Learn to diagnose the condition of save batteries
There are two ways to check whether a game has a completely flat or very low save battery.
The easiest way is to validate the presence of saved games. Otherwise, play and go to a save point in the game, save your game, remove the cartridge for a few minutes from the console and test it again. It's important to remove the game from the console for a few moments to perform this test, since a console that is turned off sometimes retains a sufficient electrical charge in the circuit to maintain a save file. For some games, this technique can be quick, while for others it will be long and tedious. Also, not all games use a battery to maintain saves.
The best way is to get a voltmeter or multimeter to test the voltage of the internal battery of your games. A new battery full of energy should show you between 3.1 and 3.3 V. I recommend changing it when it's under 2.5 V, because at this point, the integrity of the battery is at stake and it risks weakening faster when playing. With this last method, you can at the same time inspect the condition of the electronic components inside your game cartridges... Remember that a battery that hasn't been used for a very long time or that has been improperly stored may leak and damage the electronic circuit board (PCB) of a game.
What is the expiration date of save batteries?
Let's face it, the batteries back to time were really good qualities and sometimes still work very well, even after 20 to 30 years of use. This makes it really hard to know the perfect time to change the batteries in old game cartridges. On average, these batteries are well past their expiration date, but as long as they're still holding around 3V of energy, there's not much point in replacing them.
As a general rule, popular games that were intensively used back then may need a new battery today: Pokémon games on Game Boy, Game Boy Color and Game Boy Advance, Super Mario or Donkey Kong Country games on Super Nintendo, Legend of Zelda games on Nintendo (NES) and NHL games on SEGA Genesis.
Introduction to electronic component soldering and desoldering
It's time to roll up your sleeves and learn how to do your first electronic soldering. Before doing battery replacement in your Nintendo and SEGA games, it's important to learn a few theoretical notions for your safety and the durability of your equipment.
Equipment: You will need a soldering iron (pencil type preferred), soldering flux (liquid or paste), desoldering braid or pump and soldering wire (mixture of tin and lead / acid-free / with resin or integrated flux). There is no need to have a $300 or more soldering station; a small set from $30 to $50 will be perfect for small jobs that don't require precision.
Caution: All metal parts of a soldering iron become extremely hot when in use. The fumes released during welding are toxic and may be harmful to your health (presence of certain metals such as lead in the welding wire).
Precaution: Wear safety glasses or a face shield to protect your eyes from metal spatter, dust or welding fumes. Always wash your hands before and after carrying out welding work. Work on a clean surface and in a well aired or ventilated area.
Soldering iron use and care: Always melt some tin on the tip of your soldering iron for better heat distribution. Don't leave your iron on without using it for more than 15 minutes. Clean the tip of your iron after each use with a wet sponge or wire-type cleaner (brass wool).
For a nice and good weld: Make sure the iron is hot enough. Never blow on your solder to make it cool faster. Heat the metal components to be soldered and then apply the tin. Don't heat the electronic components for too long, otherwise you could damage them.
Best technique to replace save batteries in game cartridges
WARNING: The following steps will delete your game saves if you still have any. There are ways to not lose your savefiles, but they're a bit complex and that's not the purpose of this tutorial.
BATTERY CHOICE: I suggest you use brand new batteries with "legs" since the surface of a battery is very difficult to solder.
- Open your retro game "carts" with the proper tools;
- Inspect and test the save battery voltage (if you have a multimeter);
- Clean and inspect the game printed circuit board (PCB);
- Unsolder the old save battery;
- Apply a little of flux to the surface to be desoldered (optional/recommended);
- Add some tin to the tip of your iron;
- Lightly press the tip onto the solder to be removed. Let the tin melt and lift each leg of the battery with pliers;
- Remove remaining tin with a desoldering pump or braid;
- Clean the surface with rubbing alcohol (90% and above) to remove flux or other residues.
- Solder the new save battery;
- Apply a little of flux to the surface to be desoldered (optional/recommended);
- Add some tin to the tip of your iron;
- Lightly press the tip on the components to be soldered (the legs of the battery and the metal parts of the printed circuit). Touch the solder point with the tin wire and allow enough of it to melt;
- Pull lightly on the battery to test the strength of your welds;
- Clean the surface with rubbing alcohol (90% and above) to remove flux or other residues.
- Let cool and/or dry before closing your retro game "carts";
- Test your game.
Technique to avoid to replace save batteries in game cartridges
Over the years, I've seen several unusual ways to replace a save battery on used Game Boy, Super Nintendo or SEGA Genesis games... Some people use tape or gum to hold a new battery instead of welding it. This tip may work and help out in the short term; on the other hand, if you drop your "carts" and the contact between the battery and the printed circuit breaks, you risk losing all your savefiles...
Last word
As you will probably have to change the batteries again in 10-15 years, it can be advantageous to install battery holders in your Nintendo or SEGA cartridges. Unfortunately, this won't be possible in Game Boy cartridges given the limited space.