IPhone 4 battery trouble after screen replacement?

I had my iphone 4 screen changed about a week or two ago because i had cracked the original one. I purchased a new screen and had a friend replace everything. The phone has been working fine up until now. Last night it began shutting down like it was dying and not turning back on until i plugged it in. It would stay on for a while then repeat the process. Now, it will not stay on at all unless plugged in. It still shuts down and restarts randomly, even when plugged in. Even when on, it shows constant "Searching…" for serve and "100% battery" when i know the battery isn't fully charged. I have no idea what's going on… Help?

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