Every few months we have an issue where the battery in my 2006 Audi Avant dies. I've taken it to a battery place and they say that it's perfectly healthy. So I bought a BLE battery monitor and now I'm watching the battery do it's thing. I do know that the battery might not be getting fully charged after each drive since I usually only drive for about 5-10 minutes at a time. After several weeks it finally did the big drop again and I have data recording what happened.
You can see that the last time we drove the car was May 19 and then didn't drive it at all on May 20, then sometime during May 21 the battery level collapsed.
What might cause something like this?
What else besides this battery monitor might I use to diagnose this problem?