OMG!! Those "charger"s are really just power supplies. You are lucky the battery
didn't explode. It is a protected cell (chip inside), but you should not rely on that.

What you want is usually called a "desktop charger" which has a chip to regulate the charging of Li-Ion cells, with both current and voltage limits.
There are plenty of cheap ones on ebay etc. Search on 'E61', which takes the same battery. $6 delivered, on ebay.

http://www.google.com.au/search?hl=e...esktop+charger

But I don't use an external charger, as it means a shutdown and reboot for the N800 to change batteries.