Light Aircraft Association

Hi all,
I have a 912uls fitted to an Ikarus C42.I replaced a time expired 12v battery last October.A couple of weeks ago when I went to fly the (new)battery was fully discharged.Took the battery home,charged it overnight,put a voltmeter across it which read12.5v.
Fitted the battery earlier today.Started Ok,got airborne and noticed that the RPM gauge was fluctuating.I checked the digital voltmeter on the transponder which was registering 15 to 18.5 volts with an overvoltage message in the scratchpad.Terminated the flight immediately.
My question is;
is the battery the problem or is it a problem with the voltage regulator?
Battery details;
Odyssey PC310.12v.cyclic charge voltage 14.4-14.8v @25c.
Cheers
Tom

It's definitely the regulator. A fully charged battery would read around 13.5v but the alternator can produce a no-load voltage considerably higher than that. The fact that the battery discharged it'self when nothing was switched on suggests some form of diode breakdown (I'm not familiar with the Rotax alternator, but most alternators have some sort of diode network or similar. It's a common prob with car alternators when the diode plate is faulty, allowing a direct drain to earth.