Brand new replacement battery for Samsung Galaxy S2 dies fast?

I got a new battery for my Samsung Galaxy S2 because the one I had before had a life span of 2 hours. When I first put the battery in (when I took it straight out of the package a week ago), it started at 50%, then went up to 52% without charge, then it began to act normal. Well, I've had this battery for a week now. Yesterday it lasted about 10 hours. Today, it began draining just as fast as my old battery, but it lasted for 4 hours today. It went all the way down to 18%, then it died without warning. No vibration or turning off screen or anything. I noticed that the light in the screen was flickering before it died. I don't know if it's the phone or the battery, or what. And just so you know, I got this battery from my dad, so I've only had this phone since February of this year and my dad has had it for about a year. I added a few apps to the phone, but I only use Facebook, Facebook Messenger, and a few other apps. I constantly clear all applications that are running in the background.

Yesterday it would go down 10% every hour.
Today it went down about 1-2% every minute or so, just like my previous battery.

What I have tried:
I took out the battery for about 30 seconds, put it back in, but it didn't turn on.
I took out the battery and held down the power button for a minute. I put the battery back in, but it still wouldn't turn on.

Battery:
Replacement For i777
3.7 VDC 1800mAh
Rechargeable Lithium-ion Battery
Li-ion 3.7V

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