most computer have a small battery in many cases,the battery is soldered directly onto the motherboard,but the battery is usually in some sort of holder so it is easy to replace. computer are not the only things that have a small battery like
this camcorder and digital cameras often have them, too. just about any gadget that keeps track of the time will have a battery in your computer ( as will as other gadgets),the battery powers a ship called the real time clock chip.
the RTC is essentially a quartz.watch that runs all the time whether or not the computer has power. the battery power this clock,when the computer boots up, part off the process is to query the RTC to get the correct time and date. a little quartz clock like this might run for five to seven years off of a small battery. then it is time to replace the battery.