My car (jag xk 2006) went dead when I left it parked for 3 weeks. Understandable. I charged it and all ok. I made the same mistake a couple more times. Then one day when I was pumping up the tires at the garage it went totally dead. No lights. No dashboard. Nothing. I got it jump started and at home I charged it again. I then spent £200 for an auto electrical place to check it for 4 days, telling me that the car had no parasitic drain and the alternator is fine, and the battery is good.So, all good, £200 please. Then it happened again whilst filling with petrol at the service station. I've put a CTEK gadget on it and it charges up perfectly, but I know its going to happen again. What to do? Could the battery actually be clapped out and not hold a charge despite them testing it and claiming its good? I don't drive very much, and mostly short journeys, but it never used to happen. Anybody intelligent out there with any ideas?
Sounds like your Battery was already old, not sustaining charge over 3 weeks. On top of that, when you completely drain you battery it can get damaged further. What you describe sounds like that.
I had the same behavior with my Toyota Aygo a while ago. The battery could not stand the load of the starter motor any more. Changed the battery and all is fine now.
(My Mechanic called it an "internal shortage", read about it here)
I suggest you replace the battery, should be a lot cheaper then spending £200 on finding nothing and actually solve your problem.