Instructables
Picture of Replace an Apple IIGS clock battery
If you use an Apple ][ you've probably noticed that the clock setting and your Control Panel settings are lost whenever the power is turned off. That's because these settings are stored on a volatile SRAM chip, which requires power to retain data. When the computer is turned off this power is normally supplied by a lithium battery on the motherboard. The battery lasts a relatively long time, as the drain is small–on the order of microamps (µA) or less–but nevertheless, it does run down eventually, and after so many years it's more likely than not to be completely dead. This Instructable will show you how to replace it.

---IMPORTANT NOTE: This procedure is much more complicated than replacing the battery in a modern system. Please read through the entire Instructable before starting. Also, this repair is designed to meet the original factory specification for this computer. If you just want to change the battery–and don't mind slightly changing the appearance of a rarely-seen area inside the computer–then there are much simpler options. See "Alternatives" at the end of this Instructable.
 
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Step 1: Supplies

–Your Apple IIGS (duh…)
–Replacement battery, 1/2AA, 3.6V, axial leads. (Tadiran TL-5101/P)–Digi-Key, www.digikey.com, P/N 439-1003-ND
–Soldering iron, 20-40 watts, fine tip–Radio Shack
–Solder, 60/40 tin-lead–Radio Shack
–Desoldering bulb, wick, or pump–Radio Shack
–Screwdrivers

NOTE: DON'T substitute other battery types. Most seemingly similar batteries (even with the same nominal voltage) won't work properly, may damage your computer and could even pose a fire hazard.

Step 2: Getting Started

Picture of Getting Started
Shut down the computer, and disconnect all external cables (power, peripherals, etc.) If you use the internal game port or legacy interface cards this will mean opening the top. Place the unit on a table where you can work comfortably, and make sure you have good light.

The components are very sensitive to static electricity, so use an antistatic wrist strap or floor mat connected to a good ground.

Gather all your tools and materials.