Mac PRAM, NVRAM, CUDA/PMU & Battery Tutorial
What, When & How?

Last Update on 9 October 2009

The information in this tutorial primarily applies to legacy Macs, through the G4 models, using OS 7.x, OS 8.x and OS 9.x . However, some of the information also applies to legacy Macs operating on OS X.

Your Mac has a microcontroller/memory chip which stores data that is important to its successful startup and operation. This data is maintained when you shutdown or unplug the Mac by a battery which provides power to the chip. This chip controls the Parameter RAM (PRAM), Non-volatile RAM (NVRAM), the real-time clock and Apple Desktop Bus (ADB). Also the chip sends a constant signal to the power supply, and if this signal is not within specifications, the Mac will either shutdown or freeze.

When Mac users are having problems with their Mac, troubleshooting suggestions are frequently made to: "Zap the PRAM", "Zap the NVRAM", "Reset the CUDA (or PMU)", or "Replace the internal memory battery". Then the question arises: What is the PRAM, NVRAM, CUDA/PMU and Battery? When do I zap, reset or replace? How do I zap, reset or replace?

This information has been compiled from the Apple Care Knowledge Base, Apple's manuals, numerous web pages, and several Mac books. Also included are links to additional information and several useful utilities. 

The table below provides a list of many (but, probably not all) problems/symptoms relating to PRAM, NVRAM, CUDA/PMU and the internal memory battery. If one matches your problem/symptom, refer to the noted section and corrective action. There may be other hardware/software anomalies, which require separate corrective action, that cause the listed problems. For what it's worth, read the MacGuru site Mac Troubleshooting....What Works....What Doesn't....... Summarizing, the site states:

"Apple did an in-depth report of troubleshooting techniques and their effectiveness. This is what they found [Covers cases with repeat (more than four) calls on the same issue during an 8-week period, where the issue was crashing and freezing.]:

1. Extensions troubleshooting worked 56% of the times it was tried. 

  • Extensions troubleshooting is the most widely applicable troubleshooting step. It is appropriate for any error type 1, 2, 3, 10, 11, 25 or freezing in addition to miscellaneous weird behavior. If the issue occurs on startup or in multiple applications, this is the best bet.
2. Clean installs worked 28% of the time they were tried.
  • When all third-party hardware and software have been eliminated and the issue persists on startup or in multiple applications, reinstalling system software may be appropriate. 
3. Disconnecting SCSI devices worked 21% of the time that it was tried.
  • Disconnecting SCSI devices is appropriate when a computer starts with a only gray screen and a pointer, but goes no further. Random hard freezes and recurring directory corruption can also be signs of SCSI chain problems. Many users do not realize that all SCSI devices must always powered on before the computer is started and then left on at all times while the computer is being used.

These are all valid troubleshooting steps. They have a high likelihood of resolving your problem. In contrast, the following troubleshooting steps are tried way too often.

1.    Rebuilding the Desktop worked 0% of the time that it was tried. It was tried 54% of the time.

  • Rebuilding the Desktop should only be tried to resolve generic file icons. A single generic icon is often a file level problem (such as a bundle bit), that rebuilding the Desktop won't fix. In rare cases, application/document connection problems can be fixed by rebuilding the Desktop. 
2.    Deleting Preferences worked 3% of the time that it was tried. It was tried 38% of the time.
  • Moving Preferences should only be tried when an issue is isolated to a specific item (Finder, Control Panel, application). Usually, the program will crash on startup or the application-specific settings fail to "stick" when you quit the application.
3.    Zapping the PRAM worked 5% of the time that it was tried. It was tried 77% of the time.
  • Resetting the PRAM should only be tried in cases where PRAM-resident Control Panel settings are not "sticking". Keep in mind that resetting the PRAM resets all of the Control Panel settings to their defaults and requires you to reset any settings you changed. Resetting PRAM can affect ADB and serial port issues."
[My Comment - Re: Above statement that "all SCSI devices must always powered on before the computer is started and then left on at all times while the computer is being used." This may be true on some Macs, but not my PM 6500. I have two external SCSI devices connected as follows: Mac > Scanner > Zip Drive > SCSI Terminator. I always leave the Scanner powered Off when not being used and I also disconnected power from the Zip Drive. The Mac starts up properly and the internal SCSI devices (CDR, Floppy Drive and second HD) still operate.]

This came from another Mac site.


Never DEFAULT install Norton Utilities on your system. The default install puts what I consider crap all into your system. Crashguard, Filesaver, and Disklight cause far more problems than they are worth. If you have already done this disable from the extensions manager: crashguard, filesaver, disklight, and the auto update features. I love Disk Doctor and Speed Disk but I run them from the CD when needed. 

