12v battery drained in 48 hours whilst locked

[Joningy]

ADMIN may repost this elsewhere, but thought all EV owners need to guard against the 12v battery failing.

I had the 12v battery failure on my MGS5 after no use for 48 hours, something had totally drained the battery, yet the car remained locked throughout.

I ran a battery test, the battery was totally flat, a reading of just 3 volts, I connected a smart charger overnight, but it didn’t do the job.

I used my small 12v jump starter which I keep under the seat along with a headtorch (for use if dark) it worked perfectly, temporarily got the car on the road.

I connected the car to the Ohme pro charger & charged for about 1.5 hours before we drove the 12 miles to Salisbury, the car had lost adaptive CC, plus all driver aids like lane keep assist etc, but drove just fine.

I drove straight to the main dealer, Penton MG in Salisbury who were fantastic, they took the car in straight away, got a taxi for my wife and I to run the mile or so into the town centre, they kept the car for about 4 hours, I had a phone call to say the car was ready and the offer of a taxi back to pick it up, but I briskly walked off lunch in less than 10 minutes, all faults reset and the 12v battery charged, all worked perfectly on the way home.

It’s on a scheduled charge tonight to ensure the 12v battery is totally charged.

I would have been totally stuck without the jump start kit, it was also a good learning exercise to open the driver’s door with the tiny manual spare key, it was straightforward, but daylight.

A lesson to all EV drivers, get a small 12v battery booster, saves hours waiting for a breakdown callout, but DO NOT keep it in the boot, it MUST be accessible from the driver’s door, mine is under the seat as the rest of the car is locked tight until the jump starter performs its magic.

 

[LenEV2016]

I know many advocate the battery booster but I am on my 3rd EV and never had one or needed one, cars been left on numerous occasions for two to three weeks without issue.

MG 4s and now S5s seem to be experiencing random issues with this.
I dont think MG place much faith in their 12v batteries as they only cover them for 12 months, still doesn't explain them suddenly going dead, there has to be something draining it

 

[Joningy]

I know many advocate the battery booster but I am on my 3rd EV and never had one or needed one, cars been left on numerous occasions for two to three weeks without issue.

MG 4s and now S5s seem to be experiencing random issues with this.
I do t think MG place much faith in their 12v batteries as the only cover them for 12 months, still doesn't explain them suddenly going dead, there has to be something draining it

Well that's good news Len and I suspect the case for most EV drivers, my original MG4, went 2 years without a problem, although I always carried a battery jump start booster, but out of the blue my S5 12v battery lost everything.

I was very pleased to have a booster on board (under the driver's seat)

We live in a pretty rural spot in Hampshire, miles from the nearest garage or town, so the £35 or so quid it cost was a real bonus in the long run.

It certainly saved me a whole lot of time waiting for recovery as the car was parked on the drive at the side of the house and it wouldn't have been easy to jump it from our other car without extra long jump leads.

 

[Ian Key]

I replaced the battery on my MG4 after 2.5 years just as a precaution and will probably do the same with the MGS5 but luckily never had the battery go flat.

I keep my battery booster under the centre armrest as I don't want it suddenly shooting forward if I do an emergency stop.

 

[Joningy]

I replaced the battery on my MG4 after 2.5 years just as a precaution and will probably do the same with the MGS5 but luckily never had the battery go flat.

I keep my battery booster under the centre armrest as I don't want it suddenly shooting forward if I do an emergency stop.

That's a good idea Ian, yes it does shoot forward on occasions, but neve caused a problem, thinking of some self adhesive velcro, there's less room under the S5 seat than on the MG4.

But the important thing for new folks to the idea of a battery booster is it must be in an easily accessible place, it's NO GOOD in the boot as it is virtually inaccessible if the battery is flat, I guess another alternative would be under the bonnet somewhere, perhaps in a small plastic box, but as already mentioned elsewhere here, I keep a cheap headtorch with the booster, sods law it will be dark when needed!

 

[Ian Key]

That's a good idea Ian, yes it does shoot forward on occasions, but neve caused a problem, thinking of some self adhesive velcro, there's less room under the S5 seat than on the MG4.

