Battery drain or faulty battery

[b3oty]

I only bought my 2001 3.0d a couple of weeks ago and the battery has been dead twice in the morning (both times I was playing with the sat nav the night before with the engine off). But even if I just lock it up and leave it until the next day after using it, starting is a bit slow suggesting the battery voltage is quite low.

I've read a lot on forums about parasitic battery drain, but I don't know if this is the problem, or if it just needs a new battery as I have no idea when it was last replaced.

So I gave the battery a full charge yesterday and when I removed the charger last night, it was showing 12.82 volts on my multi-meter.

I'm told a fully charged battery should be 12.60 volts after 24 hours of rest. I couldn't leave it that long as I need to use the car, but after 12 hours of rest the battery was only showing 12.40 which I gather is only about 75% charged.

So I presume that either the battery is past it's usefull life and can no longer hold a charge and therefore needs replacing, or something is drawing power when the car is locked and left overnight.

Can anyone tell me how I can tell which is the case please?