First, the Tennokoe Bank's battery *can* go dead, and it does *not* recharge while in use in the Turbo system - so the original, 20-year-old batteries are probably basically dead or untrustworthy.
However, it can be replaced with a new battery of type CR2016.
Note that the battery was not originally considered replaceable, and it is soldered in place rather than a battery holder.
CR2025 and CR2032 could also be used, and are higher-capacity batteries, but are thicker and would make the cartridge bulge even more than it already does. I suggest buying a battery with leads already welded to it, as the leads will accept solder more readily than the battery itself would... and heating the battery directly with a soldering iron could be dangerous. This type was used in the original cartridge as well.
For example:
http://www.batterymart.com/p-rc2016-3v-75m-lithium-battery-solder-tabs.htmlThe plastic cover on the cartridge is held on by super-thin double-sided tape. I once had pictures of the replacement process and may even have made a webpage for it at one time... but I've forgotten where I put them.
OK, and the second thought I had was about the Nanami tool.
I really like it, but I had a few ideas for its improvement - though I likely don't have enough time to implement them myself:
1) Instead of requiring the user to painstakingly transcribe a long data stream, a 2D barcode could be displayed, and scanned by cellphone. Not sure if this could make a file directly, but it could certainly go to a website (including QUERY data) to convert/generate such files for download.
2) Individual games' data should be separable and recombinable. This is not so difficult, since the storage format is pretty well-understood. There are two reasons for doing this: a) so that indivdual game saves could be archived, traded, studied, etc. (like the unlock of Force Gear in Tokimeki Memorial stated above), and b) to reduce the size of data displayed by the barcode, to make it less problematic.