I just popped my old copy of Final Fantasy that I bought brand new in 1990 in my Retron and it still has a valid save game!
I'm just impressed that a 26 year old game still have a working battery (remember, this was before the discovery of flash memory...).
The same isn't true for my 1986 Legend of Zelda, which lost its saved data, unfortunately.
The save data isn't the problem for older games, but the battery that maintains them. Games that didn't use them (like Final Fantasy, for instance) the save will last forever, while ones like Pokemon (which did in 1st and 2nd Gen) die in ~12 years or so.
I assume that the battery can't even be replaced without the savedata disappearing (unless you go to extraordinary lengths to maintain current to the chip while replacing the battery).
The save data isn't the problem for older games, but the battery that maintains them. Games that didn't use them (like Final Fantasy, for instance) the save will last forever, while ones like Pokemon (which did in 1st and 2nd Gen) die in ~12 years or so.
Final Fantasy uses a 3V primary lithium to retain contents. Current consumption to retain SRAM in these carts is less than the self-discharge rate of a primary lithium cell.
Primary lithiums tend to leak with age. It is not a bad idea to replace the 3V coin cell these used to prevent damage to the game PCB. Lifetime is expected to be 10-20 years, so you are certainly over the shelf life and on the lucky side.
It IS possible to read data out, replace the battery, then load it back in.
With care, it is also possible to attach a 3V PSU while replacing the battery to retain contents.
