Well if you have tested out different cables and you are getting results with the proper nokia cable...
then you've ruled out the cable being a culprit.
use a 32bit linux as was suggested by pichlo...
the battery would only be an issue if after fully charging it drains rapidly upon start up ...and doesn't have enough charge to continue either proper start up or flashing...
meaningngggggg...you need a battery which will hold a properly charge...either a refurbed oem bl5j ...a oem bl5j that has been little used and still has a good charge...a brand new official bl5j (not very common anymore) ...or lastly take your chances with a knockoff.
But ..
unless your batt can't hold a proper significant level of charge...
which doesn't seem to be your problem..(from what little exact info you have given) ...since you haven't said "it shuts down BEFORE having the opportunity to flash..."
I would surmise it isn't your battery.

I've tried another Nokia original USB cable=> same result the flasher waits for the device but no recognition is made.
the battery manages to power up the device in usb mode (no light in the sceen) i would agree that the battery is ok.
I've tried a 32 ubuntu system but still no responding.

and you are
-1 : have the keyboard slider open.
-2 : have the cable at the ready plugged into the pc but not the device yet.
-3 : you are holding down on the "u" key on the keyboard and then proceeding with plugging in ...
-4 : you "should" have the usb icon in the upper right of your device screen

If the above has been done correctly...
then you have therefore done everything right up to this point and eve if at this point the device isn't recognized by the os...
I would hazard it is an issue with the os (did you try the old buntu distro cd that pichlo specifically mentioned or are you using the latest one with Unity?)...
have you tried flashing on a different computers even...
(to rule out the possibility that your problem is n900 device related?...)
in specific as Pichlo has mentioned (that he had to use a variety of computers before finding one that wanted to play nice...the same issue has been noted with other people....for myself I haven't had a prob flashing with the few computer brands I favour as much as had a problem with specific os's in my time...) there can be singularly persnickety os's and computer brands which can make things feel that nothing is going right...

for myself dell, toshiba and ibm and ibm/lenovo and lenovo as brands (pre-2013 ...don't know how the latest are preforming..) always worked for me.
32bit debian or buntu (even the latest lubuntu 32bit works fine..just did a test yesterday to see how the latest would perform...), win xp (win 7 had to be tweaked a bit)
always seemed to perform without a hitch...

I've used ubuntu 12.1
I follow the plunging steps right i'm getting the usb logo (upper right).
I've tried 3 machines so far. but i'will try with others.

odd...
it should be working...
battery isn't any problem...
the device doesn't seem to be the problem...
the device is registering that it is instigating flash mode (usb icon)

it could be the computer in question...

or it still could be the distro ...
lubuntu 32bit latest works fine as I mentioned...since I did a test flash with it yesterday..

or it could be the cable is pooched (...that is still highly possible...)

or it could be the pc usb port (unless you have no probs with the port and rule that out)

I've tried another machine with xp sp3 but same result.

hm...
win xp even...
has to be the cable then...
after assessing all possibles and process of elimination...
it can only be 2 possible things...

1- you are absolutely "sure" the n900 micro usb port is ok? right?

if it is a perfectly ok micro usb port on your n900 ...

2- it has to be "that" specific nokia cable you are using...(it's pooched)...


pertaining specifically to your post "here"
"I brought a new USB cable a samsung one but the phone doesn't react to it.
I'm thinking may be the old nokia one a have is good because he makes the phone go to update mode.
I' suspecting the battery. the one i have is a BL-5J is it the adequate one for the N900?"

1- no...a different cable meant for a different device means that it likely will not work...
BUT...even a generic usb to micro usb should work...but expecting samsung periph's to work with nokia is a huge stretch...
So in reflection ...it really could be the nokia cable you are using since the only other you have tried was meant for a different device completely ...
2- BL-5J is what is meant to be used in your n900...
If it holds a decent charge and can power up the device ...all good.

out of curiosity ...Got bootloops goin on....right?...
have you taken the battery right out of the device for ...say 15 minutes? Battery removal for a few minutes may stop the looping...I say "may"...no guarantee there...it is worth a shot though.

found the problem, the usb port was in a bad condition.
I knew that to late, now the port is broken and it seems to be impossible to solder back

not impossible. but you need to find a really good electronics repair guy. i fixed my two N900, that being my first SMD project. Do not trust a person who uses soldering iron + solder only, no matter how much "experience" they have. Hot air is the best way. If soldering iron is the only option, lots of flux and braid are not optional.

I disagree. Experience and good eyes are more important than tools. I strengthened the USB port and replaced the RTC backup battery on four N900s using only this crude soldering iron.

(I would probably not embark on trying to re-attach a fallen off one with that though. That would require much finer tools. Especialy the eyesight - my eyes are not quite up to that kind of job any more.)