There is a 2032 coin cell battery on the motherboard.  It is backup power for the CMOS Memory (BIOS).  These batteries, generally, last 5 years.  Replace the battery.  
heyso basically, I tried to get back this old Optiplex 760 from 2009 that I have in my good old logs. To my surprise, the system "battery voltage" was low. Honestly, I didn't know what that meant. Can anyone help me.I checked the old ones, they are from 2003. I need a updated version ASAP.
Battery Voltage Low