Power problem

[Bb86]

Hi guys could really use some expert advice, first off I got a 8L a3 1.8t agu, ok so went to start the car and nothing completely dead, so charged the battery and it started but after about 4-5 minuates the dash lights dim the abs light comes on then it just stops, with no power at all not even hazards, I've replaced the alternator and battery and checked the wires which run from them but still can't solve the problem. Any help would be great

 

[<tuffty/>]

Check the earths under the battery tray.... common for them to corrode

<tuffty/>

 

[Bb86]

Checked that and all was ok but got my vag diagnostics scanner today and it read supply voltage b+ sporadic signal to low any ideas I'm new to all this lol.