I have a couple of old Genesis games with game saves still on them. The batteries have to wear out eventually and I'm actually quite impressed they are still working. I know that the act of removing the battery and replacing it with a new one would wipe the memory but surely there is a way to avoid this memory loss.
Why couldn't the old battery simply be left in place and a new battery installed on top of it? By wiring positive to positive and negative to negative the same 3V electrical input would be maintained and as the old battery wears out the new one will be there to maintain the electrical input. If my old ones have lasted 10+ years, surely a new one would continue to maintain the memory for about as long. It seems the only trick would be to insulate the new battery from making contact with other components on the board but a little electrical tape should do the trick. There is plenty of space inside the typical cart for a small extra battery so such an operation couldn't be too hard.
I'll admit that with a cart which has already lost its memory due to a bad battery removing the old and soldering or clipping in a new one in its place is the best solution but there just has to be a way to maintain existing memory if it's there through a little extra effort. I have a Sega CD RAM cart which has a few useless saves on it so I think I'll get a new battery and see what I can make happen. If the memory is lost there no harm done.
Can I get these lithium batteries at any Battery Patrol store or some similar store? Walgreen's carries a wide variety of button batteries but I don't know if they will have these batteries in a lithium version. I'm pretty sure the lithium nature of these batteries is why they last so long.