If you have a cherished game save on an old Nintendo cartridge & know the battery inside can’t last much longer, then here’s the simple way to add a new battery without losing that old save.
You will need these things:
Safety glasses (as always)
2 pieces of small copper strand wire each around 4" long.
2 new CR2032 batteries (one for temporary power during exchange & one for the actual exchange).
CR2032 battery holder if you don't have one already installed.
Needle nosed pliers to remove original style battery and tabs.
Knife to remove about eighth of an inch of wire insulation on each end of wire.
Electrical tape to secure wires inside cartridge housing
Glue if you want to permanently attach the battery holder inside housing.
Gel pen to write date of when you change the battery inside the housing
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Step 1: Solder the Temporary ( or Permanent ) Wires in Place.
This cartridge I had already installed a CR2032 holder inside so I switched it simply by holding the wires in place with my left hand & then used my right to remove the other. If you have a friend to help it would be easier. Be sure nothing makes contact with each other & be sure it's kept wired in parallel with + to + and - to - to keep 3V because otherwise it would be 6V and probably destroy the game altogether. I had sunbeams in it originally from the dollar tree but they had an exp date of 2018 and they don't last near a long in my wii fit meter as the one it came with so I chose to go to walmart and pickup a couple of energizers (one for zelda II, one for zelda ) and they have a use by date of 3-2024 so I expect them to last a very long time. They only costed $4.37 plus tax so decent price too. If your cartridge still has the original soldered in battery then just make the wires you soldered on opposite side of board from it permanent & solder a battery holder in place & go ahead and install your new battery & then either cut or use pliers to get the old one out & break the tabs off. This leaves the back side totally intact & there's plenty of room above in the nes cartridge to install the holder. I also have a holder on a zelda cart that is soldered directly to the board which I will soon be posting an instructable of but this method is a lot simpler & just as reliable.
Step 2: Saves Should Last Many Many Years.
Here you can see I have my energizer installed & save is still intact. I recommend using a gel pen to write the date of the new battery install. If you want to get a bit more obsessed with never losing that old game save then you could always wire 2 CR2032 holders inside & that way you could switch the battery out easier anything you wanted. This would be particularly good idea if you use cheap batteries and want to switch them out once ever year or so just to be on safe side.