Dead 12v battery - any help and advice please?

[rogerco]

MG5 SR late 2020. Just got back from 5 days away having left car standing on drive as usual.Car was turned off and locked.

Keys (both) wouldn't unlock the car. Appeared completely dead. Unlocked manually and car would not turn on. Opened bonnet and removed cover. Disconnected 12volt battery - it measures 2.5 volts open circuit...

Ok so something has caused the battery to go flat. I have it on charge now which may or may not recover it.

The question is how will it be safe to reconnect it and how much current should be drawn from the 12v with the car off?

What can have caused the battery go flat? Has anyone else experienced this? When I left it last week the state of charge on the HV was about 50% - of course I have no way of telling what it is now, I can't unlock the front flap to plug in the charger (which is what I was going to do as I need it fully charged in the morning.

I'm fingers crossed it will at least wake up when the 12v has had a couple of hours charge - but it is very worryng that simply leaving the car for 5 days it will go dead - we have left it for much longer before so something must have gone wrong. Any ideas what. Anyone else had similar?

Just to add that the car as left unattended for 2.5 weeks late june-early july with no problem. The 12v battery state normally shows 14+ volts - and was doing so last week. Never seen it lower than 13.5v. All seemed normal week ago. Seems unlikely that the battery simply fell off a cliff in 5 days.

 

[rogerco]

Hurmph, After 1.5hrs charge battery reading 11.7 volts off charge. Disconnected charger reconnected car. Spark when connecting the +ve lead and a clunk from lowdown in engine compartment. After two secs the horn started beeping (not popular I imagine at 10pm on a Sunday) and the car would still not unlock so can't get it onto HV charge. Oh sheet. Dealer call in morning.

Meanwhile reading other threads here I realise that I have probably never seen the true 12v battery state as always start with foot on the brake to go straight to ready mode.

Also perhaps I've done quite well to have had the battery last 2.5 years and 25k miles. Is anyone with an earlyish original MG5 still on the original 12v battery?

Has anyone tried fitting a leisure battery (which seems to make more sense for me as even with everything possible on (AC, lights etc) in non-ready mode the drain should still be considerably less than an ICE cranking drain?

 

[johnb80]

Hurmph, After 1.5hrs charge battery reading 11.7 volts off charge. Disconnected charger reconnected car. Spark when connecting the +ve lead and a clunk from lowdown in engine compartment. After two secs the horn started beeping (not popular I imagine at 10pm on a Sunday) and the car would still not unlock so can't get it onto HV charge. Oh sheet. Dealer call in morning.

Meanwhile reading other threads here I realise that I have probably never seen the true 12v battery state as always start with foot on the brake to go straight to ready mode.

Also perhaps I've done quite well to have had the battery last 2.5 years and 25k miles. Is anyone with an earlyish original MG5 still on the original 12v battery?

Has anyone tried fitting a leisure battery (which seems to make more sense for me as even with everything possible on (AC, lights etc) in non-ready mode the drain should still be considerably less than an ICE cranking drain?

I dont know specifically about MG but I have had some issues with my i3. Car systems in general should go to sleep a short time after locking the door, mine takes 10 minutes. During this time of non sleep it is actually charging the 12v battery at 11 amps, when it finally sleeps theres a very small discharge taking place. My issue's started when I went over to Octopus Intelligent, they are constantly updating the car with a charge schedule, when they update it, it wakes up the ECU's and current is drawn from the 12v for a few minutes but this happens several times per hour. The 12v is only small (20aH) and it soon goes flat. My solution is to switch off Octopus Intelligent only enabling it when I can connect a PSU to the 12v overnight / day etc. The alarm going off I suspect is a normal feature of connecting the 12v, probably reset by manually locking the door and unlocking it again.

