12v battery must be replaced - warning after car repair

[Yev000]

So just got my car back from 3 weeks fixing dented doors and it has "12v battery must be replaced" warning - Fastlane, the repair shop, said its because its reporting 11.5v. They had the car in service mode for 3 weeks - and apparently that's why - not sure I'm convinced. Told me I should drive it for a while and it will sort itself.

Anyway, logged it with them before I left so they can fix it if need be.

What do you guys think? Car is 6 months old...

 

[NewbieT]

Wonder if in service mode they kept the HV contactors open whilst repairing, ahem waiting for parts. Without the inverter the computer will run down the 12v battery. We all know what happens to car batteries if you fully discharge them. Might recover. Might be cooked.

 

[Durzel]

Yeah, the HV battery needs to be connected in order for the DC-DC converter to do its thing. It’s possible that if the HV battery charge was too low that it didn’t kick in either.

You might get lucky and it corrects itself after you’ve let it charge it back up again, but I think there is also a process you need to do to clear that message anyway, no matter how good the battery is.

@Mr H is clued up on this sort of thing.

 

[Tam]

So just got my car back from 3 weeks fixing dented doors and it has "12v battery must be replaced" warning - Fastlane, the repair shop, said its because its reporting 11.5v. They had the car in service mode for 3 weeks - and apparently that's why - not sure I'm convinced. Told me I should drive it for a while and it will sort itself.

Anyway, logged it with them before I left so they can fix it if need be.

What do you guys think? Car is 6 months old...

Tesla advises that it should be plugged in but I doubt many repair shops would follow that advice because they don't have enough outlets for all the cars they have in their shop.

The 12V battery would not be recharged if the main battery goes down to a certain level. As others said, maybe "service mode" would prevent 12V recharge as well.

So the risk of 12V battery failure is real when the car is left in a repair shop for a long time without proper cautions on how to keep both the main and 12V battery recharged.

Just hope that your 12V battery will recover because it is now under your care.

 

[Yev000]

I'll pop in there today I think. Since I drove it about 100 miles since then.

The HV battery never went below 60% in the shop.... If it's cooked it's entirely their fault.

 

[GlynG]

The battery will be fine - 11.5 volts wont be an issue and it will recharge from the main battery, wouldn't give it another thought. In order low voltage could damage a battery it would need to be sub 6 volts and left for a long time - months in fact for any permanent damage.

 

[Durzel]

I would tend to agree. The battery is probably fine. If the car detects excessive voltage drops on it then it will assume that it’s wearing out, and throw that message.

As said before, however, I don’t think you’ll be able to clear that message without effectively tricking the car into thinking a replacement battery has been installed, by following the HV disconnection process.

If you feel up to it - this is the process for replacing the battery:

 

[Yev000]

Thanks, it's back with them. They gave me another bloody Mercedes - I can't for the life of me understand why people buy these heaps of rubbish anymore. Like going back to a Nokia phone. Makes me feel like Scotty - lifting up the mouse and going "Computer"....


They told me they will put it on charge for ~ 4 hours and that should fix it.


On another note. Told them to sort the tyre pressures out. They refurbished the alloys, but pumped the tyres up to random pressures ..... Supposed to be 40 Psi and I got 42, 44 and 48 on 3 different wheels. Joy.

I bet they charged about 20k for the repair as well (2 new doors etc.)

 

[Yev000]

So got the car back, battery is fixed... but now the tyre pressures are 44, 44, 48, 48 ..... Why they think thats perfect I have no idea. At least the battery is sorted.

 

[Darmie]

I'm glad it's fixed for you but I find it weird that the dc to dc from the traction battery would not charge the 12v battery correctly. Just odd that it needed a separate charge.

 

[Yev000]

Maybe I needed to rive it for longer? I did ~ 120 miles. And to be honest there is a lot going on with regen braking etc. No clue at what point the 12v gets charged.

 

[MrT3]

Maybe I needed to rive it for longer? I did ~ 120 miles. And to be honest there is a lot going on with regen braking etc. No clue at what point the 12v gets charged.

The 12v battery is charged when needed from the main battery pack, independent of whether the vehicle being driven or not.