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PRAM and PRAM Battery - About - How To
David Anders,The Computer Guy, a freelance systems engineer in Seattle, WA.
Resolves computer problems, assists with production issues, designs and installs networks.
All Computers have a battery (usually on the motherboard) that powers a memory chip that stores the computer setup information (PRAM - parameter random access memory).The Battery can last as long as 7 years and as short as 3 years.

PRAM stores: Status of AppleTalk, Serial Port Configuration and Port definition, Port Speed, Alarm clock setting, Application font, Serial printer location, Autokey rate, Autokey delay, Speaker volume, Attention (beep) sound, Double-click time, Caret blink time (insertion point rate), Mouse scaling (mouse speed), Startup disk, Menu blink count, Monitor Depth, 32 Bit addressing, Virtual Memory, Ram Disk, Disk Cache.

On newer Macs, it also powers the circuit that allows startup from the Power Key on the keyboard.
 
Symptom: Mac will not reliably startup when PowerKey is pressed. It requires several presses to start.
Cause: the back-up 3.6v lithium or 4.5v alkiline battery provides power to the logic board to activate poweron circuit.
Solution: replace the PRAM Battery.
 
Symptom: Mac switches to black and white mode, changes the desktop pattern, the date reverts to 1956 (or 1904), and it loses printer settings, Chooser settings, and time.
Cause: the back-up 3.6v lithium or 4.5v alkiline battery provides power to the logic board to save the control panel settings and is exhausted.
Solution: replace the PRAM Battery.

Symptom: Low Battery Can Cause No Video

The loss of video signal on some Macintosh computers can be caused by a low voltage lithium battery.
Macintosh models which can be affected: Quadra 605, Performa 475, Performa 476, LC 475, Centris 660AV, Quadra 660AV, Power Macintosh 6100, and the Performa 61XX computers.
Diagnostic Steps:
Step 1 If possible, swap the monitor with a known-good monitor and video cable to isolate the issue to the computer.
Step 2 Reset the Parameter RAM (PRAM) by restarting the computer while holding down the Command-Option-P-R keys - OR use Techtool to ZAP all the PRAM, saving the settings before the ZAP.
Step 3 If the computer still does not display video, restart the computer off a known good startup disk or CD-ROM disc. If video returns, reinstall your system software.
If you still do not have video, your computer should be brought to an Apple authorized service provider to have the PRAM battery voltage checked. Below 3.0 volts - the battery should be replaced. If the battery voltage is above 3.0 volts DC, the logic board is likely the cause.
Apple TIL #20328: "Macintosh Computers: Low Battery Can Cause No Video"


Note: When replacing the PRAM battery, use TechTool from Micromat.com to save the PRAM settings (read the Techtool splashscreen information) - when the battery is removed, the PRAM is set to factory defaults. This includes the "hidden" parts of the PRAM that store the number of hours it has been on, the date it was finished, etc.

To replace the 3.6v lithium batteries:

  1. First make sure you are well grounded.
  2. Next take the cover off of the Mac.
  3. Next touch the power supply to ground yourself.
  4. Squeeze both sides of the battery cage - lift straight up.
  5. Note which way the battery is inserted. The bottom of the battery holder indicates the positive and negative poles of the battery.
  6. Remove the old battery.
  7. Insert the new battery - FACING THE SAME WAY.
  8. Replace the battery cage.
  9. Put the Mac cover back on.
  10. Plug it in, turn in on.
  11. Use Techtool to reestablish the saved PRAM Settings

OR

  1. Reset the Monitors control panel to color, double click speed, etc.
  2. Reset the date and time, and check that you are in 32 bit addressing.
  3. Open Startup Control Panel and set your Boot HD
  4. Open the Chooser and reselect the Printer.
  5. Open the Network or Appletalk Control Panel and reselect Ethernet, if necessary.
Dispose of the old battery.

Why do different Macs reset to different dates?
Most Macs reset to the birthdate of one of the designers, Aug 27th, 1956. Others reset to the default date - the date that the clock shows when the clock registers are filled with zeros.
"What is the default date if the clock contains all zeros?"
It's midnight, Jan 1, 1904. This date was selected because the Mac's clock (which counts in seconds) can encompass a period of about 136 years. Selecting 1904 as the start date means that the 136-year period covered by the clock (1904-2040) includes the birthdate of nearly every Mac user, and extends well past the expected lifetime of the Mac OS. It also means that the simplest rule for leap-years can be used (every fourth year has an extra day), which simplifies day and date calculations. They didn't choose the year 1900 because it was not a leap year, and so would have complicated matters.
 

LC Series/Quadra 605
The LC, LC II, LC III, and LC475/Quadra 605 are low-profile versions. The 3.6v lithium battery, RAM, VRAM, etc are in the open. With a dead battery, these models fail to start up - the hard drive may spin for a moment, the screen remains black.
Centris/Quadra 650
The 3.6v lithium battery on this model is on the front, right corner of the logic board, under a bracket that holds the floppy drive. An internal CD drive is right over the battery.
Quadra/Performa 630 Series
This series (630, 635, 636 or 637) has a Rayovac 840, 4.5v Alkaline battery instead of the 3.6v lithium, held on the logic board with velcro.
Quadra 700
The Quadra 700 is a CI on it's edge, and needs the power suppply and drive holder removed to replace the 3.6v lithium battery.

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