AA Rescue needed New Years Eve

[satfan54]

Hello all and happy new year.

I took my 1990 auto to Blanchardstown for a spin. I headed home from there, the only difference being the headlights on. I stopped for ciggs in a garage on Kilmore Road and when I came out the car would not turn over. I had no tools so called the AA who arrived in 15 minutes. He boosted the battery and the car started. He checked and confirmed the voltage from the altinator as correct. After a few minutes he got me to stop the enging and restart. This went well and the car was running as normal. I then turned the headlights on and as they began to pop up, the car died completly with the pop up lights partly raised. The battery voltage was down to 10.5volts. Another boost to the battery was needed and the car started ok. I turned on the head lights on with the booster attached and when it was removed the car stayed running and I got home without stopping.
The AA man had confirmed the connection from the altinator to the battery, and suspected the battery. The battery is only 6 months old and when I had it tested after Christmas it was fine.
I am at a loss as to why the battery failed. It is bigger than whats required, a wet cell, and I feel the car should have been running on the altinator when started and therefore should have not cut out when the pop up lights were turned on. My altinator was reconditioned last year.
If anyone has had a similar issue I would be gratefull if you could let me know as I am not driving the car with this issue present.

Thanks.
I have not driven the car since and would