I never got into the SNES. My NES is around somewhere. Doesn't work very well anymore though and I had to replace the rubber pads for the controller buttons.
said by C0deZer0:
If I remember right, many of the Nintendo cartridges and even some of the flash carts that enmulate the function of the backup save use a watch-style button cell battery, which can be had pretty cheap. Also, I would think if you were enterprising enough (and skilled enough with a soldering gun), you could theoretically replace the button cell battery with a rechargeable, if you knew there was a way to give it enough current to keep its battery charged. I can't speak for the SNES cartridges, but I know this was mentioned as a solution for the Dreamcast's internal battery (that acted as a CMOS).
Are you referring to the Dreamcast't VMU battery or battery in the console? The Dreamcast's internal battery recharges every time the console runs and I have not noticed any serious degradation for the battery on any of my Dreamcasts. The VMU batteries on the other hand I gave up replacing many years ago. I had a couple batteries I put in only when I needed to use the VMU for something, such as the playing minigame Skies of Arcadia. But they still did not last long. I would be very interested in finding a way to make the VMU run off a rechargeable battery.