So I just got this today and while it was working fine, I went to replace the hard drive with a different one. When I finished putting it in, I noticed the laptop wouldn't boot up from the battery anymore. When plugging it in, it booted up fine, but the battery still wasn't detected.
I confirmed the battery is in fact plugged in and I tried using that pinhole on the bottom to see if that would do anything (it didn't). I've tried the pinhole at varying durations, with and without the battery connected, and with and without the AC adapter connected. I also tried a BIOS update, which didn't do anything.
I've found I'm in a similar situation to this post:
http://community.acer.com/t5/Ultra-Thin/Aspire-M5-581T-6405-Battery-Not-Detected/m-p/97515#M3863
with the only difference is I had no intention on doing anything to my battery, so I don't know why it isn't working. It was approximately at 40% charge before I decided to replace the hard drive.