never flash N900 with empty battery .. even if said battery is new!

you need to charge that battery somewhere else (universal charger, Nokia 5800, etc.)

also, N900 will not draw a lot of charge current on charger that does not follow the specification to the letter, i.e. a charger needs to have a short on the D- and D+ pins. If it doesn't have that, no matter the rating, N900 will draw less than 500mA on it