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. 

[daveyboy1967]

Just for reference Martin, mine just now with engine running.

[Reavley]

Thank you Davey

[daveyboy1967]

It could be something as simple as a loose belt, you're getting some output from your alternator as the voltage is higher with the engine running than when turned off, but it's nowhere near enough. You should definitely get it checked by a decent mechanic before you end up ruining what could be a perfectly decent battery.

[Magnet]

Martin, could you please clarify your ‘engine on’. 
Thanks, Gareth. 

[daveyboy1967]

2 hours ago, Reavley said:

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

I'm pretty sure he means running Gareth.

[Magnet]

Thanks Dave, but I don’t do pretty sures, we need to deal only in confirmed fact on here, otherwise they are assumptions. 

Kind regards,

Gareth. 

 

[daveyboy1967]

He would see a voltage drop with the ignition energised but with the engine not running when compared to an at rest reading.

[Reavley]

Apologies for the ambiguity Gareth... I did indeed mean engine running when I said engine on.

I'm no fan of Halfords but I have messaged a local independent mechanic, who I've used in the past and had positive experiences with, to book the car in for him to look at.

I note Cliff's point about having the battery coded to the car and will ensure if I do end up needing a new battery I will make sure it is somewhere that can code it to the car.

Really appreciate your notes on a potential belt or alternator Davey. Handy to have a few potential causes in mind when the mechanic gives their opinion.

 

[Magnet]

Does indeed now sound like an alternator issue. Beware of replacement aftermarket alternators - as opposed to getting yours repaired. 
Kind regards,

Gareth.