I normally have my phone charging during the night so that it is fully charged when i wake up.
But the problem I have is first that when I look at the battery's status with Battery Eye I see that is is only charged to around 92% and after it is fully charged it declines gradually EVEN THOUGH it is still connected to the charger.

Is my Battery defect or does it stop charging after fully charged?

And must say that my Battery time is REALLY bad. I need to charge it twice a day when I hhardly use it.
Maybe it is defect!

No, all N900s do this, I won't go around spreading false stuff but it's either:
1. Made not to charge over this, since that might damage the battery.
2. lshal/bme might not be updating the current real quick, so it keeps reporting 92-94% till it starts decreasing.

Either way, it's not a problem with your N900.

Thanks for the fast reply. Is there any way to get past this, the battery being fully charged and declining thing?
Is there already a thread for this? Haven't found one...

By the way, when the phone is fully charged and indicates GREEN, it cuts off the circuit and starts to discharge even though it is still connected, right?

exactly.

li-ion and trickle charging isn't very long lasting combination.

yes, but trust me you don't want to do it

Wow this is new for me. Is it true for all Li-ion batteries?

In this case, what is the usual loss in charge when you plug-in the phone at say 00:00 and disconnect it at 08:00???

There is a cut off after it's fully charge and it won't re-charge until the battery is below certain level (most probably below 85%).

If I'm not wrong, this is actually to protect from over-charging your battery.

As with Li-Ion and Li-Poly technology, you should always keep your battery charged, never let it drain completely as compared to older battery types. If you let the battery drain completely, you risk damaging the battery or shorten the lifespan of the battery.

i learned this the hard way with my Motorola MPX200, I left till the phone turn off itself almost everytime before re-charge and the battery died on me right after 6 months, which is the warranty period for the battery. After that, did some research and found out that I shouldn't have drain the battery.

Frequent usage and top up will make the battery more efficient too. This is also from my experience with my PSP when I played multiplayer games with my friend.Both of us have full bar or at least 90% battery, and his PSP died half way while my PSP still have 2 bar left. After some talk, found out he always drained off his PSP battery.
You can also read some other examples from the web, someone did a test on N95 with different type of charging cycle.

If you charge the phone overnight, turning off your Internet connection (WiFi or 2G/3G) and switching to GSM from 3G will make the loss of battery charge negligible. Remember, your N900 can go almost for two days on total standby so a few hours doesn't mean much. The CPU downclocks itself a lot, the screen is off. Well, at least, that's my experience.

And if you use (provided it works with your N900) the ulv kernel the loss is even smaller as it will give you lower voltage which uses even less power at minimum clock speeds.

if the battery is fully charged, it still consumes power if left plugged.

normally, n900 chargers that i've used charges the battery for 2 - 2.5 hours

in USB charging, it can take up to 4 hrs, if you sleep when charging, i suggest charging via USB or a charger that has a long period of charging time.

i have a wall battery charger for BL-5J that charges for 5 hours, i always left it overnight and the battery charge is constant at 97%