[yard2010]

Agreed. But keep in mind that is not perfect either, some games that use a clock feature depend on a small battery in the cartridge which is depleted after a decade or so

[Kwpolska]

Same goes for saving games on the old systems — that's using a RAM chip kept alive by a battery inside the cartridge.

[Uvix]

Depends on how old. NES and Game Boy were all batteries, but by the N64 and Game Boy Advance they had started to use Flash memory or EEPROMs in many games instead.