Car not starting after 2 months

[Hammonda]

Hi - apologies if this one has come up before (it probably has): After around 2 months idle, the car isn't starting up. The main battery has been charging from the mains for about 24 hours now but with no impact on the auxiliary that does the ignition. If I keep the charging the main battery for long enough, will it supply some energy to the auxiliary to allow the car to start? Or is one of them potentially caput (the car is from 2016, but mileage is not huge, at c.50,000). Or would jump starting work? Local garage was unwilling to help: I'd possibly need help from the dealer in Swindon.
Thanks

 

[DaveL59]

Worth measuring the voltage on the 12v aux which can be done from the engine bay. Do that before trying to jump start it and see if it'll boot. Good chance the 12v Aux is flat and perhaps terminally so and may need replacing. If the 12v is low and not charging I'd not drive the car since it may chuck up lots of random errors and could leave you stranded.

Is the car actually charging though, since that'd also charge the 12v at the same time?
Thought it'd need the 12v aux to have enough power left to energise the contactors for the main to charge up.

 

[Hammonda]

Worth measuring the voltage on the 12v aux which can be done from the engine bay. Do that before trying to jump start it and see if it'll boot. Good chance the 12v Aux is flat and perhaps terminally so and may need replacing. If the 12v is low and not charging I'd not drive the car since it may chuck up lots of random errors and could leave you stranded.

Is the car actually charging though, since that'd also charge the 12v at the same time?
Thought it'd need the 12v aux to have enough power left to energise the contactors for the main to charge up.

Hi thanks. The main battery does seem to be charging, yes. Maybe if I give it a lot longer time, like a few days even, it might eventually get the auxiliary going?

 

[kernelpanic]

DaveL59 is correct, with no power in the 12v Aux battery nothing will happen, the Drive battery won't be charging.
You can "jump" the Auxiliary battery in a way similar to what is done in Older cars. The procedure is in the Manual, here's a copy

 

[ArthurUK]

I keep a Cigar Plug USB with integral DVM plugged in, its usually above 14V.
I had to jumpstart mine once (as described in the manual) having left the car on what I thought was "Accessory" with the radio on in the garage for a few hours.
So clearly even in this mode the 12V battery is being drained by more than just the radio, although it is a puny battery.

 

[DaveL59]

I went a different way and added an extra USB outlet with voltmeter:



Powering to ACC shows the 12V AUX battery level, once Ready it sees the 12V converter output which will jump to 14.5v and gradually drop to 13.5v ish once the aux battery has charged while driving.