To me it just seemed like my battery life was shorter than expected (~8 hrs or so). I guess I was just underestimating how much power my earphones (pfe111) would suck up - plus the odd hi-res, turning screen on, etc. I ran a benchmark similar to the advice Fiio gave (flac on repeat, this time using my gr07be) and I got something like 12-14hrs before it shut down, so it seems like everything is all fine and good. I might still end up getting a battery as a spare just in case (it is only $15 USD after all).
You emailed Fiio about battery replacement for the X3 and they sent you instructions on how to do it I think you said in a previous post. you said it seemed as easy as using a screwdriver. So there would be no soldering or anything like that? Just removing the enclosure and replacing the battery? I guess you'd need a tiny screwdriver too. I've had my X3 for a year now and I can't tell if the battery life is shorter or not but I might want to get a spare too if it's easy to replace. Either that or a spare X3.