarm, telematics, keyless entry) are covered

What

• It is not a constant charge
• If the 12 V battery is:
  • Old
  • Sulphated
  • Internally failing
    …the system may not rescue it

In those cases the car can still end up with a flat 12 V battery despite a full traction battery

​

---

When the car wakes itself up

The S5 EV will briefly energise systems:

• After locking
• At scheduled intervals
• When remote services / telematics check in
• When HV battery SoC is healthy

You may hear:

• A faint click
• Relays engaging
• A brief whirr

That’s the DC-DC converter doing its job.

​

---

Important edge cases to be aware of

HV battery very low

• If the traction battery is critically low, the car may stop maintaining the 12 V
• This is a safety strategy

Long storage (many weeks)

• MG still recommends:
  • Driving periodically or
  • Using a smart 12 V maintainer if the car is parked for a long time

After software updates

• Some MGs temporarily increase standby drain post-update
• This can expose a weak 12 V battery

Even though the handbook says that the 12vdc battery is maintained by the HV battery, in reality it has never happened I had a Facelift MG5 for 2 years before getting my S5 in June and it was supposed to do the same but again it never did. I read somewhere that Mg admitted that this facility had never been implemented on the Fl mg5 (and I believe also on the Mg4) so I suspect that the S5 is the same, despite the hand book saying on both models that it does.
If you think about it if this system was working the Hv battery would be down to 20% everytime you had a flat 12vdc battery, 20% being the cutoff for VTL so I would assume it would be the same for charging the 12vdc battery if it ever gets turned on in any future software update.

 

[tsedge]

My MG4 maintains the 12v battery just fine, regularly topping it up when parked and locked or while driving. We have left the car for weeks at a time and had zero problems. After 3 years it is still on its original 12V battery - the MGS5 should inherit this implemented system.

As this seems to be a pattern with the MGS5 quite a few people have, I would suspect:

• A software bug is causing parasitic drain that overwhelms the 12V topup.
• There's a batch of faulty hardware modules causing the drain.
• There's a bad batch of 12V batteries.

Is the battery lead-acid or a newer smaller Lithium-Ion type? If they have changed the chemistry maybe they need to change the management software or maybe they undersized the capacity? The MG4 uses a conventional lead-acid battery,
mine is 60Ah IIRC.

Have there been many software updates for the MGS5 yet?

 

[dx4100]

In October I went away for 7 days and came back to a working car.
At New Year went away for 8 days and came back to a working car.

I had the 12v battery fail once but I think its down to me switching the car off rather than the infotainment screen while I was fixing a screen protector. Got a 12v warning and next day car was dead. I think that was user error not the car.

If my car can maintain itself over a few day inactivity while on holiday then surely every MG MS5 should be able too. What is written above about how the car should maintain itself makes perfect sense. Not driving the car enough is clearly rubbish.

One thing I have come to know over the last couple of months from my own experience and reading around forums and facebook group is MG dealers, servicing and customer service is appalling and they don't really understand their own product.

Get yourself a battery tester like the KINGBOLEN BM550 and see what the state of the battery and if that has an issue. I am guessing you find the SOH of the 12v will be just fine and will let you discount it.

You can keep an eye on the SOC and how and when its draining.

 

[dx4100]

Good battery management habits isn't a bad idea though.

If you are sat in the car for extended periods of car and not moving e.g. waiting in a car park etc. Hit the brake pedal to put the car in READY mode. This make sure the car is awake and drawing power from the main battery and not the 12V.

 

[Ian Key]

Even though the handbook says that the 12vdc battery is maintained by the HV battery, in reality it has never happened I had a Facelift MG5 for 2 years before getting my S5 in June and it was supposed to do the same but again it never did. I read somewhere that Mg admitted that this facility had never been implemented on the Fl mg5 (and I believe also on the Mg4) so I suspect that the S5 is the same, despite the hand book saying on both models that it does.

My MG4 like @tsedge did maintain the 12V battery often showing 14.7V in the app indicating it was being charged. The only reason I replaced the battery was that these readings were becoming more and more frequent.

I very rarely let the main battery drop much below 40% before charging back up to 80% unless I was doing a calibration charge from under 10% to 100%.

I seem to remember some MG4 owners did have a software update to "fix" their 12V batteries charging problem.

 

[T1 Terry]

It would be an interesting test to put a battery maintenance charger on the failing 12V battery over night, still connected to the vehicle electrical system, and see if the charger is enough to keep the battery charged .....

T1 Terry