Why does charging a mobile slows down towards the end?

I believe you are referring to charging a mobile phone. If this is the case, mobile phones change their charging technique at something like when they are 90% full. Instead of being in full charging mode, they switch to trickle charging mode. This is an effective means to charge a battery over a long period of time without hurting the battery- i.e. if you accidentally leave your phone charging all weekend, instead of the required 2 hours.