Alert! System battery voltage is low.
Strike F1 key to continue F2 to run the setup utility
Press F5 to run onboard diagnostics

When you see this alert you need to follow a sequence of steps to get your system to boot again. The steps listed here work for a Dell Optiplex 755, and I have used very similar steps for other Dell models. Just try to get the gist.

1. Unplug the power cord from the computer.
2. Remove the cover.
3. Take out the hard drive (make sure to unplug the cables connected to the hard drive — should be two of them)
4. Locate the battery. On the small form 755 it will be under the fan. There is a screw that holds the fan cradle in place, so remove that screw and slide the tabs out of their places on the sides of the blue cradle that holds the fan (be careful because there are many things plugged into the motherboard that you do not want to have to figure out). Then you can raise the fan up and see the battery.
5. Remove the battery and inspect it to see what type battery it is.
6. Ascertain a replacement battery. Make sure the replacement battery is the same type as the one you remove. The 755 uses a 2032.
7. Place the new battery in the battery slot (if there is tape on the bottom of the battery remove it). The flat part that has the brand, model, etc., etched on it should be facing up.
8. Replace the cradle and secure it with the screw you took out previously
9. Replace the hard drive and plug in the cables
10. Replace the cover
11. Plug in the computer’s power cable. That may start the computer. If so, you can let the computer power up. You will get two or three messages, including the alert about the system battery voltage being low. Don’t worry about it. You need to change some settings to reset the system
12. Press F2 and go into the BIOS setup.
13. On the Dell System 755 there is a menu on the left. Use the DOWN arrow to go to Maintenance.
14. With the Maintenance item highlighted press ENTER. On the drop down menu go to Load Defaults and press ENTER.
15. On the right side of screen press the RIGHT arrow and go to Continue and press ENTER.
16. Under the title “Faculty Default Acknowledgement the OK should be highlighted. Press ENTER and you will be taken back to the previous menu. The Load Defaults item on the Maintenance submenu should be highlighted.
17. Press the UP arrow until you get to the Drives menu item and then press ENTER.
18. On the Drives submenu go DOWN to SATA Operation and press ENTER. You will probably see on the right side of screen the AHCI item highlighted. You want to highlight the ATA item so use the RIGHT arrow to highlight ATA and press Enter.
19. You will see a popup menu that tells you the SATA Operation is being changed. Press the RIGHT arrow to ACCEPT the change. And press ENTER.
20. SATA Operation on the left menu should now be highlighted. Press the UP arrow to get to Module Bay menu item and press ENTER
21. Under Module Bay title, you will see On highlighted. Press the LEFT arrow to highlight IDE and Press ENTER
22. The Module Bay menu item should be highlighted now. Press the UP arrow until to get to the System menu item. Press ENTER and use the DOWN arrow to get to the Date/Time submenu and press ENTER. (Actually, on my 755s the System submenu is available when I go into Setup. Your may or may not be, but whichever it is just find the Date/Time menu item).
23. With the Date/Time menu item highlighted press ENTER.
24. Set the proper date and time and press ENTER.
25. You are now finished with Setup so press ESC.
26. Use RIGHT arrow to highlight Save/Exit on the right side of screen. Press ENTER.
27. The system will reboot. When you see the Dell Logo window press CTRL-P. This is the Management Engine BIOS Extension.
28. When you get the blue and black screen notice that you are asked for a password. The factory default is ‘admin’ so type that into the space asking for the password and press ENTER.
29. Now an item at the top of the page will be highlighted. Use the proper arrow to select the Change Intel(R) Password and then press ENTER.
30. Type in a new password. The system requires a strong password, so you must include at least one upper case letter, one lower case letter, a number, and a special character (I always use an exclamation point !). Also, the password must be at least 8 characters long. After you type it you will be asked to type it again to verify it.
31. Once verified you will be taken back to the menu at the top of the page. Select EXIT and press ENTER.
32. You will be asked whether you are sure you want to exit. Press Y.
33. Your system should now reboot. If you were repairing a disk or doing some other maintenance you may have to shut the system down and do a hard boot.