It may be that your battery is dead. But I don't own one of those M3 Lite Perfects.
I believe the mechanism goes as such:
1) Saves are written into the RAM
2) The save is written into your MicroSD card when you restart your DS.
Saves aren't written into the SD during game-play to maintain good performance.
To keep your save in the RAM 'alive', a battery is necessary to keep the RAM with a current.