Today I found all my three Mini 2 batteries are fully discharged. They have been sitting in the battery hub for about 3 month. It was about 80% when I store them.
According to the manual this could cause damage to my almost new batteries!

Hello there Mickey33. I am sorry for the trouble this has caused and thank you for reaching out. DJI do not recommend fully discharging your DJI Mini 2 intelligent flight batteries. In addition, the DJI Mini 2 intelligent flight battery has an Auto-Discharging Function to prevent swelling. The battery automatically discharges to approximately 96% of the battery level when it is idle for one day, and automatically discharges to approximately 72% of the battery level when it is idle for five days. It is normal to feel moderate heat being emitted from the battery during the discharging process. In addition, I will post a link where in you can find the DJI Mini 2 Disclaimer and Safety Guidelines user manual ( https://dl.djicdn.com/downloads/ ... y_Guidelines_12.pdf ). Thank you.

You misunderstood it. Of course I know it is bad to fully discharge the battery.
Three month ago, I left my battries (all about 70-60%) in the DJI Mini 2 Two-Way Charging Hub !!!
I didn't touch them for 3 month.
But yesterday, I found them all have been discharged to 0 (can't turn on).
It means the Mini 2 Two-Way Charging Hub consums the batteris slowly!