NEVER RUN NORTON 3.5 or earlier on an HFS+ Formatted Drive. It will destroy the contents of the drive. You may be able to recover it with the patch someone put out for Norton called Drive Fix 1.1. A proper version of Norton 5.0 or later will probably fix it as well. 

NEVER RUN NORTON Disk Doctor 6.0.2 or earlier on an OSX system. Norton 6.03 or later for OSX systems is okay and it has to be run from OS 9 or by booting from the CD. Use the Get Info from under the file menu after selecting the Disk Doctor icon to see what version you are running."

Refer To
Zip disks do not appear on the desktop or eject PRAM
CD-ROM or DVD-ROM disc does not appear on the desktop PRAM
Flashing '?' at startup, or Startup is not normal, or Computer malfunctions; starts up from wrong startup disk  PRAM
Hard disk won't mount or boot-up PRAM
Freeze when booting-up
Tip -  Most likely cause is related to SCSI devices/connections or corrupted Finder preference file
Mac is very slow when booting up
Tip - If zapping PRAM doesn't work, trash Finder preference file and reboot.
No sound from one or both of the Apple Pro Speakers PRAM
iMac: Address and Illegal Instruction errors during startup PRAM
USB-equipped computers encounter a gray screen when waking from sleep or a "Finder has unexpectedly quit" error when going to sleep PRAM
Remote Access alert when receiving an incorrect response, or no response from the modem PRAM
Mac OS 9: Modem makes no sound PRAM
Getting Type 10 or 11 errors repeatedly PRAM
Every time the computer starts up, a text-based command-line-interface appears that says: "To continue booting, type "mac-boot" and press return. To shut down, type "shut-down" and press return. PRAM
Startup problems after crashes, freezes or power interruptions (improper shutdowns) PRAM
When Shutdown selected, Mac Restarts
Tip - Energy Saver on some Macs/OSs can cause this problem. Set Energy Saver to Never.
Corrupted data to output devices like printer, modem FAX. i.e., Printer prints "garbage" characters. PRAM
Hard drive won't spin up or you get "Sad Mac" error
Tip - Defective RAM or bad RAM card connection also causes this. If you install new RAM, you should zap PRAM.
Virtual Memory won't stay Off PRAM,
Serial port problems; requires reset
(See Tip in PRAM section)
Selected printer in Chooser won't stay selected PRAM,
Can't access any external connected SCSI, FireWire or USB device. PRAM,
Nothing on display, or no power at all PRAM,
Open Firmware not booting normally into OS NVRAM
Display problems NVRAM
Battery replaced, but will not boot from HD. Will boot from CD. CUDA/PMU
Battery replaced, but will not boot & no display. CUDA/PMU
Nothing works, no power (appears to be failed power supply)  CUDA/PMU
Startup fails or shuts down after adding RAM, PCI expansion card, or processor upgrade card. CUDA/PMU
USB port(s) not operating
The date/time is reset to 1956 or 1904 Battery
Depressing keyboard power switch won't start up Mac Battery
Mac (some) won't start up; or you hear a startup chime, but the screen remains blank Battery
The graphics hardware in some Macs will fail to work correctly; boots but won't display anything (no video) Battery, 
AppleTalk is turned On, when it was Off, whenever you shutdown and then startup Battery
Selected printer no longer prints; it gives you an error saying the port is in use (by AppleTalk). 
Tip - To possibly avoid this problem, go to AppleTalk control panel and set AppleTalk to Remote instead of Printer port.
Boots from the wrong startup disk when started up Battery
The selected AppleTalk port has been changed Battery
Monitor gets set to Black & White 
Tip - If Mac boots to B&W and then reverts to color, trash Finder Preference file and restart.
Can't access internet (because date/time incorrect) Battery
Can't access external hard drive Battery


PRAM is a small amount of memory continually powered by the internal battery to retain its contents even when the computer is shut down or unplugged from AC power. PRAM maintains information such as background color, default video selection, network information, serial port information, and default highlight color.

Even though you may not know what in PRAM is causing issues on your computer, resetting this information can often be an important troubleshooting step. This is particularly true when serial devices are connected to the computer, or when networking software is in use.

