What are symptoms that show that iphone battery is getting weaker?

And should be replaced?

The recharge gets much quicker whilst battery life becomes much shorter. Your phone will start losing charge much faster after you unplug it. Towards the end of the life of your phone battery things like playing games/video will become the very bane of your phone as it will die so quick. (Always keep in mind when replacing your Iphone battery that the most humane thing is to take it out back and shoot the old one so it doesn't feel offended or unloved when you get a new one.)

Unreasonably short battery life and rise and lowering in battery percentage allthogh they aren't the best for battery life to begin with.