Hi. I drove 90 miles. Stopped. Engine wouldnt turn over, message screen said all sorts of things wouldn't work. Got a call out from a mechanic, jump started the car. Ran for 10 mins to get me half a mile to where I wanted to be. I could jump start from there to get home. Alternator is pushing 12.3-14.7. After 10 mins on jump start 3 days later tried starting engine worked straightaway. Has worked fine ever since. It's not the battery

 

Is it the computer? If so can I reboot iir? 

How do you know it’s not the battery John? 

Because it's now working fine. Driven 100 miles today started fine multiple times. After it went 'flatt' was charged enough to drive 10 mins. Started fine 3 days later. 

Good, but back to your question - why wasn’t it performing its function when it let you down? 
The only way you are going to get proof positive that it is fully serviceable is to get it tested. Halfords stores will do that. 
Get the alternator output check at the same time, since 12.3 volts is way too low. 
Until you get both these checked, sorry, but you cannot eliminate either as being a culprit. 

Lol I drove porsche 911s for 25 years I constantly check the alternator with a 12v socket plug in as 911 batteries die at short notice. Engine starts 12.3v after 30secs it's up to 14.7v but after half an hour it drops to about 13.5v.then goes back up after a while. 

I'll get the battery checked but I think it's something else it's too inconsistent. A dead battery boosted for 10 mins that starts 3 days later doesn't make sense to me. It's been fine since then. 

John.

Believe what you want, but 911s have positively old fashioned electrical systems compared to your A3, which requires a stable electrical source to avoid erratic and seemingly unrelated electronic problems. The voltage you measured tells you nothing about the real condition of your battery, so Gareth's recommendation to get the battery professionally tested is spot on. That means a high rate of discharge during the test in the region of 250 amps rather than the 0.005 amps of a simple voltmeter.

Cheers. Tell me about old 911 electrics you get in turn the engine on and hope you don't get a burning smell 😂😂😂 hope your wife doesn't try to open the door as you press the unlock key as that jams the locks hope the cold hasn't seized up the immobiliser contacts and hope the battery hasnt just died (annual occurrence) 

Much as I loathe Halfords ill give them a visit. And take the advice of the rescue bloke turn the stop start off and buy a normal battery. 

Do let us know the outcome.

Modern alternators don’t supply maximum charge at all times, they supply enough charge when needed to keep the battery maintained between a set range range, typically when engine braking the alternator produces maximum output and when accelerating produces little unless the battery requires it, this is to improve mpg as a alternator producing charge also produces drag, common battery failings are a broken cell, typical symptom is the battery will take a charge fine but will lose it very quickly over 24/48 hrs and this time of year is common for this fault 

Any progess on this John?

Hi. Battery seems fine. Charging at 14.7 constantly. I think it was a glitch wouldn't start jump start got me going then 3 days later startered first time. 

So what was the result of the Halford’s test on your battery then John?