Apple IMAC Service Source - Page 153

Logic Board Battery, Testing the Battery

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Logic Board Battery 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 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. Testing the Battery 1. Remove the user access door on the bottom of the computer. 2. Locate the battery test point. 3. With the computer powered off, measure the battery voltage. Does the battery measure at least +3.5v? If no, replace the battery and reset the PMU. If the battery measures +3.5v or higher, reinstall the battery and reset the PMU as above. 8 - iMac (USB 2.0) Troubleshooting General Information

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8 -
iMac (USB 2.0) Troubleshooting
General Information
Logic Board Battery
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 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.
Testing the Battery
1.
Remove the user access door on the bottom of the computer.
2.
Locate the battery test point.
3.
With the computer powered off, measure the battery voltage. Does the battery measure at least +3.5v?
If no, replace the battery and reset the PMU. If the battery measures +3.5v or higher, reinstall the
battery and reset the PMU as above.