I have a brand new heavy duty Varta battery and it still does it. Using it the other night with headlights and heater on, it was reading 12.5v. If I turn the engine off and just leave the ignition on, it drops quite rapidly to around the 11v mark. I think there is something draining mine. I got Si at A1 Rally Sport to check it over last week, but he couldn't find anything and at most times his ammeter was reading 13.7v... when I have time, I will take it into an auto electricians, as it really pees me off having to put it on a trickle charger every time after driving it. My car does still have the original alternator, so maybe it is that, after all it is over 11 years old.