Battery problem? (Percentage issue)

I have an iPhone 6 bought in 12/2014 and in March this year started to have some problems related to the percentage of the battery when it is near 20/25%, the phone turns off and says "no battery" but if I try to power on, fifth/sixth attempt later, boot with is 20/25% usually, sometimes the battery jumps from 25% to 10% and stuck on that until the battery really reach 10% when I connect the charger the percentage back to their normal, it all started when I upgraded my iPhone from iOS 9.0.2 to 9.2.1, I tried to clean install iOS 9.3.2 on iTunes but the problem continues ... this can be a problem in the battery or is a bug on iOS? I have less than 500 charge cycles.