the Supercard series does not have battery backed up RAM. For NDS saves it simply saves the file to the memory card. No harm no foul. For GBA games though, you must manually press a button combination to enter "save mode" and save to ensure your game data is recorded (aka you need to save your data in game, then enter save mode and save for the supercard to write your .sav onto the card). Boy is that a mouthful!
With the M3 series, there is battery backed RAM that automatically keeps the data in its RAM until you use it again. This means there is less trouble for GBA games as it records all the data as soon as you restart your ds. However this means it has to do this for NDS games too. Also, after the battery dies (which probably will take years), in order to save you need to quickly turn off your NDS and then restart it hoping the RAM doesn't lose any data for it to save.