Important: Apple highly recommends removing the battery when handling the logic board. Make sure to use proper ESD protection when handling modules.
The battery on the logic board controls the stored system settings, such as date and time. It is only necessary to test the battery when you can't power on the computer, or the date and time are reset every time the AC power is removed.
The battery is also used to power the PMU chip (because the PMU chip keeps time and must always be running) when the computer is unplugged from the wall (AC power). The PMU is very sensitive and touching any circuitry that is connected to the PMU can cause it to crash. If the PMU crashes, the battery life goes from about five years to about two days if the PMU is not reset. Once the battery goes dead, the PMU will reset the time and date to 12:00 AM 1/1/04 every time the AC power is removed. To fix this situation, replace the battery and reset the PMU (refer to "Resetting the PMU on the Logic Board" mentioned earlier in this chapter).
If the computer has a "No Power" situation, check the battery before replacing modules. When checking the battery, make sure the computer is unplugged. Measure the voltage across the battery's BT1 positive and ground terminals. The battery should read 3.3 volts to 3.7 volts. If it's lower than 3.2 volts, replace the battery and reset the PMU chip.
Note: The iMac uses a lithium battery that must be disposed of in accordance with hazardous waste regulations.
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