Since first writing this FAQ many years ago, battery clips and holders are now widely available. Instead of soldering the battery directly to the board, you can follow the instructions below using a clip instead of the battery itself. Just remove the old battery, solder in the clip, and you can pop the battery into the clip! Not only is this safer than soldering the battery itself directly to the board (and no hacking off clips from the old battery!), but speeds up future battery changes as well. So do a web search for "cr2032 clip" and you should be able to find one pretty easily. Just make sure it has the proper tabs and a flat profile, or you might have difficulty closing up the cart.
The batteries in NES cartridges can last as long as 20 years (Scroll to the bottom of the page for FAQ about batteries.); however, for various reasons, some can conk out early, making it impossible to save your game (and even worse, deleting your old, hard earned progress). When this happens, you have three options: Get a new cart, keep the system on for as long as you're playing a game (which can be several days), or change the battery. The first two options aren't particularly good, as some games can be expensive, and I don't think anyone wants to keep their system on for days. Changing the battery, however, will let you play as normal, without going through much trouble.
Ninteno used to provide this service for 10 dollars, but if you ask them nowadays, they'll just tell you to buy their current system. Gee, thanks Nintendo. Being that I like to tinker with things, I opted to try and change the blasted things myself. Below, I've shown the procedure I use. Now there are several different ways to handle a battery change. However, most sites that offer advice usually provide info on how to jury rig a new battery in using electrical tape. This is precarious, as the tape might not hold over several years and your battery will fall out. Other sites describe methods using wires or paperclips, and these too are risky methods. The way I do it is about as close to a factory job as possible, permanent, and very functional.
Please note that the procedure described below is just my method. It works for me. If you like other methods, that's fine. I'm not telling you that you have to do it this way, I'm just sharing what I do. In other words, if you butcher your cart by trying what I do, don't blame me for your broken cart and cut fingers, especially if you don't follow instructions and advice closely.
Now, onto the procedure.
What you need:
-3.8 mm security screw bit. You can get these on eBay for a few bucks.
-Soldering iron with a thin tip.
-Solder. The thinner the better. (Thinner solder melts faster). I use flux core.
-Safety goggles (You have been warned. Battery acid and eyeballs don't mix.)
-2032 lithium battery. I use CR2032; I cannot vouch for other prefix letters.
Any time you're digging into delicate circuitry, you must be very careful. It's easy to ruin chips. Before you try and fix your gem mint Final Fantasy or Dragon Warrior IV cart, I suggest you go out and buy a shabby Dragon Warrior or Ultima Exodus cart (or any other cheapo with a battery) for a couple bucks, and practice a couple times. The first time I tried this, I ruined the cart. Practice makes perfect. Once you get the hang of it, then you can do your more expensive carts.
I suggested safety goggles because you'll be working with heat and a battery, and those don't go together. If you're not careful, the battery will explode. (Actually, it will just pop, but still. It will send acid flying.)
Also, be very careful with the knife. When trying to pry off the clips, one slip will badly cut (or even sever) your finger. X-Acto knives are dangerous. I've seen several model-making friends make a trip to the emergency room after being careless.
I've placed the important cautions in boldface text. Read and heed!
By choosing to follow this procedure, you agree that I am not at fault for any injury to yourself, your game, or anything else.
Step 1: Open the cartridge using a 3.8 mm security screw bit, and take out the circuit board. The battery is the silver disc shown in the inset.
Step 2: Remove the battery from the board. The battery is held to the board by two clips which are soldered to the board. (You can see these clips in the photo. The soldering iron is pointing to one of them.) Turn over the board and heat the solder until you can open the clips (i.e., bend them away from the board). Be very careful not to hold the soldering iron to the board for very long!
If the board gets too hot, you can ruin the chips. After the clips are bent up, let things cool down.
Step 3: When the board is cool, heat the solder again while gently prying the battery away from the board. When the solder melts, the battery (and the clips) should come away from the board. In the picture, you can see the battery and clip assembly that has been removed from the board. Note the clip alignment.
Step 4: Now you need to remove the clips from the battery. This can be a tricky step, because the clips are spot welded to the battery (see the diagram in the photo. The arrows are pointing to the welds.) A soldering iron will not help remove the clips. You're going to have to pry the clips off. My tool of choice for this is an X-Acto knife. Use an old blade, because it will be ruined.
Step 5: Insert the X-Acto knife between the spot welds and "cut" into the weld. Be very careful! This is very dangerous. I suggest using a vise to hold the battery, or you risk badly cutting your fingers.
The welds should pop off after some struggle.
Step 6: The clips are removed from the battery. Notice the clips are still in good shape. Get your new battery ready. Now you've got to solder the old clips onto the new battery.
Step 7: This is another tricky step. What I do is melt a small amount of solder onto the battery (long clip goes on +), then hold the battery and clip together with a pair of pliers. I remelt the solder while squeezing the pliers. Be careful NOT to heat the battery! It can explode or leak acid, which can cause injury.
Repeat the procedure on the negative side. Make sure the clips are aligned properly.
Step 8: The picture shows the old clips soldered onto the new battery. Now we're ready to put it back into the board.
Step 9: There should still be plenty of solder around the clip holes to work with. Make sure you have the + and - correctly aligned.
Place the tip of one clip over the hole, and melt the solder from the other side. Again, be careful not to heat the board too much.
When the solder melts, the clip should poke through. Bend the clip back into place. You can see in the picture the negative side is back in place. Do the same for the other side.
Step 10: The picture shows the replaced battery assembly. It might not look as pretty as a factory job, but it will work just as well.
Step 11: While you have the board out, you might want to clean the contacts with a pencil eraser. Place the board back into the casing, and screw the cart back together. You're good to go for another 15 years!
Q: How long to batteries last?
A: Well, Nintendo says 5 years, but they're just covering their *****. Lithuim batteries in an NES cart last, on average, 15-20 years.
Q: Does saving often drain the battery faster?
A: Yes and no. Yes, it will drain some juice, but the amount is so incredibly small, it won't matter. You should be able to save over 100,000 times with no significant drainage.
Q: How do batteries work?
A: NES carts with batteries operate analogously to a watch: A watch always "knows" what time it is, and a NES cart "remembers" where you last saved. In essence, a cart with a battery is always "on." The battery provides the energy to keep your save in memory.
Q: Are NES batteries the same as those in Dreamcast VMU's?
A: Yes. But a VMU will drain them in a week because it uses up so much juice. However, even a "dead" VMU should keep your save; you just won't be able to play the VMU games. Darn