I have been using the ac charger to power up the phone but after a few times of noticing the battery always low I caught what was happening, after plugging in the charger it would say charging the battery and a few seconds after "disconnect from charger to save power" and thus stop charging...battery was at 1/4.

What works for me now if I just use the usb connected to a pc or laptop it starts charging immediately. AC charger gone bad?

is the ac charger covered under warranty? i mean, we are talking about a charger which is 1 day old!
Your problem seems to be a charger-phone connection issue, somehow, due to voltage difference or anything, your phone doesn't accept the chargers output or sth...
I mean mine can be the same, but WHY?
WHY?
I mean, what kind of time do these people spend before marketing a 500$ device...
This is an outrageous mistake.
Lets gather up here, and obtain data about what is really happening and write to nokia care center at the same time.

I have some news for you guys.
So, the N900 does not charge on A/C but USB,
I called Nokia, he told me I can ask for a replacement from newegg.
But I said no, They will call me tomorrow and send me a new charger...
I will let you know how it goes, it was pretty quick...

I thought that to be strange since I have been exclusively using the ac charger since I got it to charge the phone, just the past week its no longer reliable. Ive tried charging with the phone completely off which I thought worked for 1 day, but after not any change seeing it at last red bar after it was plugged to AC with the phone off a whole day. So like you I am relying on the USB (plugged into a PC)to charge it. *sigh* just making past a month of ownership and this just annoying.

I had the same problem. Put my phone onto charge the first night I got it. Woke up in morning and it was completely dead.
After a brief panic I plugged it into car charger where it charged fine.
Went back to the Nokia store, they tested the charger, said it wasn't working and immediateley gave me a new one. Has charged fine ever since

Yeah, same problems here.

Plugged into the wall socket - takes around 12-14 hours to charge. Plugged into the computer, takes much shorter, though I haven't left it in long enough to make a full charge.

Sounds like the wall charger's faulty then - it should take half the time to charge via the wall socket than via the computer.

Same issue
purchased new n900 from newegg
out of the box had a charge didnt realize it was not charging from wall adapters i had it connected to usb cable

Got low battery plugged in and its not charging
I have a 2nd n900 and connected it to the cord
cord is fine to charge other n900

Then for giggles plugged it in to usb and it charges form the usb

interesting enough It came with a USB 12v mobile adapter as a combo from newegg this will NOT charge the phone from USB
either one of my phones says insufficient power but its in a cigarette lighter socket with at least 12volts maybe 13.5volts

I did play around with this newest phone and the wall adapter and got it to come on once by wiggling the micro usb a little bit
I have an MRA for replacement so did not want to toast the brand new phone under warranty but there appears to be some issues with perhaps a short or cold solder joint ion the connecter anyone take one of these apart send me a pic of the connection for reference

I think that the N900 has a flaw in the micro usb port but on the newest phone the older n900 (7 months old) works great still ?

Phone did transfer all data and contacts etc on mass storage so the usb connection has to good you'd think ?

For the record this new N900 is completely stock no apps no tweaks no upgrades prior to noticing the charging issue
The only added was contact VCF file and a memory card with
files from old phone upon it

Troubleshot to the DEVICE itself
It is not the wall adapter cord
It is NOT the battery (swapped em same issues)
It is not the memory removed
It is not the Sim (Swapped em same issue)

Process of elimination...........the phone' left !
works on USB charges and data transfers
Fails on known good/tested wall charger's several tried

Jim

so the battery is the problem

Its not your charger Its the phone !
Got a phone like that also form newegg out of the box same issue wont charge from wall adapter
have other n900 swapped adapters around bad phone wont charge
on either good phone charges on both adapters
swapped batteries too for giggles still same issue its in the phone somehow ......I have replacement coming form newegg for this
hope its not the same