Can i charge Nikon D5000 battery while it's 70% charged?

I always wonder if i could be able to do so or not? It's a lithium ion battery and i need the battery to be fully charged and at the same time it's like 70% charged.

Yep. It can be charged.

Lithium ion batteries can be charged at any time. The charger is smart enough so it won't overcharge the battery. There are those that say you should run the battery down to maximize the battery life. However, it's better to have a full charge than to miss out on shots just because you want to extend the full life (as in when you need to replace the battery after a few years of service) of the battery by a few percent.