guys i bought a new battery for my n900 and bought a cheap external charger but before using it i heard it from a friend that its not worth using and can cause problem to the battery can anybody suggest to me how can i charge the extra battery? or which is the appropriate external nokia battery charger??/

P>S> can anybody tell me the link to check if my battery if genuine!?

I use an old Nokia 5230 (aka Nuron) to charge my extra batteries. Very inexpensive phone that you can probably find used on craigslist. Since it uses the same batteries, if you buy a complete phone, you'll have yet another extra battery for your N900.

If the external charger doesn't charge too quickly, which is unlikely since the cheap chargers tend to charge slowly and doesn't try to charge it over 4.2V it won't damage the battery. If you have a multimeter maybe you can check it's output voltage to see if it is ok.