 

[rogerco]

I dont know specifically about MG but I have had some issues with my i3. Car systems in general should go to sleep a short time after locking the door, mine takes 10 minutes. During this time of non sleep it is actually charging the 12v battery at 11 amps, when it finally sleeps theres a very small discharge taking place. My issue's started when I went over to Octopus Intelligent, they are constantly updating the car with a charge schedule, when they update it, it wakes up the ECU's and current is drawn from the 12v for a few minutes but this happens several times per hour. The 12v is only small (20aH) and it soon goes flat. My solution is to switch off Octopus Intelligent only enabling it when I can connect a PSU to the 12v overnight / day etc. The alarm going off I suspect is a normal feature of connecting the 12v, probably reset by manually locking the door and unlocking it again.

Ooh that sounds nasty. I cant go Oct.Intel. cos I have zappi
Hopefully that is an id3 specific issue, and anyway not related to my current problem. Useful info anyway. Thanks

 

[ensmith007]

30 month old MG5. 45k miles. I had a thing that I believe (from the AA man) is a regular problem.
I had “radio initialising” problem. Radio was left on one dab channel no control.
Left it 3 days whilst I was away. When I came back totally flat. It had been drained by this fault.
Anyway recharged it and all’s been well since - including the faulty radio sorting itself out.
AA also said he had changed lots of MG lead acid batteries. So I guess it depends on cold damage incurred during winter. Mines still top notch though.

 

[Wibble]

Mine had an odd 12v battery issue a few weeks ago. I trickle charged the 12v battery for 4 hours and there have been no issues since.

 

[rogerco]

Ok, back on road. Found that the old battery after recharge and back in car was dropping volts overnight (car off and locked) - not a lot, about 12.4 down to 12.2v on the dashboard but I felt enough to be a concern if leaving the car for a week or more.

Difficult to measure the current drawn by car from 12v in off mode - I was still getting 5A after a couple of minutes standing there holding the meter probes in place, but couldn't see what state the car was in without letting go. 5A sounds way too much, it obviously takes a while to go properly to sleep.

Replaced the battery anyway with an overnight replacement from Tanya - very good service, wide selection available, ordered lunchtime and here (in Cornwall) by 10am next day. The old one I can use for other purposes.

Reminders:

1. keep a 10mm ring spanner in the glove box in case you ever need to disconnect and reconnect to clear a problem (eg with rogue public chargers)
2. remember how to access the emergency manual key (under cover to rear of drivers door handle) - not good forgetting and the handbook being in the glove box of locked car!
3. if you unlock manually, open the bonnet, disconnect the battery and then reconnect then the alarm will sound. Not if you relock the manual lock before reconnecting.
4. after a 12v disconnect you are likely to get a whole load of dashboard warnings, even to the extent of shutting down the HV. If the HV shuts down turn car off and on again (TIOAOA). Clear alarms by gently driving about 5 metres, and then if necessary TIOAOA
5. when replacing the battery you will need an 8mm socket and extension to remove the gubbins mounted in front of the battery and rotate it towards the centre, and a 10mm socket to remove the bolt for the battery clamp that was underneath it. The battery may be very stiff to slide towards the front to release it from the rear - a flat blade screwdriver to gently lever it from the back will help.
6. Your new battery may well have extension pieces on top of the side rails - these are not needed in this car, remove by levering up with flat blade screwdriver before putting the battery in.
7. The 12v display on the dashboard doesn't tell you anything very useful when the car is in ready or drive mode. It only tells you the static battery level when the HV is off (ie press start twice without pressing the brake)

 

[Jonhklee]

Are 12v Battery not covered by warranty for first 2 years?

 

[PCR]

Ok, back on road. Found that the old battery after recharge and back in car was dropping volts overnight (car off and locked) - not a lot, about 12.4 down to 12.2v on the dashboard but I felt enough to be a concern if leaving the car for a week or more.

Difficult to measure the current drawn by car from 12v in off mode - I was still getting 5A after a couple of minutes standing there holding the meter probes in place, but couldn't see what state the car was in without letting go. 5A sounds way too much, it obviously takes a while to go properly to sleep.

