Oic, nice to meet you. I asked because I was wondering where could youget a genuine N900 with non-genuine battery.

What you should really worry is when the battery was made by malicious fakers, and they would make them with lowest quality materials possible. If it's the case then not only it'd give incorrect readings, it is dangerous to use as well.

Therefore, I'd suggest you get a genuine Nokia's battery, or OEM like Goki or Mugen.

However, you'd like to note that sometime OEM battery would exhibits similar behavior, such that when it showed 100% it might actually be at pretty low battery level.

Neverthelss, big OEM battery makers usually make batteries with pretty good quality, at least, you're exposed to lesser chance to face the danger of the battery being exploded in your trousers' pocket.