On the PCB of the game you will see a little IC labeled "6735" with a lot-trace code underneath. That is the game's real-time clock. Like any digital clock, this chip needs to always be powered-up in order to keep time while the cartridge is sitting about unused. Rather than try to fit an extra battery into the cartridge, the solution chosen was to power the clock chip off the same battery that maintains the save ram. Naturally this clock chip uses more power than the save ram and the battery gets drained much quicker as a result.