There is of course no way to retrieve those lost saves once it's gone, but replacing the battery for a new save is a lot easier than I originally thought.
In this post I will show you the step by step on how I replaced the battery. I must stress that this is not a tutorial post, it's just a post on me following this video tutorial. I did find a lot of tutorials out there while researching, and I feel that this one uses the most simplified way of doing things and it was explained the clearest.
First off, the tools of the trade:
The game cartridge, duct tape, a brand new battery (thankfully Nintendo uses readily available batteries in their carts) and my Gamebit tool makes another guest appearance. I knew I would be using it again soon as it works for a load of consoles and cartridges. Also not pictured is a flat headed screwdriver.
From there I set about removing the dead battery. In theory this was simple enough; use the flat headed screwdriver to separate the battery from the contact tabs which were spot welded together. In the video the guy was popping them like there was no tomorrow. You could tell he had done it before, but it must of took me about 5 minutes per spot weld (there were four all up). I didn't want to apply too much force for fear of cracking the circuit board, so I just kind of chiseled away at them until they broke. Probably not the most practical way of going about it, but at least nothing was broken in the end.
Finally all the spot welds were broken, and the dead battery was free:
On closer inspection I noticed that the negative side of the dead battery was marked "01 01". I can only presume that this means it was installed January 2001, which seems like sort of a short life span when you think about it. I usually see forum posts now about people wanting to replace dead batteries in NES period games and maybe even SNES period, and they're 10-15 years older than this cart! Maybe Nintendo cheaped out on batteries for this game, who knows.
The next step was to install the fresh battery. The idea for this was to physically tape it into the contact tabs. Sounds a bit crude I know, but it actually works a charm. Originally I was just going to solder it in, but after some research I was advised against that as it could make the battery explode! Makes sense really; an extremely hot tip against a battery. Hurrr.
As you can see in the picture above, the new battery is securely fastened in with the duct tape. After that it was just a case of putting the lid back on and screwing the security screw back in with the Gamebit.
Mission successful! The game now holds a save, and many more hours can now be enjoyed out of the game!