If your laptop is consistently receiving CMOS error messages or resetting the date and time, chances are you need to change your motherboard's CMOS battery. Your computer's CMOS is a chip that does not require outside power, and is often powered by a battery. The CMOS chip stores basic information such as the date and time. The CMOS battery looks similar to a large watch battery and is located near your Acer 5720 laptop's wireless card.
Turn off the laptop, close the LCD screen, disconnect all cables and peripherals and remove the battery pack from the bottom casing of the laptop.
Place the laptop vertically so the rear hinges are facing upwards. Remove the two Phillips-head screws from each side of the back edge of the laptop.
Close the LCD screen, and then place the laptop face-down. Remove the two Phillips-head screws securing the RAM and wireless-card compartment cover to the bottom centre of the bottom laptop casing. Remove the cover and set it aside.
Locate the CMOS battery on the left side of the wireless card located near the bottom edge of the laptop. Remove the CMOS battery from its socket and set it aside.
Install the new CMOS battery by going through the previous steps in reverse order.