I will live xD It would be hard to find one with a working battery nowadays anyway right? They are very old.
Sorry for posting about it again.
You are not missing out on too much. You wont be able to grow berries, tides in the shoal cave. Also the lottery wont be working, and there will never be Roof sales with the stuff for secret bases. Also, unless the day when the battery died is the day Mirage island shows up, which is not likely, you'll never see Mirage island as well, the same goes for the rooftop sales. etc. You can find more on google :)
The battery you need is CR1616 for Ruby/Sapphire/Emerald. I believe they aren't hard to get, but it can be tricky to replace it. You have youtube tutorials so check them out. I changed batteries on my Silver and Crystal, it's not as hard as it might look at first. But you have to be careful of course.
To sum up, my Sapphire's battery died as well, but I hope that now when I get Emerald, the game will be still working, I need those items from the Roof sales, and I really like the berries :D
I finished the game pretty much. Done with E4 and Sevii islands.
I'll train Scyther (94) to lvl 100 and will let the day care train Rhydon to 100. I'll try to fill up my pokedex a bit more too. But that is pretty much it.
Anyone else active in FR/LG?