[Markulous]

Li-ion, which are what're used predominantly in camera battery packs, have in-built charge level detection and will 'switch off' the charger (turn light green and prevent further charging). This is to prevent overcharging (for which there's a safety issue). I have never seen evidence to suggest it was better or worse to leave a battery 'on charge' for long periods of time

They can be charged from any charge state as they don't suffer 'memory effect' but do not like being totally discharged (again, in-built protection against this) which is why they're supplied half-charged (they also retain their charge if not used, unlike NiMH). However, they have a limited life, whether used or not (2-3 years) from date of manufacture