I got a tray from AliExpress and the booster fits underneath it.

 

[Joningy]

I got one of these trays from AliExpress and the booster fits underneath it.

Auto-Aufbewahrungsbox für MG S5 MGS5 EV ES34 2025 ~ 2027 2026 Gummimatte Mittelkonsole Organizer Armlehne Kleinigkeiten Tablett Teil Zubehör - AliExpress 34

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

Brilliant Ian, I'll order one, didn't realise they were made !

 

[Ian Key]

Brilliant Ian, I'll order one, didn't realise they were made !

Just put MGS5 EV accessories into AliExpress and all sorts of items will show up. The link I included is just an example of the item, you might find it cheaper from another seller

 

[Gomev]

These things never happen until they happen. First time in my EV life the boot hadn't shut properly and overnight drained the 12v. Had the booster in the frunk thankfully.

 

[T1 Terry]

Your smart charger didn't charge your battery, because it needs at least 10v to turn it on .... how's that for smart If you had put your boost battery on the 12v battery at the same time, the charger would have kicked in and charged the battery.
Reason for battery failure, most likely an internal battery short. They are made as cheap as possible these days because lead acid battery manufacturers know their days are numbered.

A word of warning regarding drop in 12v batteries, a friend lost his motorhome, fortunately they weren't in it at the time, 2 weeks after installing it. It was in his son's front yard, had sat there for two days with the battery isolator switched off.
It just decided that was as good a time as any to burst into flames, safe to bet it wasn't an LFP battery.

While he was talking to the firey, it burst into flame two more times, they called in a hazmat drum and filled it with water and throw it in there, they could see it glowing under the water ......

T1 Terry

 

[Joningy]

Your smart charger didn't charge your battery, because it needs at least 10v to turn it on .... how's that for smart If you had put your boost battery on the 12v battery at the same time, the charger would have kicked in and charged the battery.
Reason for battery failure, most likely an internal battery short. They are made as cheap as possible these days because lead acid battery manufacturers know their days are numbered.

A word of warning regarding drop in 12v batteries, a friend lost his motorhome, fortunately they weren't in it at the time, 2 weeks after installing it. It was in his son's front yard, had sat there for two days with the battery isolator switched off.
It just decided that was as good a time as any to burst into flames, safe to bet it wasn't an LFP battery.

While he was talking to the firey, it burst into flame two more times, they called in a hazmat drum and filled it with water and throw it in there, they could see it glowing under the water ......

T1 Terry

Well that's interesting to know @T1 Terry , I've certainly learnt a few things regarding flat 12v batteries in the past few days, that's probably why the lights on the charger were alternating between charging modes, which I couldn't understand.

@MikeJS I wonder where you got the info that the 12v battery is lithium?
My info tells me my MGS5 is bog standard lead acid, why would a budget motor vehicle as MG use expensive Li-ion batteries to run the electrical system, when a far cheaper lead acid does the job, despite the lead acid downside of weight and longevity?

 

[Joningy]

Brilliant Ian, I'll order one, didn't realise they were made !

I replaced the battery on my MG4 after 2.5 years just as a precaution and will probably do the same with the MGS5 but luckily never had the battery go flat.

I keep my battery booster under the centre armrest as I don't want it suddenly shooting forward if I do an emergency stop.

Ordered Ian, thanks for the heads up!
Got the glove box and one for under the centre console, both for £12.49 including delivery from China, that's if I ever see them of course.

3 years ago I ordered a large bank of LiFePO4 batteries for my DIY Solar from China through Ali Express, several thousand quid, they never turned up had to do a bank chargeback, it worked fine, but always made me very cautious with Ali Express, I now use PayPal to buy from them. Cheers

 

[MG Clive]

@MikeJS I wonder where you got the info that the 12v battery is lithium?
My info tells me my MGS5 is bog standard lead acid, why would a budget motor vehicle as MG use expensive Li-ion batteries to run the electrical system, when a far cheaper lead acid does the job, despite the lead acid downside of weight and longevity?

