Can i replace/add a cellphone battery to my mp4player?
 

Cellphone batteries have the same voltage (3.7v) as the ones on mp4players. so no worries there. Cellphone batteries also have 800 mAH and higher.

The only thing keeping me from adding the battery is the thought of charging the battery. Will the mp4player charge then Cellphone battery without problems?

This is the one i plan to use:
http://www.e-mobilecentre.com/images/Nokia%20BLC-5C.jpg

yes u can :d
batterie MP4 is BL-4C and the batterie of Nokia for exemple is Bl-5C but the two battries have the same voltage is 3.7 so nothing dangerous ;)
i'm using MP4 player with cell phone Batterie :)

Did you remove the old battery or just tapped the New one with the old one?

My limited electrical knowledge tells me to remove the old one becuase it only has 240+ mAh. while the cell battery has over 800mAh. So i think it would be bad to use both...

Are my assumptions correct?