I've been using my Lenovo A328 for less than a year, when its battery started suddenly dropping its charge from ~80% to 15%, then even to 3% and less and the phone switched off. I've now replaced the battery, but after several charge/discharge cycles it started to behave very similarly. Here's its typical charge history:
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Since the battery has been replaced very recently, I conclude that it's not the battery's fault. But what else can be the reason? The phone lies in my pocket most of the time, and I occasionally take it to make some calls or use GPS or WiFi. It doesn't seem to be a heavy use, and even if it were, I'd expect battery meter to show a smoother charge curve.
Can it be an OS fault (but I did upgrade firmware to latest version, without any difference)? Or does my device need to be repaired?