My battery doesn't seem to hold charge, and it has previously vented. I hooked up my voltmeter, and it was showing 12.5v (ish). I started the engine, and when the alternator was running the voltage was getting as high as over 17v and consistently over 16.5v. Is this bad news? I expected the voltage to be more around 14.5v when charging.

Sounds like its cooked the battery, which has vented.

Charging should be around 14.4 - 15V. 17V consistently would probably have killed the battery.

Sorry bud.

ETA: Thats what you get for having a dig at me all week!!

[Edited on 26-9-10 by MakeEverything]

Alternator unwell, charging voltage 13.9 14.5 ish

sounds like your regulator is buggered

As above, it is likely to be your regulator on the blink. You can sometimes find a replacement at alternator repair companies - or simply replace the alternator.
Unfortunately it may have taken your battery with it, so its best to get it checked out at a battery specialist who can see if it has done any lasting damage.

[Edited on 26-9-10 by RazMan]

Starting with a battery with decent charge ie showing 12.5 or more volts accross the termminals with everything switched, measure the battery voltage cranking the engine (best done cold cold & ignition coil disconnected), if the voltage acrross the battery is less than 9 volts the battery is goosed.

[Edited on 26/9/10 by britishtrident]

Looking back at the megasquirt logs from my MOT yesterday, I stopped at a garage afterwards and it wasn't keen to start. Voltage dropped to 8.5v during cranking, so it is officially more goosed than Meg Ryan's character in Top Gun