Last Updated: March 13, 2021
Many older Nintendo and Sega cartridge-based games had internal batteries which were used for storing saved data. These games are 25+ years old and as they continue to get older, the original batteries they came with are starting to die and fail to hold save data.
Buying replacement batteries in bulk is cost efficient and can bring the cost down to around 25 cents per battery. That is a small price to greatly increase the value of a game. An item listed as "new battery" will have an icon to identify it as such, and is significantly more likely to sell compared to listings without a confirmed new battery.