So I' ve got problem now. I deinstalled all apps that can softwarely cause the problem like Advanced power mangement before. So when I plugged my n900 to charger after a trip it got to the boot loop. I mean it booted to the charger mode, diode started to blink normally and again several times. Then I felt asleep. Next morning I woke up and it didn't turn up. I tried to plug it again but this time it didn't boot. The yellow diode was blinking shortly and it now happens always when I plug i to the charger and after few hours blinking stops and I still can't start n900. Any ideas?

Get a fully charged battery from a friend (5800 has BL-5J too) and reflash your phone.

Can I use smaller fitting battery?

No, BL-5J battery only, or a custom made one for N900.

I believe that the quick blinking means the battery has no juice left at all, so if it continues to do that then maybe your charger is faulty.
Try charging from a computer.

I reflashed. Even bought new battery. I discharged it thinking that everything will be ok. But now again it starts to blink normally then boot starts and again... what's going on please tell me!

Your device is trying to boot itself up after it receives some external source of power but because your device is semi-borked (you noted bootloops) this is the fundamental cause. You will need to plug your N900 into the computer and do a reflash of your device.

Each time your device reboots due to errors somewhere critical within the filesystem/kernel, your device is not getting charged enough and is rapidly draining the battery.

I would advise that the best idea is to reflash the device as soon as you can whilst your battery still holds enough charge. There would be no point in plugging N900 into a power point.

http://talk.maemo.org/showthread.php?t=73308

Here are some of the symptoms i experienced, seem pretty similar to yours.
There is lots of suggested causes and solutions in that thread.