The complementary metal-oxide semiconductor battery plays a role in storing system startup settings when you power down your computer. When you boot your computer, the BIOS accesses these settings to determine the system time and date as well as the boot sequence for your computer. Some models, like the Acer Aspire 5520 model, have the battery soldered to the motherboard. In these instances, you should enlist the help of a professional.
You might run into CMOS-related problems when you don't use your computer for an extended period of time or when the battery fails. If you have a bad CMOS battery, you may get a notice of an incorrect system clock, the computer might fail to keep time or your computer may have difficulty finding the primary hard drive. You might also get an error messages when you have a bad CMOS battery. Common messages include "CMOS Battery Bad" and "CMOS Checksum Error." Acer recommends contacting your dealer or an authorized service center to correct these errors.
Before replacing the CMOS battery, make sure to unplug the computer, remove the battery and wear an anti-static wrist band to prevent damaging your computer. Wearing shoes with rubber soles and grounding yourself before working on your computer protects both you and the computer. Make sure you remove any jewelry and give yourself a clean and clear work environment. If you don't have experience repairing computers, take the computer to a certified professional. Check to see if you have an active warranty that would cover the repair.
Replacing the Battery
Remove the screws holding the computer casing together and then lift up and remove the cover. Carefully remove the hard drive and memory modules by spreading the latches on the side and gently unseating the memory. Place the components on an anti-static sheet. Turn the computer over and remove the keyboard from your system. Remove the small, round CMOS battery from the motherboard and replace it with a new one. Don't try to solder a new CMOS battery to the motherboard. Applying heat and attempting to solder in a new battery to your computer can cause serious damage to the components.
Changing CMOS Settings
Shut down your computer and hold down "F2" while the computer boots. Release the key when the Acer logo disappears and the BIOS setup screen appears. If you have trouble accessing the BIOS on Windows 8 systems, press "Windows-C," select "Settings" and choose "Change PC Settings." Click "General," then "Restart Now" and select "Troubleshoot" from the list of options. Choose the "Troubleshoot" menu, select "Advanced Options" and click the "UEFI Firmware Settings." Click "Restart" to enter the BIOS. Once inside the BIOS, use the arrow keys to move between items and menus. Press "F9" to access the setup defaults.