|The cmos battery holds the charge for the cmos which is the part of the bios the user can change. CMOS (complementary metal oxide semiconductor) uses fleld effect transistors (FETs) at their voltage input. FETs have very high input resistance. In circuit analysis programs they're usually modeled as having infinite resistance. Because of this they pull very little current and that's why cmos batteries usually last for years.|
Cmos batteries don't get damaged unless they're mishandled. As with any battery in use their rated voltage drops and eventually they need to be replaced. My experience has been that the 3 volt coin-sized cmos batteries most often used now will hold the cmos settings until their voltage drops to abouit 1 volt.
If you need to check the status of the battery just put a DC voltmeter to it. You don't really need to put a load to them when checking the voltage since their current draw is negligible and the meter input resistance is high.
You can run a PC without the cmos battery but because the cmos holds the date and time when the PC is off those features will default to the original bios date. So if for some reason you are running it without the battery--or with a dead battery--you need to set the date and time after booting the computer. Web certificates and some software will give errors if the computer date is too far off.
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