Hardware Reference
In-Depth Information
Figure 18.42. Cutaway view of a CR2032 lithium coin cell.
Table 18.29
lists the specifications of the common 20mm diameter lithium coin cell batteries you
might find in a PC.
Table 18.29. Common 20mm Lithium Coin Cell Specifications
Estimated battery life can be calculated by dividing the battery capacity by the average current
required. For example, a typical CR2032 battery is rated 220 mAh (milliamp hours), and the
RTC/NVRAM circuit in most current motherboard chipsets draws 5 μA (microamps) with the power
off. Battery life can therefore be calculated as follows:
220,000 μAh ÷ 5 μA = 44,000 hours = 5 years
If a thinner (and lower-capacity) battery such as the CR2025 is used, battery life will be shorter:
165,000 μAh ÷ 5 μA = 33,000 hours = 3.7 years
Battery life starts when the system is first assembled, which can be several months or more before
you purchase the system, even if it is new. Also, the battery might be partially discharged before it is
installed in the system; higher temperatures both in storage and in the system can contribute to shorter
battery life. All these reasons and more can cause battery life to be less than what might be indicated
by calculation.
As the battery drains, output voltage drops somewhat. Lower battery voltage can impair the accuracy
of the RTC. Most lithium coin cell batteries are rated at 3V; however, actual readings on a new
battery are usually higher. If your system clock seems inaccurate (it runs slow, for example), check
the voltage on the CMOS battery. The highest accuracy is obtained if the battery voltage is maintained
