Hello, so, I found a not-very-new-but-original BL-5J battery. It solved the not-booting-on-charger problem I had before, but it created a much more minor one:
When I plug in a charger, ANY batterymeter shows the same percentage as it was before plugging the charger in UNTIL the battery charges completely, when everything goes back to normal. Is this solvable?
P.S. can you use a BL-4U battery from a Nokia 500 in the N900?

That is an expected behaviour. The N900 is about the only phone I know of that does not show the battery charge kevel while charging.

Really? The previous battery I had showed everything perfectly!
BTW, when it's full, it sometimes displays around 45 instead of 100% (no impact on batterz life)

That happens with mine too. I was worried too. But now I thinks that is normal. Though I do sometimes run into problems. For instance, I am watching a video and the battery was 12%. After 5 mins, 0.5%. And many times, I have reached home safely after 0.5-1 hrs with 0.5% battery. :P