I have a dead battery, dead enough that I can't boot enough to charge it. I do not have enough money for an external battery charger, but I do have a bunch of electronic components (resistors etc.) lying around, and was wondering whether it is possible to rig up a circuit that will charge the battery far enough that the N900 can take over?

you don't have the boot n900 to charge the battery.
It wil also charge when phone is off.
Just put battery in and connect charger, then the orange led will begin blinking, when it's green, the battery is fully charged.

Firstly what kind of power adaptors do you have, like 12V, 5V etc...

You can do it with a resistor but here you want a low voltage to begin with like 5V, so if you have any Mains to 5V, or just an old ATX power supply laying around you can use that

GND/- to - on the battery
and between + from power supply and battery you can put a 3.3Ohm 1/4 watt resistor for 5V input, this will charge with 250mA

If you plug the phone to a power source while off, the phone actualy boots in charge mode. If the battery is dead empty, this doesn't happen anymore and you need an external charger.

For the OP, do you have friends or access to a nokia phone that uses the same battery?

if you want a really really simple charger well , get a usb cable , cut it in half , remove part of the wire cover .

get a multimeter , plug the usb cable into your pc , and measure the short circuit current of the usb port , should be 500mA or less, if your pc turns off , or you get crazy currents , do not proceed .

assuming the current is internally limited (I am 99.99% sure it is) , check the cable polarity , usually red =5v , black=ground , but that isn't standard so check with a multimeter .

connect the +ve cable to the +ve of the battery , connect the -ve of the battery to the multimeter , then the other terminal of the multimeter to the usb cable , set the multimeter to measure the current .

ensure every thing is connected well , and then plug the usb cable into the pc .. check the battery is actually charging by monitoring the current , keep your face away from the battery at all times , and don't place it on a carpet , charge the battery for 5 minutes , then measure it's voltage , it should have increased a bit , repeat till the voltage rises above 3.3 , that should be enough for the battery to be charged inside the phone .


a simple safe charger :

you can make a simple charger maybe a single quadrant chopper , add R sense on the low side, get feed back voltage into a controller to set duty cycle of the chopper to control the charging current .
you will have to program that controller of course..
I don't know what electronics you have , but you can get an old 8-bit micro-controller for almost nothing now , make your own programmer .
this stuff costs little , requires moderate experience (isolating the microcontroller , implementing over charge , over current protection) you can find this easily on Google but building will consume time .

well i think he just wants a simple charging with simple parts

And yes the usb cable is a good idea, but for safety use a 3.3ohm resistor here also, i should work with a direct connection from usb to battery, but can have consequences in worst case scenario, like the usb shutting off and on because it senses to much current, and if it's a "cheap" computer/motherboard that just uses fuses that usb port might stop working.
I have experienced the off/on syndrome before, and this will slow charging down to nothing.

So just to be safe, use a 3.3ohm resistor.

i was honestly under the impression you could leave it plugged into the mains - it coninuously reboot but would charge a bit each time until there was enough and eventually would boot? amiwrong?

im pretty sure mine has got down so low it wouldnt boot - never had to do anything drastic like cutting up usb cables

I have asked around and no-one else has a Nokia phone. I left it plugged in for three days and it didn't get anywhere, so I assume it has discharged past that point.
Which contact is which? The battery has three pins, do I ground the third?

I forgot that your battery is FLAT please use a 15ohm resistor not 3.3ohm

1 pin is + (closest to the "edge")
2 pin is - (GND)
3 pin will not be used for this

From USB cable use RED wire to 1 pin, PLEASE remember to use a resistor or there is a chance of blown USB or no charging because of protection
The BLACK wire goes directly to 2 pin.

NOTE: if you connect a battery with a voltage of 2.5-3.3V directly to 5V it will try to draw in excess of 1A (1000mA), this is why i recommend that you use a resistor, since this will drop it to a safe level.

EDIT: if you want to totally safe use a 27ohm resistor

If the battery is dead empty, it should be considered dead and replaced for safety reasons. However, the hw will attempt to charge it anyway..

If the cell is not completely empty, but in the area between 2.2 and 2.8 volts (2.8 is N900 hw cutoff, 3.2 is shutdown threshold, 3.4...3.5 low warning), then what happens is:

1) Status led shines steady bright orange. Emergency charge in progress.
2) As voltage rises. N900 is gradually powered on. GAIA is activated and makes a decision to boot the CPU into the bootloader NOLO.
3) NOLO takes over charging. The led changes from steady to pulsating.
4) When NOLO thinks there's enough power to boot, it gives up control of charging and boots Linux

Obviously if you ****ed up your install into a reboot-loop, the battery wont get charged.

However, recovery from very low battery is perfectly possible as long as the OS is intact.