The included N900 "charger" is a power supply only. It outputs 5V. Any "charger" that connects to the MicroUSB port is a power supply. Also "chargers" that connect to 2mm port on older Nokia phones are power supplies.

It is important to never connect power supplies directly to batteries.

As you can see from dr_frost's picture, he has taken the cable from a car charger, and connected it to the output of a Li-Ion charger. A real charger, since you can see it's a cradle designed to connect directly to 18650 Li-Ion batteries.

Again, never ever connect power supplies directly to batteries. It can result in what the industry calls "venting with flame", or what normal people would call "explosion".