6.5V * 1.0A = 6.5W
5.0V * 1.2A = 6.0W

The N800 battery will be pretty worn down by now. My own N800 battery's capaity is about 700mAh today. My N810's original battery is at about 600mAh, and the replacement original Nokia battery had been on the store shelf so long it only gives 900 mAh.

I use the charger of Jawbone, 550mA, it charges the phone to full.

2 other USB AC converters I have seem don't have the pins shorted correctly to charge the N900.

BTW the actual charging voltage will never be more than around 4.20 volts.

Correct, assuming you did mean voltage at the battery terminals.

I'm pretty sure the N900 does have internal switching step down regulator to charge the battery. By doing this it can operate over wider input voltage range and also minimise power loss (=heat generation) inside the device. Typically step down buck converters are 90-95% efficient.

As shadowjk points out, you really should look at the power source Wattage (U*I) to determine how fast the charger can charge N900 battery. This works up to a certain point where there is internal limits in N900 that will limit the charge current to prevent damage to the battery and battery charging circuitry.

Thank you all for your informative answers.

Waiting for an OEM car charger (it better work) because this one didn't:

I am rarely in a soldering mood so I will forget the usb approach.

When the OEM arrives I will connect it to a Solar Panel that worked with my E71 when connected to its car charger. It is a cheap small one designed for conditioning the batteries of RVs and tractors.

Yes, at the terminals. I thought it worth mentioning because it may not be entirely clear for everyone, and in particular I believe I saw a picture of an "ad-hoc" emergency charger someone made by connecting 5V directly to the battery.. very, very dangerous. At best it will engage the built-in overvoltage protector in the battery (unless you got a fake battery without said protector..).

Just for the record, I have a 300mA charger in my office (originally came with a Nokia BT headset) and am using it through the power->USB converter that came with the N900. If I use the N900, it's just depleting the battery slower, but still, better to tricklecharge than have an empty battery on the way home

I am charging my n900 with samsung charger 700mA. charges to full within 5 hours or so. (Who cares, i'm sleeping while that happens).