Why is my iPhone 4 dying at 25-30%?

Ok so I have an iPhone 4 and it's not really like an iPhone with a warranty or apple care or anything like that. It's bought from the street. So like. I just use it as an iPod touch I guess: P but ok I just got my screen repaired from a damaging and when I got the phone back I noticed that it has been shutting down at like 25-30% like as it would if it was at 0%. I tried resetting it by holding the power button and then turning it back on but when I do that, the battery charge display shows up as it would if it was completely dead at 0%. I didn't have this issue when I had my other screen that I broke. Any ideas?

I think you should also replace the battery too but at only 25-30% it shuts off the battery is draining but if you don't want to spend to much money since you just replaced the screen then should charge it more often i'm pretty sure you got the idea or buy a small portable charger i always carry it wherever i go it comes in handy!

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