Went out to start my car a couple of days ago, put in the key......dead as a door nail. Not enough juice to sound the open door alarm. Charged the battery overnight and in the morning the car started , drove a few miles, stopped to do an errand and when I tried to start it again the battery was almost dead, would not turn over but opened the moonroof halfway on it's own. Jumped it and got home and charged the battery again. Same result, after a short drive it will not restart but opens the moon roof. I'm guessing it's just an old battery but usually they give some warning before they refuse to work. Any other ideas before I replace it? I'm competent but no master electrician.
Thanks

It may well be your Battery, but it may be a problem with your alternator.

Easiest way to find our take the battery down to a parts store and get them to check it, they should do that for free. If its the battery thats fine (and easy) if they say the battery is fine, then you may want to start checking your alternator.

Fingers crossed on the battery because I got a feeling an alternator will be expensive !

Turns out it was the alternator.Only charging at 11 volts. Rebuilt one from Haus of Parts was $220. Not as bad as list from my mechanic at about $450. However it didn't come out easy. Still, all is well now.