when i first got the phone, the battery levels acted weird. it'll fluctuate wildly then drop to maybe 3%, then gradually "charge" itself back up magically, all while it is not plugged in. I never thought much of it and was too busy to get it replaced since it occurred only once in a while and didn't really affect the overall battery life (only the battery % readings).
recently, the phone started to restart randomly when on battery power while it shows high % of level. i thought it was a false reading again and the battery was actually depleted but showing a high %. I charged the battery up to 100% (over an hour or so using the stock charger and cable) and the unplugged it. use for a while and the problem happened again, so im quite certain that the battery, while fully charged, has some issue with providing power to the phone. it doesn't happen all the time but often enough to make me worry about having a usable phone when i leave the house.
i don't wanna drop $50+ on a new battery since I may be changing phone soon, so right now i just bring a portable charger out as a temp solution.