Why can a new battery suddenly drop its charge? 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?
There are three things at play :


IC in the battery that communicates battery parameters to the OS - not a culprit since you changed the battery recently
Hardware issue- unlikely, since you report it happening at random battery percentage values
Drivers in the OS, that read the values from the battery and amongst other things, display the battery percentage. This seems to be the culprit as you can see from the complaints in Lenovo forum. First post goes in length and after detailed investigation points the conclusion that the battery is fine but the problem lies with OS - system not showing the real amount of energy the battery still has, the reported values are much lower than actual leading to the device being shut down for prolonging battery life ( deep discharge is bad for Li Ion batteries)


This problem ideally should have been ruled out after firmware upgrade, but if the upgrade did not address the root cause ( as is evident from multiple complaints), firmware upgrade didn't help. 

I guess you can complain to the company and add your voice in the forum