Replaced the battery anyway with an overnight replacement from Tanya - very good service, wide selection available, ordered lunchtime and here (in Cornwall) by 10am next day. The old one I can use for other purposes.

Reminders:

1. keep a 10mm ring spanner in the glove box in case you ever need to disconnect and reconnect to clear a problem (eg with rogue public chargers)
2. remember how to access the emergency manual key (under cover to rear of drivers door handle) - not good forgetting and the handbook being in the glove box of locked car!
3. if you unlock manually, open the bonnet, disconnect the battery and then reconnect then the alarm will sound. Not if you relock the manual lock before reconnecting.
4. after a 12v disconnect you are likely to get a whole load of dashboard warnings, even to the extent of shutting down the HV. If the HV shuts down turn car off and on again (TIOAOA). Clear alarms by gently driving about 5m, and then if necessary TIOAOA
5. when replacing the battery you will need an 8mm socket and extension to remove the gubbins mounted in front of the battery and rotate it towards the centre, and a 10mm socket to remove the bolt for the battery clamp that was underneath it. The battery may be very stiff to slide towards the front to release it from the rear - a flat blade screwdriver to gently lever it from the back will help.
6. Your new battery may well have extension pieces on top of the side rails - these are not needed in this car, remove by levering up with flat blade screwdriver before putting the battery in.
7. The 12v display on the dashboard doesn't tell you anything very useful when the car is in ready or drive mode. It only tells you the static battery level when the HV is off (ie press start twice without pressing the brake)

Great advice thanks

 

[Les burrows]

Are 12v Battery not covered by warranty for first 2 years?

No I year only
Les

 

[Gomev]

Ok, back on road. Found that the old battery after recharge and back in car was dropping volts overnight (car off and locked) - not a lot, about 12.4 down to 12.2v on the dashboard but I felt enough to be a concern if leaving the car for a week or more.

Difficult to measure the current drawn by car from 12v in off mode - I was still getting 5A after a couple of minutes standing there holding the meter probes in place, but couldn't see what state the car was in without letting go. 5A sounds way too much, it obviously takes a while to go properly to sleep.

Replaced the battery anyway with an overnight replacement from Tanya - very good service, wide selection available, ordered lunchtime and here (in Cornwall) by 10am next day. The old one I can use for other purposes.

Reminders:

1. keep a 10mm ring spanner in the glove box in case you ever need to disconnect and reconnect to clear a problem (eg with rogue public chargers)
2. remember how to access the emergency manual key (under cover to rear of drivers door handle) - not good forgetting and the handbook being in the glove box of locked car!
3. if you unlock manually, open the bonnet, disconnect the battery and then reconnect then the alarm will sound. Not if you relock the manual lock before reconnecting.
4. after a 12v disconnect you are likely to get a whole load of dashboard warnings, even to the extent of shutting down the HV. If the HV shuts down turn car off and on again (TIOAOA). Clear alarms by gently driving about 5m, and then if necessary TIOAOA
5. when replacing the battery you will need an 8mm socket and extension to remove the gubbins mounted in front of the battery and rotate it towards the centre, and a 10mm socket to remove the bolt for the battery clamp that was underneath it. The battery may be very stiff to slide towards the front to release it from the rear - a flat blade screwdriver to gently lever it from the back will help.
6. Your new battery may well have extension pieces on top of the side rails - these are not needed in this car, remove by levering up with flat blade screwdriver before putting the battery in.
7. The 12v display on the dashboard doesn't tell you anything very useful when the car is in ready or drive mode. It only tells you the static battery level when the HV is off (ie press start twice without pressing the brake)

Good clear advice, and yes Tayna are excellent for batteries, both on price and delivery speed, I have used them twice before.

 

[Deep purple]

Interesting that Teslas have a similar problem with 12 lead acid batteries and now have changed to LFP 12 volt batteries. This change has resulted in a large number of faults and they are doing a software change to try to fix it.
For some reason EVs degrade 12 v batteries faster, my ICE cars had 12v batteries that lasted 10-12 years.

