B9 Battery voltage

[Reavley]

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

[cliffcoggin]

Those readings have little relevence, so you could indeed have a faulty battery. The symptoms you report are consistent with a dying battery which can not provide the stable power supply needed by the car's electronics. If it is the original eight year old battery I suggest you replace it as soon as practical, and ensure you get it coded to the car. See this for an explanation why:

https://www.youtube.com/watch?v=IJAScg6JrYI

[daveyboy1967]

5 hours ago, Reavley said:

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

I would expect to see somewhere around 14.4V at the battery with the engine running. It sounds like this could be an alternator problem rather than the battery.

[Reavley]

Thank you both.

Would that be 14.4v straight after turning the engine on or does it take some time to build to that?

The 12.9v I got with engine on was straight after ignition.

Thank you

[daveyboy1967]

By the time you've started, lifted the bonnet and checked the voltage you should see that voltage, pretty much as soon as the alternator is spinning.

[Magnet]

Hell Martin,

Are you talking the 12.9v with the engine running? ‘Engine on’ is rather ambiguous. 
If it were mine, I would first take the car to an Halfords store and get the battery efficiency checked there. Also ask them to check the alternator output at the same time. 
I wouldn’t take it to a local garage and get ‘diagnostics’ carried out. At best they are likely to inform you the battery is duff and probably fit a ‘best deal for them at their motorfactors’. 
If Halfords declares the battery and/or alternator to be defective, please come back to us before committing to purchase any part or repair. 
Kind regards,

Gareth.