12v Battery Charging

[GaryMG4]

Page 23 of the manual states Intelligent charging (of the 12v battery) is suspended when HV battery is in low state of charge. Sadly it doesn't state how low.

 

[Rolfe]

I think that's a reasonable thing in principle, because it's going to be easier to get the car going if it's only the 12v that's flat and not the traction battery. But if the cutoff is 50%, that's ridiculous.

 

[Tig170]

... out of interest why is the 12v battery lead-acid? - where LiFePO4 are not that much more (or why can't a small bit of the HV battery be reserved for 12v duties / backup power? )

 

[olliewebb]

... out of interest why is the 12v battery lead-acid? - where LiFePO4 are not that much more (or why can't a small bit of the HV battery be reserved for 12v duties / backup power? )

Proven and reliable technology.

 

[Rolfe]

Proven and reliable technology.

I think some owners would dispute the "reliable" part.

 

[olliewebb]

I think some owners would dispute the "reliable" part.

More an issue with the vehicle in this case though. In all the years driving cars I’ve only had a dead battery once. Funnily enough on a TF that had been stored for ten years.

 

[Tig170]

More an issue with the vehicle in this case though. In all the years driving cars I’ve only had a dead battery once. Funnily enough on a TF that had been stored for ten years

Same here not too many problems, but have just dated the 12v battery and changed after 5 years
EDIT : one thing I have been monitoring is the price of EV batteries from salvage, they are between £5000-£10000 depending on model (2023 Taycan NMC battery is the same as a 2023 MG4 NMC battery via Trent salvage )

 

[Rolfe]

Quite a number of people have had dud 12v batteries in new cars.

 

[PristDean]

That's extremely interesting, but it still leaves a couple of questions unanswered.

First, there's the issue of what SoC the car needs to be at in order for the 12v battery to be charged from the HV battery. He reckons 20% in the Ioniq, as he saw it charging at 23%. However your dive into this in the MG4 seemed to suggest that it had to be 50%. I think this is what people were questioning, not the principle itself. If you can't leave your car for a day or two at less than 50% charge, this is a problem, much more so than at 20%.

There's also the question of how fast the 12v battery is running down if it's not being topped up by the HV. He said he had just returned from a long trip, so the 12v should have been fine at the end of that irrespective of the state of the traction battery. Nevertheless it went flat in a few hours, while the car was waiting to be charged at the off-peak rate. This sounds like a fault to me.

I was struck that his monitor showed the 12v going down quite fast and being topped up frequently from the HV. Surely that's a weak battery? I know ICE car batteries are more robust, but even so. We wouldn't expect an ICE car battery in good health to fail in a few hours and surely EV batteries can't be that much worse. A decent ICE car battery will hold enough charge to turn a cold engine over for months, not days, with no boost whatsoever.

I think that, quite apart from the SoC issue, he has a dodgy battery. A good battery shouldn't be losing charge like that. I also think that if the MG4 won't top up its 12v battery when it's under 50% on the traction battery, that itself is a problem.

Read it again... I said " So possibly the main battery needs sufficient capacity (over 30, 40 or 50%)"

 

[Tig170]

Read it again... I said " So possibly the main battery needs sufficient capacity (over 30, 40 or 50%)"

...but why does it need such a capacity - its not like its cranking over the engine?

 

[Jto]

I think the video is making some sense. the car relies on the 12v battery to run the 12v systems and if the main battery runs below the percentage it charge the 12v battery then the 12v battery is running all the systems. Let’s take the long journey coming back home New Year’s Day do not know if in the day or night time, it’s going to be cold could be dull day headlight on and fan heater on and maybe the radio, all these systems draining the 12v battery which is not been charged and you may do 20 miles in this state. Park the car pug In to charge, the car still has systems running looking for the key to signal to open the are, system looking at the 12v battery all drain on the battery and if cold this has effect on the battery, so drains too low to start the car. All make sense and if this happens too many times the battery deterrents to a point where if fails.
Keep the drive battery with go state of charge should stop this happing.

 

[Rolfe]

Read it again... I said " So possibly the main battery needs sufficient capacity (over 30, 40 or 50%)"

I'm certain that you're right, but the $64,000 question is, how sufficient is sufficient? If it really is 50% as some have said, that's ridiculous. 20% is perhaps defensible.

