1. Two days from 100% to 40%? Or from 100% to almost 0% (which you NEVER do to a lithium battery unless you like paying to have batteries replaced).
2. The reason for the format is that sometimes a card can "loop" - the middle of a file can point to the beginning of the file, making Media Scanner constantly scan that file (until the battery dies or the world ends). That uses a LOT of battery. Reformatting the card gets rid of all files, so you start comparatively fresh. (Some problems, though, are hardware - so reformatting the card won't help. If it's a SanDisk card, then have live tech support at
SanDisk microSD Tech Support, and they'll replace the card. Samsung probably will too - but it's not as easy. Other vendors [not manufacturers - only Samsung and SanDisk make the chips] may or may not [usually not] replace the card.)
If you copy a looped file, the copy may hang, or it may complete with that file not really copied. (And there's nothing you can do but go back to your backup - which, of course, you have - right?)