When the PSU is plugged into the DS, there is ample power there to fully power the DS as well as provide a bit extra to the charge circuits if the battery needs charging. If the light goes out, and the cord is still plugged in - its being powered by the cord wrather than the battery (so essentially there should be 0 draw from the battery).But I'm sure someone else will give his idea about this that might actually be based on facts.
Nope.. can't be done since the DS has no battery messuring device