12v battery drained in 48 hours whilst locked

[Lovemyev]

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.

I really do like your own approach in trying to identify the root cause of the energy loss from the factory fitted 12 volt battery .
Maybe when you are not likely to be using the car for a few days, try this.
Disconnect the negative terminal on the 12 volt battery and record the voltages from the battery at a standing state, over a few days.
Is there any significant drop in the voltage now ?.
If so, this would strongly suggest that the battery is incapable of retaining energy and is faulty.
If the battery completely retains its energy, this suggests a parasitic drain by the car.
I just have a feeling that the batteries fitted to the S5 are of not great quality or they just about have enough capacity when the they are at there very best state of charge.
Any amount of drain caused by short trips or long stays of non use and they fall away quickly.
Reducing the size / weight of 12 volt batteries saves money.
EV’s weigh enough as it is due to the traction battery, so any reduction elsewhere is welcomed by the manufacturer.
I can’t help but feel that the factory fitted battery is just about doing its job, but with very little margin for any slight extra drain.
A better quality battery with more storage capacity, that offers a larger margin for discharge, has to be a better option surely ?.

 

[LenEV2016]

I really do like your own approach in trying to identify the root cause of the energy loss from the factory fitted 12 volt battery .
Maybe when you are not likely to be using the car for a few days, try this.
Disconnect the negative terminal on the 12 volt battery and record the voltages from the battery at a standing state, over a few days.
Is there any significant drop in the voltage now ?.
If so, this would strongly suggest that the battery is incapable of retaining energy and is faulty.
If the battery completely retains its energy, this suggests a parasitic drain by the car.
I just have a feeling that the batteries fitted to the S5 are of not great quality or they just about have enough capacity when the they are at there very best state of charge.
Any amount of drain caused by short trips or long stays of non use and they fall away quickly.
Reducing the size / weight of 12 volt batteries saves money.
EV’s weigh enough as it is due to the traction battery, so any reduction elsewhere is welcomed by the manufacturer.
I can’t help but feel that the factory fitted battery is just about doing its job, but with very little margin for any slight extra drain.
A better quality battery with more storage capacity, that offers a larger margin for discharge, has to be a better option surely ?.

Mentioned before that MG probably don't have much faith in their 12v batteries hence why they are only covered by warranty for 12 months

 

[Joningy]

Follow up on my flat 12v battery, since getting home from Penton MG with a fully charged 12v battery, like @emmrecs I'm checking the 12v battery, every day, sometimes 2 or 3 times per day and recording the readings.
this morning (new years day) the reading was 11.6 volts.

A measurement of 11.6 volts, taken after the vehicle has been off for a period, (≈18 hours) is a clear indication that the battery is severely depleted.

There can only be 2 reasons for the drain,.

1/ a parasitic drain on the car is causing the battery to deplete whilst it is locked and everything including headlights, supposedly off.

2/ The 12v battery is faulty and NOT holding a charge.

Listening to other EV owners, either could be the case and I'm suspecting the battery is at fault.

After I return from a family lunch today, I intend disconnecting the 12v battery and leaving overnight, testing it tomorrow to see if it is the culprit, if it has depleted I can charge it before re-connecting.

Do any electrical experts, anybody with a higher understanding of the problem think this is a safe route on a nearly new EV, I own the car outright, so not in danger of contravening any small print crap, I just want to help resolve a problem myself and not rely on MG bullshit!

 

[Joningy]

There was another thread regarding what battery to replace the original MG battery with, and a bit of a discussion came up on that thread regarding building your own LFP battery against picking up a cheap drop in from EvilBay etc.

Teslas is the only manufacturer that has gone down the lithium battery aux battery path ..... and they are paying for it big time .... exactly the same problem, inefficient balancing methods and no cell voltage monitoring BMS, an LFP cell will just vent if it goes over voltage to the point the electrolyte boils and will stink the car out and the battery die because it lost a cell, if the lithium battery is built using cells containing cobalt and one goes over voltage and get hot, fireball ..... a cell containing cobalt will generate its own oxygen during the thermal runaway, a highly flammable electrolyte combined with pure oxygen in the vapour mix, a jet flame that will rapidly heat the adjoining cells until they are all blasting out oxygen enriched fire ...... inside a plastic battery case ......

T1 Terry

I've recently learnt MG IM range, as they are called here in UK, also use lithium auxiliary batteries.
I guess the breakdown companies are equipped with the kit to deal with it worldwide today.

 

[LenEV2016]

Follow up on my flat 12v battery, since getting home from Penton MG with a fully charged 12v battery, like @emmrecs I'm checking the 12v battery, every day, sometimes 2 or 3 times per day and recording the readings.
this morning (new years day) the reading was 11.6 volts.

