My battery wont charge all the way

My iphone 5 refuses to charge to 100 percent this happened just recently and connecting to the charger mid percentage it wont raise very high and hits a random number then stops increasing and i ran it down to completely dead then let it charge for quite sometime and it ususally stops and stays somewhere at around 66 percent and i want to know what the fix is if i need to take it apart myself or purchase some equipment or something any help would be appreciated please and thank you