2005 Vauxhall Astra parasitic drain on battery

Seems you are leaving it unused for too long .Either get a battery charger and put it on a trickle charge every 3/4 weeks or so if needed , or run the car for a while instead of not starting the car for that length of time .

Thanks for your advice. Perhaps you didn't understand what I wrote - I haven't left the car for long periods of time with the battery CONNECTED - for the very reason you suggest. Since fitting the new battery mid-April, I've turned it over and run it stationary every three days or so when I haven't taken it on a run. From experience (I only drive about 4,000 miles a year max.) this has never been a problem, but to go through 2 batteries in 20 weeks just through light use is abnormal.

Your post is misleading " The new battery I connected in April " Forgetting that , you cant have a parasitic drain if battery is disconnected . If you are starting the car every few days and also running it there should be no problem . It would seem that what power you are taking out of the battery on starting and driving is not being replaced by the alternator and gradually draining the battery . You need to load test the alternator and check output / charging rate .

Yeah, the alternator is my prime suspect. Better run the checks you suggest; or maybe a blown diode? After that, I guess it's check every damned thing in the car via the fusebox. Sorry if my post misled: the battery drain didn't occur on battery #1 when it was disconnected for 6 weeks, but after it was reconnected! So I replaced it with a new battery, #2, which has never been disconnected since fitted in April, yet nevertheless it's now flat, despite the light use described. Seems suspect to me. Cheers

No problem , this covid 19 is causing a lot of folks problems with low battery / non starts . Upsetting all aspects of our lives something terrible .