I bought my Mini 9 in April of last year and it's been working just fine... Except that a few months ago, the battery seemed to only keep it's charge for about 20 minutes at full capacity. It still works fine when plugged in, so it's still usable as long as I'm near a power outlet.
At first, I thought my battery was just going belly up, so I borrowed the battery from my brother's Mini 9 and it showed the same thing. He even put my battery in his system and it showed much more than 20 minutes, which meant that the battery is probably okay.
At that point, I thought Windows 7 might be misreading the battery. So, I put the SSD
my Mini 9 came with back in, which still has Ubuntu installed on it, and it still showed ~20 minutes. I have that BatteryBar program installed on Windows that shows how much time your battery has before it needs a charge, and that says my battery wear is at 94%, which I assume is really bad.
I also tried upgrading the BIOS to A06 (I think that's the latest?) and obviously, it did nothing. I thought about possibly downgrading the BIOS, but it seems like all the programs to do that are unavailable. Plus, I don't want that to screw up and brick my system.
At this point, I have no clue what the problem could even remotely be. I have heard of the batteries on the Mini 9's being kinda finicky, but this sounds more like an internal problem than anything.