Iphone5s battery runs out with very charge quantities (with 1% or more than 15%)

Hello,

I have an Iphone5S 32Gb running latest iOS version (8.3).

Since version 8.1 I have been experiencing strange behaviours in the Battery.

Sometimes it runs until it is on 1% and even with 1% takes more than 2 hours to run out (without using it) or even 15-30 minutes when using it with 1%.

Other times it goes of with more than 15%. When this happens I put the charger and inmediately starts the system with same percentage (>15%) as before.

Under these circunstances sometimes I can keep using it for a while. I have realised that specially takes longer to run out when only wifi is connected (Sim Blocked after restart).

This would be fine if the phone would do always the same, but it is really anoying not knowing when is it going to run out.

I am quite maniatic and always wait until the phone goes off to charge it again. I would not like to have to start charging it every night (I think this is not good for the battery) but seems mine is not working anymore as it should.

Does anyone have this same problem? Is there any solution for this?