All GBA and DS cartridges have power and ground pins, and they use up a small amount of power by just sitting in there even if they're not being used. It's been speculated that slot-2 devices turn on the power-sucking RAM at all times when power is flowing through the cartridge. I don't know if this is true or not, or if it applies to the 3in1 as well, but it's an idea.i know that it kills the battery really fast but is that just having it in or using it because i dont really use it but keep it in and my battery seems to die really quick