 

[PristDean]

I think the video is making some sense. the car relies on the 12v battery to run the 12v systems and if the main battery runs below the percentage it charge the 12v battery then the 12v battery is running all the systems. Let’s take the long journey coming back home New Year’s Day do not know if in the day or night time, it’s going to be cold could be dull day headlight on and fan heater on and maybe the radio, all these systems draining the 12v battery which is not been charged and you may do 20 miles in this state. Park the car pug In to charge, the car still has systems running looking for the key to signal to open the are, system looking at the 12v battery all drain on the battery and if cold this has effect on the battery, so drains too low to start the car. All make sense and if this happens too many times the battery deterrents to a point where if fails.
Keep the drive battery with go state of charge should stop this happing.

Spot on

 

[Tig170]

I think the video is making some sense. the car relies on the 12v battery to run the 12v systems and if the main battery runs below the percentage it charge the 12v battery then the 12v battery is running all the systems. Let’s take the long journey coming back home New Year’s Day do not know if in the day or night time, it’s going to be cold could be dull day headlight on and fan heater on and maybe the radio, all these systems draining the 12v battery which is not been charged and you may do 20 miles in this state. Park the car pug In to charge, the car still has systems running looking for the key to signal to open the are, system looking at the 12v battery all drain on the battery and if cold this has effect on the battery, so drains too low to start the car. All make sense and if this happens too many times the battery deterrents to a point where if fails.
Keep the drive battery with go state of charge should stop this happing.

I must admit that I've never had an issue todate (MG4), but then again I use the car daily, and only lock it when we go to bed - Ive never seen any voltage below 13.? v (EDIT : I do use V2L from time to time, and do find that the aircon comes from the HV system)

 

[PristDean]

The HV battery disconnects from the 12v if a charger is plugged in for obvious reasons. So if left like this, the 12v can drain considering everything JTO described. So in short, the 12v will remained topped up if the HV is actually charging or the HV has sufficient charge and a charger is not plugged in.

 

[Jto]

...but why does it need such a capacity - its not like its cranking over the engine?

Simple the ICE only used for very short time and the engine starts the alternator takes over, now asking this battery to do something different. Some one said about using deep cell battery but they need an over night charge if they get low. One other was to use a lithium battery but small one can suffer from cold weather or if left low charge will go into a sleep.

...but why does it need such a capacity - its not like its cranking over the engine?

 

[Tig170]

The HV battery disconnects from the 12v if a charger is plugged in for obvious reasons. So if left like this, the 12v can drain considering everything JTO described. So in short, the 12v will remained topped up if the HV is actually charging or the HV has sufficient charge and a charger is not plugged in.

Yes I've heard that one - (Macmaster via the AA recently) - don't leave it plugged in to the wall box on the house longer than necessary

Simple the ICE only used for very short time and the engine starts the alternator takes over, now asking this battery to do something different. Some one said about using deep cell battery but they need an over night charge if they get low. One other was to use a lithium battery but small one can suffer from cold weather or if left low charge will go into a sleep.

.... now that i only really travel from home each day I'm going to give up my AA membership (with relay), after 35 years! - I still have EV recovery via my insurance company (LV) and if i get it serviced via MG it will come with 12 months AA recovery anyway - thoughts?

 

[Rolfe]

I've not had an issue with my 12v at all, including leaving the car at the airport for over a fortnight. But hearing so many stories from people who have had issues is a bit worrying. It would be really helpful if MG spelled out the dos and don'ts for avoiding problems, but I guess that's too much to hope for.

 

[PristDean]

Yes I've heard that one - (Macmaster via the AA recently) - don't leave it plugged in to the wall box on the house longer than necessary

Yep, I saw that one with his Porsche. It seems the 12v in all EV cars is under increased demand.
I experienced the same a couple months ago and posted on this forum. Good that we have enough info now to realise where we all are with EV cars and batteries

 

[JonnyG]

... out of interest why is the 12v battery lead-acid? - where LiFePO4 are not that much more (or why can't a small bit of the HV battery be reserved for 12v duties / backup power? )

I see Tesla changed from 12v Lead Acid to Lithium-ion batteries due to issues of longevity in 2021 on the Model S and X. Not sure what the Model 3 and Y have ? :-

Tesla to replace SLI lead-acid batteries with 12V lithium-ion in new EVs - Best Magazine

Electric vehicle OEM Tesla is set to replace lead-acid SLI batteries in its new cars with 12V auxiliary lithium-ion.

www.bestmag.co.uk