Most cartridge-based games that keep save-data use a battery to back it up. As we all know, batteries don't last forever and eventually they will run dry. The big question is when that will happen.
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NES
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In the case with NES games, the battery usually only powers a low-power (or a very-low-power) SRAM chip.
The battery used is in most cases the CR2032, which according to specs can provide about 220mAh at 25 degrees Celsius (which corresponds to approximately 790C). This value is when the voltage the battery provides has depleated to only 2V. Three things should now be noted:
- Very-low-power SRAM chips can usually retain data down to only 1.5V, which means that the battery may provide more charge before your savegame goes.
- The diodes blocking the batteries off during usage has a forward voltage drop-off, typically about 0.7V for most diodes, which means that the gain in charge from the previous note practically doesn't have any effect.
- The voltage-time graph in the battery specs shows that the battery's own drop-off voltage (when depleated) is sudden enough for none of this to matter.
We can then calculate the expected lifetime based on the SRAM specs. Most very-low-power SRAM chips will typically drain 0.3 to 0.4uA in standby mode. Dividing the charge on the drain therefore tells us that the average lifetime of a typically battery-backed NES cartridge is around 70 years (+/- 10) at 25 degrees Celcius. As all battery-backed NES cartridges I have seen still retains data, this seems to still be a valid estimate, and it's nice to know that they may hold data for another half century to come.