Why do video game cartridges and motherboards need a battery to maintain save data but a simple SD card does not?
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Old-school video game cartridges like those used on the Nintendo used NVRAM as a storage medium. NVRAM can hold a few bytes of information but has the downside that it needs a constant source of power (a battery) to hold it’s data. If the power source is lost, the files on the NVRAM are lost. The solution was to put a common watch battery inside the NES cartridge.
Around the same time EEPROM was invented. These are chips that included a technology called a floating gate, a trick of physics that allows a circuit to store electrons even when the power is disconnected. This allows the circuit to hold it’s data even when the power is disconnected. More importantly EEPROMs were re-programmable meaning you could erase them and upload new data.
EEPROMs were common place but expensive, and they were old school microchips meaning that the prongs or legs on the chip were very easy to snap off so they weren’t suitable for a child’s Video Game Console.
By comparison floppy diskettes were ubiquitous, cheaper, and more practical at the time. Which is why the Famicom (the Japanese NES) had a floppy drive accessory.
Modern SDcards are the evolution of this same technology. The same floating gate technology has been miniaturized and made significantly cheaper. They are also now in the form of a card (essentially a miniature cartridge) which makes them far more practical.
These are what are called Flash Storage, a reference both to the digital cameras they were originally designed for and the process of erasure which involves applying a current to the chips.
If they were to make NES cartridges today, they would use SD technology for saving data. In fact the 3DS cartridge is exactly that.
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