Battery sometimes mysteriously empty

Hi guys,

it might be just me, but over the recent months, I've been experiencíng the occasional issue with an empty or weak battery which I can't really explain.

I bought the car almost two years ago now (5-door 2005 MPI M/T). In all that time, on a single occasion this summer, the battery was so weak after a few days of not driving it that it turned the engine over too slowly to start. Initially. I let it roll backwards down the driveway, then slammed it into reverse. That didn't work but it must have somehow stirred the elctrolyte or something because after that, the starter did the trick.

Then about a week ago, after max two days of inactivity, the battery was so completely empty not even the warning lights came on and I had to recharge it over night. That worked though and the engine has started normally since then.

Now if that were all, I'd chalk it up to me forgetting to turn off a light, maybe due to not closing a door properly or something, although I failed to find a cause after both these instances. But there's more:

One evening a few months ago, I drove though the drive-through, and since there was a longish queue, turned off the engine a few times, restarting whenever the queue advanced. By the second or third time, the battery suddenly was almost to weak to start. Granted, I had kept the lights on, radio on, foot on the brake, but come on - it should easily be able to power all that for a few minutes, shouldn't it? I distinctly remember the car had started completely normally on that day when I went on that drive.

And another time, the same thing happened when I had turned off the engine in front of a long red light at a road works. That time, I even turned the headlights off and had no radio on, the only electrical consumer being the brake lights and ignition.

What am I missing? If it's just the battery dying of old age and abuse, shouldn't the trouble get worse and worse? It can't heal itself after all.