I upgraded it to KitKat a month or two ago (the official update).
Within the last week it has started just losing power when the phone is still reporting 30%+ left on the battery. It does normally seem to die when I'm doing something, rather than when it's just on standby in my pocket. When the screen comes back on after plugging it into the charger it shows the same 30% or 40%.
I have tried a factory reset (albeit using LG backup to restore everything afterwards) and it doesn't seem to have helped.
Does anyone have any information on what might be causing this, and/or how to fix it?