A measurement of 11.6 volts, taken after the vehicle has been off for a period, (≈18 hours) is a clear indication that the battery is severely depleted.

There can only be 2 reasons for the drain,.

1/ a parasitic drain on the car is causing the battery to deplete whilst it is locked and everything including headlights, supposedly off.

2/ The 12v battery is faulty and NOT holding a charge.

Listening to other EV owners, either could be the case and I'm suspecting the battery is at fault.

After I return from a family lunch today, I intend disconnecting the 12v battery and leaving overnight, testing it tomorrow to see if it is the culprit, if it has depleted I can charge it before re-connecting.

Do any electrical experts, anybody with a higher understanding of the problem think this is a safe route on a nearly new EV, I own the car outright, so not in danger of contravening any small print crap, I just want to help resolve a problem myself and not rely on MG bullshit!

In case you don't manage to get to the bottom of it, keep a log of everything you have tried and the result.
If you need to return to the dealer hand them the log, it will save time as MG are guaranteed to tell them to check all the things you will have, before the sanction any replacements.
From experience the dealer will have to report back after every step and then wait on MG UK telling them to do the next.
It can be an infuriatingly slow process which could be made so much more efficient if the dealers were just allowed to crack on with stuff on their own. Unfortunately MG are very tight with the purse strings and authorising spends can be problematic

 

[T1 Terry]

Follow up on my flat 12v battery, since getting home from Penton MG with a fully charged 12v battery, like @emmrecs I'm checking the 12v battery, every day, sometimes 2 or 3 times per day and recording the readings.
this morning (new years day) the reading was 11.6 volts.

A measurement of 11.6 volts, taken after the vehicle has been off for a period, (≈18 hours) is a clear indication that the battery is severely depleted.

There can only be 2 reasons for the drain,.

1/ a parasitic drain on the car is causing the battery to deplete whilst it is locked and everything including headlights, supposedly off.

2/ The 12v battery is faulty and NOT holding a charge.

Listening to other EV owners, either could be the case and I'm suspecting the battery is at fault.

After I return from a family lunch today, I intend disconnecting the 12v battery and leaving overnight, testing it tomorrow to see if it is the culprit, if it has depleted I can charge it before re-connecting.

Do any electrical experts, anybody with a higher understanding of the problem think this is a safe route on a nearly new EV, I own the car outright, so not in danger of contravening any small print crap, I just want to help resolve a problem myself and not rely on MG bullshit!

Disconnect the battery and charge it to 14.4v and until the charging amps drop to only an amp or so ..... disconnect the charger, wait 10 mins and check the battery voltage ..... it should be better than 13.8v.
After an hr, check the voltage again, it should not have fallen any further. Check it again in the morning, if it is less than 12.6v, the battery has an internal short.

It can take a number of days on a slow charger to fully charge a 12v lead acid battery, a fast charger will only get it to between 70% and 80% charged, the rest is a very slow process ..... there is a reason why lead acid batteries are as crap as they are, 19th century technology and these days, made as cheap as possible with very thin plate material, a lead acid battery made to last, like a Concorde or Rolls, will last you 10 yrs treated properly, they will also cost you more than $1,000 ...... how many $1,000 batteries do you reckon a vehicle would be willing to fit as an auxiliary battery

T1 Terry

 

[Lovemyev]

In case you don't manage to get to the bottom of it, keep a log of everything you have tried and the result.
If you need to return to the dealer hand them the log, it will save time as MG are guaranteed to tell them to check all the things you will have, before the sanction any replacements.
From experience the dealer will have to report back after every step and then wait on MG UK telling them to do the next.
It can be an infuriatingly slow process which could be made so much more efficient if the dealers were just allowed to crack on with stuff on their own. Unfortunately MG are very tight with the purse strings and authorising spends can be problematic

After looking at your detailed results, the dealer will of course need to relay their own findings back to MG CS.
But they COULD apply a bit of common sense themselves while waiting for an answer back.
Installing a new different battery from a donor car, then after a couple of days of conducting the same voltage checks that you did, it will soon become clear if it's the original battery at fault, or if not, the car does indeed have a parasitic drain.

 

[Martinonline]

Unless the 12V battery is really knackered the car should be maintaining it at a level suitable to allow the car to start, given a sufficient HV battery level. If that charging circuit is faulty there should be the error warning described in the manual. However, the charging circuit in the absence of a 'active' HV battery source (car powered up) may be the cause of the drain.

 

[Rolfe]

Anybody able to put a number on "sufficient" in that context?