I’m looking at buying a second hand Mini 2 and the seller hasn’t touched the batteries for over a year. I’ve asked for the battery info from the app and have received this,
Also found out that one of the batteries has died (blinks 3 times).
Would it be safe to use the two working batteries?
Since 2021, the batteries have been fully charged 2 and 3 times 
If the battery’s charge and is not blinking a fault then the BMS is saying they are ok, that’s really the only way to tell.
You could also monitor the flight time
If you do purchase it make sure it’s been unbound from their account and I would hover test first if worried about the batts check times temp etc I’m sure it’ll be fine if their smart batts and the firmware has been updated on them
