When your computer gives the warning "System Battery Low" at the boot up it may be time to replace your battery. The CMOS battery is located on your computer's motherboard. It maintains computer configuration, date, and time information. You may have to repeatedly reset time and date information after turning on the computer when the battery runs out of power. After a while it may refuse to boot at all, displaying a "CMOS Checksum Error" or "System Configuration Lost" message instead.
You should save your system's current BIOS setting before replacing the battery. Enter the system setup program and restore the settings you recorded after the battery is replaced.