We've got our threads crossed here I think mate, @MikeJS post was referring to the IM6 (this post), which uses a 12V low-voltage lithium battery system.

 

[Rolfe]

I had exactly this problem with my Peugeot 306. It was never solved and it even did it the morning it was booked to be delivered to the scrappage scheme in 2009.

 

[emmrecs]

A lesson to all EV drivers, get a small 12v battery booster, saves hours waiting for a breakdown callout, but DO NOT keep it in the boot, it MUST be accessible from the driver’s door, mine is under the seat as the rest of the car is locked tight until the jump starter performs its magic.

1000% agree.

Like @Ian Key I keep mine under the centre console, at least I do now! Having had the (slightly) humiliating experience of calling MG Assistance to a totally flat 12v battery (AA measured it at 4v) and being unable to access the booster I bought two years earlier (and never needed with the previous MG5) and having to admit to the AA Technician that it was in the boot!

(Just as an aside, the technician arrived at my house in rather less than 1 hour after calling the assistance line, so absolutely no complaints there!)

But, on a broader point, @Joningy is yet another MGS5 owner experiencing this 12v battery problem.

Since my recent experiences of needing to use the booster four times, and a full 2-day diagnostic check at my local dealer (which found no fault, of course) I have been making at least one check of the state of the 12v every day and recording the voltage shown by my 12v charger (accepting it's likely to be not "wholly" accurate, so just an indication of changes). The figures reveal that, following a full charge by the 12v charger on December 22, the voltage was given as 12.8. Since then the figure has gradually reduced until yesterday afternoon, December 30, it registered as 12.0. There was one "increase" in the figure, December 28, when it read 12.3 (from 12.1 the previous day) after the HV battery had been on charge overnight.

This morning it read 14.7 (!!). Car was not "on" or even "booting up" so that suggests, I think, that the HV battery was carrying out the "Intelligent Battery Charging", perhaps? No overnight HV charging this time either.

But if it was the Intelligent Battery Charging, why has it not done the same before now???

EDIT, to add: all those battery voltage figures are part of the evidence I intend to pass to my dealer when the car goes for its next diagnostics check, next week.

 

[Joningy]

1000% agree.

Like @Ian Key I keep mine under the centre console, at least I do now! Having had the (slightly) humiliating experience of calling MG Assistance to a totally flat 12v battery (AA measured it at 4v) and being unable to access the booster I bought two years earlier (and never needed with the previous MG5) and having to admit to the AA Technician that it was in the boot!

(Just as an aside, the technician arrived at my house in rather less than 1 hour after calling the assistance line, so absolutely no complaints there!)

But, on a broader point, @Joningy is yet another MGS5 owner experiencing this 12v battery problem.

Since my recent experiences of needing to use the booster four times, and a full 2-day diagnostic check at my local dealer (which found no fault, of course) I have been making at least one check of the state of the 12v every day and recording the voltage shown by my 12v charger (accepting it's likely to be not "wholly" accurate, so just an indication of changes). The figures reveal that, following a full charge by the 12v charger on December 22, the voltage was given as 12.8. Since then the figure has gradually reduced until yesterday afternoon, December 30, it registered as 12.0. There was one "increase" in the figure, December 28, when it read 12.3 (from 12.1 the previous day) after the HV battery had been on charge overnight.

This morning it read 14.7 (!!). Car was not "on" or even "booting up" so that suggests, I think, that the HV battery was carrying out the "Intelligent Battery Charging", perhaps? No overnight HV charging this time either.

But if it was the Intelligent Battery Charging, why has it not done the same before now???

EDIT, to add: all those battery voltage figures are part of the evidence I intend to pass to my dealer when the car goes for its next diagnostics check, next week.

You've certainly gone down the route I and a few others are currently starting out on it appears @emmrecs .
Also Like me having a similar negative response from MG re the adaptive cruise control not always turning of with the switch and the crappy automatic headlight dipping.

MG have said they are driving aids and not necessary for safety, which is boll***, it's just another cop out, but my local MG garage gave an amazing service when I took it in yesterday.