iphone 4 randomly shuts off has to be plugged in

iPhone 4 randomly shuts off, has to be plugged in.
location: modmyi.com - date: July 8, 2012
So up until yesterday, I had a perfectly functional iPhone 4. I happened to drop it, and it decided to shut off. The curious part was that I had to plug it into a charger to get it to reboot despite the fact that it had 90% charge remaining. Now, it commits suicide in the same manner whenever it's not in use. I've concluded that it must be a hardware issue although there is no visible, external damage because I have since restored the phone to 5.1.1 through iTunes. If it's as simple as a battery replacement, I'll be ecstatic, but as of now I'm unsure what I should do since it's no longer under warrenty.
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