When to charge the batteries?

[Yellow Elephant]

I've heard of pp saying if u charge ur batteries when it's fully drained, the lifespan of the battery will deteriorate very quickly (I.e. Efficiency drops). You should charge it when it's ~30-40%?
Myth or fact?

 

[Octarine]

Myth, imprecise and generalized. First: which battery type? Deep discharge of Li-Ion will destroy the battery, period. Charge when empty (battery symbol) or whenever needed (e.g. preparing for next day even if it still has charge). Older NiCd batteries needed the discharge to remove memory effect. NiMH batteries don't need anything like that. Recent chargers recognize the battery type.