Hardware Reference
In-Depth Information
at 3.0V or higher. Lithium batteries normally maintain a fairly steady voltage until they are nearly
fully discharged, whereupon the voltage quickly drops. If you check the battery voltage and find it is
below 3.0V, consider replacing the battery, even if it is before the intended replacement time.
Caution
Coin or button lithium cell batteries are extremely dangerous when swallowed. Once in the
body, the voltage combined with fluids causes severe burns in as little as two hours. Children
under 4 years old are at the greatest risk, and the symptoms may be similar to other illnesses,
including coughing, drooling, or other signs of discomfort. If you suspect that a child has
swallowed a battery, immediately go to the emergency room. For more information on this
growing problem, see
www.thebatterycontrolled.com
.
Obsolete or Unique CMOS Batteries
Although most modern systems use 3.0V coin cells, older systems have used a variety of battery types
and voltages over the years. For example, some older systems have used 3.6V, 4.5V, and 6V types as
well. If you are replacing the battery in an older machine, be sure your replacement is the same
voltage as the one you removed from the system. Some motherboards can use batteries of several
voltages, and you use a jumper or switch to select the various settings. If you suspect your
motherboard has this capability, consult the documentation for instructions on changing the settings.
Of course, the easiest thing to do is to replace the existing battery with another of the same type.
Some systems over the years have used a special type of chip that actually has the battery embedded
within it. These are made by several companies, including Dallas Semiconductor and Benchmarq.
These chips are notable for their long lives. Under normal conditions, the integral battery lasts for 10
years—which is, of course, longer than the useful life of the system. If your system uses one of the
Dallas or Benchmarq modules, the battery and chip must be replaced as a unit because they are
integrated. Most of the time, these chip/battery combinations are installed in a socket on the
motherboard just in case a problem requires an early replacement. You can get new modules directly
from the manufacturers for $18 or less, which is much more expensive than the coin-type lithium
battery found in most modern systems. In fact, due to their expense and the fact that most motherboard
chipset manufacturers have integrated the RTC/NVRAM functionality into the motherboard chipset,
few if any modern PCs use these chip/battery modules.
Some systems do not use a battery. Hewlett-Packard, for example, includes a special capacitor in
some of its systems that is automatically recharged anytime the system is plugged in. The system does
not have to be running for the capacitor to charge; it only has to be plugged in. If the system is
unplugged, the capacitor powers the RTC/NVRAM chip for up to a week or more. If the system
remains unplugged for longer than that, the NVRAM information is lost. In that case, these systems
can reload the NVRAM from a backup kept in a special flash ROM chip contained on the
motherboard. The only pieces of information that are actually missing when you repower the system
are the date and time, which you have to re-enter. By using the capacitor combined with an NVRAM
backup in flash ROM, these systems have a reliable solution that lasts indefinitely.
Many older systems use a separate battery that plugs in via a cable or that can even be directly
soldered into the motherboard (mostly older, obsolete systems). For those older systems with the
battery soldered in, a spare battery connector usually exists on the motherboard where you can insert
