Since I bought my Octavia II from new, I have quite often heard a tone over the speaker system in the car which I have taken to mean the battery is low. It happens when I leave the radio on for more than 10 mins, doors open for same, interior lights... etc. I am even wondering if leaving a phone charging is having the some effect.

Now however, I keep finding myself with a flat battery and unable to start. It seems that just 30 mins with the radio on is draining the battery?

I am wondering if, even after I switch the engine off but don't take the key out, are my lights staying on longer but I thought they went off after a period even with key in ignition but not turned.

The AA have checked my battery and alternator twice and found they are both 100% and charging normally.

Today, a friend in an Audi S3 found her battery was flat but she had just parked overnight and had nothing on in the car since she parked it.

 

Does anyone know if something else is causing a high drain on the battery?

it may be charging OK but needs more than a run 'round the block to fix properly  ideally it needs removing and load testing after a good long charging session for pref' with something like a C-Tek