Battery or phone problem?

My phone will start charging then think that I unplugged charger about 7-8 times. then it will proceed to charge normal when powered off. When I unplug it it wont fully boot it will bootloop but when plugged in it boots fine. When its on it usually says the battery is at 0% although the battery was at 100% before boot. When I unplug charger it shuts off then bootloops. It does this on another phone but it has a bad charging port so I have no way to see if it will boot fully. I dont have another battery at the moment but just want to make sure it is the battery before I buy a new one, Thanks!