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i dont want my saves to perish at any time, so i want to make my cart run on a li-po cell (one of those tiny ones used for rc helicopters and the like).
now the li-po cell outputs 3.7v, so ill have to use a 3v regulator for that. charging ciruit boards are also incredibly small (not much bigger than the four soldering points on them).
i only have to figure out two more things:
-how do i charge it?
-do i need a battery meter / at which voltage does the save perish?

the first point is pretty easy, these are some options i can think of:
-use a usb cable
-use a solar panel
-modify the gameboy so the cart plugs into a charging connection when inserted

the last option will probably be the most feasible, but needs some more thinking: do i want the battery to be charged constantly?
the answer is no, because it will damage the battery over time. so you need a small switch to enable/disable charging. youll want the cart to be chargeable even when the gameboy is off, so the charging connection of the gameboy should be directly connected to the batteries.
now if you can charge the cart battery at will youll also want to know when you should charge it. this is why we have to know at which voltage the save goes byebye. i dont have the equipment to test that, so ill welcome anybody who would bother to test that for me.
if we know the critical voltage, we can build a battery led that lights up a little while before the critical voltage so we can react and charge the battery. this should be at least a day, so we dont have to stay alert 24/7.
so the voltage tester could either do a lot of measurements to know when the battery still has about a day of charge, or he calculates the power consume of the gameboy cart and from that we calculate the voltage of the day before the voltage drop.

this is a lot of text and im sure its not all in a logical order, but youre very welcome to help
when the project is finished we could make a simple tutorial for everybody to copy.

and yes, we will probably have to mod our cartridge, because i highly doubt well find enough room in the cart for all the extra parts (namely the battery, the 3v regulator and the battery meter, the rest could be in the gameboy).