B9 Battery voltage

[Brian74656]

On 4/21/2025 at 11:06 AM, Reavley said:

Hello,

I have a 2017 B9 2.0 petrol. I've recently had dash warnings the start/stop and drive system fault. 

The first few times the faults cleared with turning it off and on again. But now the engine dash light is permanently on yellow. I will take it to a local garage for a diagnostic but wanted to have a bit dig myself too. I saw it could be an issue with the battery so thought I'd test the voltage with a cheap multimeter I had.

The readings I got were 12.5v with engine off, and 12.9v with engine on.

Are those readings ok or could there be an issue with the battery?

Cheers

Martin

 I would put an agm 95 mah battery on it cured my stop start fault and epc  warning light on 2016 a5 tfsi

[Reavley]

I haven't added an update as I don't have an update to provide at this point. Nothing has changed since my previous post.

[cliffcoggin]

I agree with you Brian, but it is proving difficult to persuade people away from the old argument that if the battery has 12 volts and cranks the engine then it must be OK. That was indeed true before the advent of electronic controls a few decades ago, but it is no longer a good enough criteria to judge the condition of a battery, as you and I and many others have discovered the hard way.

[Magnet]

So we take it the asked for advice which results in:- 

Getting the alternator output checked.

Getting the full battery efficiency checked. 
 - has not been followed, and prejudging, is unlikely to. 

[Reavley]

I appreciate all the contributions in response to my post. And I take the information provided into consideration in the decisions I make.

I will provide an update as an when there is new information to share. You don't need to try and prejudice what I may or may not do with my car.

[Magnet]

Absolutely Martin - it’s your car, and you are free to do with it what you will, and my feeling is that any advice given ( when asked for) which doesn’t concur with your views, will be ignored. 
I certainly won’t be chasing you again for a thread-closing update, since I feel the tread will remain as ‘unsolved’ anyway.