I didn’t know this feature existed. I knew that when the phone was on, I’d get notifications that my Lumia would charge more slowly because of a slightly incompatible microsUSB charger (a non Nokia – usually the Apple/iPhone or Palm charger – yes I said Palm).
By using a Nokia charger or the equivalent MicroUSB wall outlet charger, the red flashing light indicator (when the phone doesn’t have enough power to turn on)flashes faster. When using a slower charger (some portable chargers) it would flash more slowly.
I don’t think I’ve ever seen my 1520 dead. 1020 is a different matter.