 

[Lovemyev]

For some reason EVs degrade 12 v batteries faster, my ICE cars had 12v batteries that lasted 10-12 years.

ICE cars usually have an alternator that is used to charge the 12 volt battery, over the years batteries have become smaller in their capacity and size as powerful alternators are able to replenish the battery really quickly.
This has been both a cost and weight saving for the manufacturers of course.
I feel generally that 12 volt batteries last longer in ICE cars due to the high efficiency of the alternators.
I think the older batteries life cycle is being extended by a very effective charging system.
I personally think the 12 volt charging system in an EV is not as efficient as that of a ICE vehicle and therefore less forgiving if pushed to its limits.
A 12 volt battery plays a more important part in an EV than a ICE vehicle.
You can’t “bump start” an EV like you can with an EV .

 

[Jto]

Hi all
could I ask are you all short journeys and not used every day.

 

[Jto]

Would think have a solar panel that plugs into the 12v out let to keep up the battery when away would work?

 

[maskull]

Would think have a solar panel that plugs into the 12v out let to keep up the battery when away would work?

I should imagine that fitting one would be a problem , plus it would take a long time . Better option to buy a Noco Boost or similar

 

[setanta]

All modern cars draw on the battery when idle, it might have more to do with the health of the battery than anything else, a sign that it's got an issue, had the same thing happen to me years ago in my ice car after a holiday. Evs are charged by the HV battery, but only when it's in use. When the car is off the HV battery is dormant, if you run a diagnostic right after using the car and again a few hours later you'll see that the voltage will have dropped in the 12v battery. I Saw a video recently online about how to prevent this. While you're away open the ismart app occasionally and make the car do something remotely. This will wake up the HV battery which will then boost the 12v battery and keep it topped up.

 

[Madgermusso]

We left our MG5 LR 2021 for 2 weeks whilst cruising - no issues for us?

 

[Mundiz]

I've just had the same problem. After leaving my car parked for 5 days whilst visiting friends in Germany. I found my 12v battery was flat. I used the key in my key fob, a 10mm spanner and a new battery and the job was finished in ten minutes. The old Chinese 12v battery would not hold a charge. It was 2 years old.

 

[brianharries]

MG5 SR late 2020. Just got back from 5 days away having left car standing on drive as usual.Car was turned off and locked.

Keys (both) wouldn't unlock the car. Appeared completely dead. Unlocked manually and car would not turn on. Opened bonnet and removed cover. Disconnected 12volt battery - it measures 2.5 volts open circuit...

Ok so something has caused the battery to go flat. I have it on charge now which may or may not recover it.

The question is how will it be safe to reconnect it and how much current should be drawn from the 12v with the car off?

What can have caused the battery go flat? Has anyone else experienced this? When I left it last week the state of charge on the HV was about 50% - of course I have no way of telling what it is now, I can't unlock the front flap to plug in the charger (which is what I was going to do as I need it fully charged in the morning.

I'm fingers crossed it will at least wake up when the 12v has had a couple of hours charge - but it is very worryng that simply leaving the car for 5 days it will go dead - we have left it for much longer before so something must have gone wrong. Any ideas what. Anyone else had similar?

Just to add that the car as left unattended for 2.5 weeks late june-early july with no problem. The 12v battery state normally shows 14+ volts - and was doing so last week. Never seen it lower than 13.5v. All seemed normal week ago. Seems unlikely that the battery simply fell off a cliff in 5 days.

Having had this (well documented) issue with my previous Nissan Leaf, I couldn’t believe it that within 2 days of ownership, the same thing should happen with my new MG4. I was directed to the Apple Car Play wireless dongle I had fitted (permanently connected to my USB-C socket) that was constantly searching for my i-Phone causing the battery to discharge overnight. Removing all cables from the USB sockets rectified the problem, and I have never had another issue since - even leaving it parked up for a two-week period whilst on holidays, with absolutely no issues at all. I hope you might get your answer as easily as I did.