Technical information

Troubleshooting Logic Board Battery - 14
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 bat-
tery 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 situa-
tion, 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 bat-
tery’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.