This article explains specifically changing the CMOS battery for Dell Precision 3431 desktops, but Dell desktops of similar model numbers will be very similar.
Power off the machine, disconnect all cables.
Remember that some USB devices, such as printers, are picky about which USB port they are in, so make sure to label which USB cable came from which port, to avoid issues.
2. Locate the latch on the rear of the machine, and using (most easily) fingers and thumb of left hand, slide the latch toward the finger well shown in the picture. Fingers will be pulling directly away from the back of the machine, in line with the sides of the machine, so that the entire side slides back about an inch or so, and can then be lifted off. Note that there are tabs and notches along the edges of the frame and the inside edges of the side that comes off. These will need to be realigned to put the side back on later.
3. Locate the CMOS battery, if possible. It is a coin-battery, a little bigger than a nickel. In the case of a Dell Precision 3431. It is underneath the additional video card. Similar models, such as the 3430 are likely to have the battery in a very similar location. This means the video card (if present) will need to be removed in order to access the battery.
4. IF NO VIDEO CARD PRESENT, SKIP TO STEP 6. To remove the video card, locate the tab/latch on the back of the frame near the end of the video card. It has a blue rectangle on it, as shown in the picture. You’ll need to lift the end that is inside so that it unlatches. It is on hinges. Lift it so that is folds straight back, to get it out of the way. This will expose the frame-insert of the video card.
5. Grasp the video card firmly, near the frame as shown, then lever it up and out of the computer. It may be necessary to pull the lever-latch (pointed out in the second image below) in order to release the other end of the card. Set the card aside carefully, taking care not to touch the connectors at the bottom edge.
6. Locate the battery latch (if present) on the battery housing. There are a couple of types and I have highlighted the one usually present on this model. Another common version of the battery housing has no visible latch, and only requires that you squeeze the battery out from under a lip of the housing. If the latch is present, press it away from the center of the battery, toward the outside edge. The battery should pop up slightly. Some models have a spring strong enough to flip the battery entirely out of the computer.
7. Latch the new battery in place, plus-side up as shown in the picture for Step 6.
8.IF NO VIDEO CARD PRESENT, SKIP TO STEP 10.Realign the video card to click back into the same slot it came out of. It will click or pop back into place, but does not require a lot of effort. Just firm fingertip pressure. Note that the silver bracket that goes next to the frame has a tab that slides down into the gap between the motherboard (mobo) and the frame of the computer. The connector tab on the bottom of the card has notches to accommodate the edges and divisions of the blue housing that it slides into. The latch should easily move itself out of the way.
9. Flip the frame tab/latch back over and click it into place.
10. Realign the tabs and notches on the removed side of the machine and the frame. The side should slide down onto the frame, approximately 1/2 to 3/4-inch back from the front of the machine. Once it settles into the notches, slide it toward the front of the machine until the remaining gap is closed and you hear the latch lock into place.
11. All done with the battery replacement! Reconnect the cables, taking care that any USB cables for devices such as printers or scanners go back into the port they came out of.