It takes a 3.6V Lithium 1/2AA PRAM battery. Unplug the power cable from the CC. Remove the logic board by taking out the two retaining screws on the back panel, then pressing on the tab at the top and pulling the panel out. Grasp the logic board firmly on each side and pull out directly.
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You will see the battery on the logic board. Use a sponger under it to remove it noting which end is positive. Push the new battery into place. Plug it back in and start up. You will need to reset the date and time and reselect any printer that hooked up to it. Most of time these are lost when the battery dies. Happy computing ;-)