The real problem is that the N900 really wants 500mA to do any significant charging, preferably up to 1A if possible. Most car converters are cheep and only supply 200-300mA, which is what most devices want.

I did find that the "generic" Sprint micro USB car charger (available at most sprint stored) puts out ~750mA if pushed, but it will whine and get warm doing so. If you can find a charger with specs, make sure it can put out a sustained 5V@500mA and it should work. If you can find one that does up to 1A, even better.

You'd think with most cars providing 12V@5A-15A, converters would be able to hit 5V@1A without thinking twice. But the circuitry to regulate 12V to 5V is often low amperage in nature.