Also had this problem on an 8 GB Tesco Moto G.
I was playing around with the USB cables yesterday after having problems with XP SP3 recognizing the device and suudenly the phone was displaying 0%. Left it on charge for 10 minutes and it was back to 100%! Really strange.
Also, don't seem to be getting much life out of the battery today. I let it discharge to 10% then re-charged it with a mains charger. Battery level droped 20% after an hour or so just playing Candy Crush for about 30 minutes.
---------- Post added at 03:49 PM ---------- Previous post was at 03:43 PM ----------
Hi, not sure if this helps, but I also had problems with XP SP3 recognising the Moto G. I'd already got the 'Motorola Device Manager' installed but no sign of it in 'My Computer'.
It turned out that it was because I was using the default Windows Media Player. As soon as I updated it to Media Player 11 and restarted XP, it now recognises the phone as an XT1032 and I could then transfer to and from the phone with no problems.
Presumably WMP installs some updated USB drivers or whatever?
Hope this helps.