Zapping the PRAM resets your Mac to the factory defaults, so anything you have customized (like background or highlight color) is reset, the alert sound may be different, and AppleTalk will most likely be turned Off. Date and Time is not reset. Generally, you only only need to zap PRAM if the PRAM-resident settings are not "sticking", however, system crashes can result in corrupted PRAM settings. Zapping PRAM can also affect ADB (ports that connect the keyboard and mouse) and serial port issues. Use the control panels to set the computer back to the way it was before the PRAM was reset. Some Macs may not have all the settings described below.

  • AppleTalk Control Panel (& Chooser) - Status of AppleTalk 
  • Chooser - Serial Port configuration and Port definition, Serial printer location 
  • Appearance Control Panel - Application font 
  • Keyboard Control Panel - Autokey rate, Autokey delay 
  • Mouse Control Panel - Double-click time, Mouse scaling (mouse speed) 
  • Startup Disk Control Panel - Startup disk 
  • General Controls Control Panel - Menu blink count, Caret blink time (insertion point rate), Folder Protection 
  • Monitors & Sound Control Panel - Monitor depth, Attention (beep) sound, Speaker volume 
  • Memory Control Panel - 32-bit addressing (OS 7.x), Virtual memory, RAM disk, Disk cache
  • Date & Time Control Panel - Set Time Zone, Set Daylight Saving Time (but not Date and Time)
  • Energy Saver Control Panel - Sleep settings, Scheduled Startup & Shutdown
The following control panels are not affected by zapping PRAM: Energy Saver, File Sharing, Text, Numbers, Speech, PPP, TCP/IP and several others.

Also see Mac OS X: What's stored in PRAM

There are several Apple Care Knowledge Base documents related to resetting PRAM:
Resetting your Mac's PRAM and NVRAM

When to reset NVRAM or PRAM
RAM Disk: Resetting PRAM Causes Loss of RAM Disk Contents

CAUTION - Zapping PRAM will erase all data in your RAM disk. Backup any important RAM disk data before zapping PRAM.

According to Apple, there is no value in resetting the PRAM as a troubleshooting tool for software problems. There have been a few, unconfirmed reports that resetting the PRAM helped to reset a NuBus video card when the monitor displays only static. These reports are not verifiable and there are no PRAM values or settings for NuBus slots.

PRAM vs. the Finder Preferences File -  The Finder Preferences file also stores some user-selected settings. In particular, the Finder Preferences file stores: the Finder version; all settings from the Finder Preferences window except Label colors and names (which are kept in the System suitcase file); last window size and position for Clipboard, Copy windows, and the About this Computer window; where Sherlock was last located; and the location of the Trash. It also stores the On or Off status of the "Warn Before Emptying" check box (located in the Trash's Get Info window). These settings are separate from PRAM settings and are not reset when you zap the PRAM. Problems with these preferences settings are usually solved by replacing the Finder preference file.

A freeware utility called PRAM Inspector lets you see exactly what is stored in your PRAM, so you can detect PRAM problems by yourself. The figure below shows the PRAM Inspector window and PRAM contents.

Tip - Make a desktop picture (Command+Shift+3) of PRAM Inspector's window, print and save it for future reference when you have problems.

Tip - Before you zap PRAM, try this corrective action since it may solve your problem. Start up (depress Shift key) with Extensions Off . Trash the following Preference files (don't worry, new ones will be generated) and restart: ASLM, Finder, Mac OS, System. Some of you Control Panel settings may change. For example, if Virtual Memory was Off, it will now be On.

There are several ways to zap or reset the PRAM.

A. The Apple OS steps to zap/reset PRAM

Note: This procedure may not work with some keyboards from manufacturers other than Apple.

  • 1. Make sure the Caps Lock key is not engaged.
  • 2. Restart your computer and immediately hold down the Command (Apple)+Option+P+R keys.
  • 3. Keep holding down the keys until you hear the startup sound/chime a second time, then release the keys. (Tip - Others suggest that you hold down the keys until you hear 3 or 4 chimes.)
  • 4. The PRAM is now reset.
  • 5. If you had special settings in any Control Panels, open those Control Panels to reset the desired settings. 
Tip - If corrupted PRAM prevents you from starting up from the hard drive, startup using a system or utility CD/disk and then zap the PRAM.

Tip - Some third-party software programs also store information in PRAM. You may have to reconfigure video cards after resetting PRAM, and other aspects of your work may be affected.

B. Using TechTool Lite

The freeware utility TechTool Lite from MicroMat--makers of TechTool Pro--does a very good job of helping you reset your PRAM (but not NVRAM). It also offers you a way to save your previous PRAM settings if you ever want to revert. This is the best way to zap PRAM. The figure below shows the TechTool Lite window.

Tip - For a description of how to reset the Power Manager and PRAM in a PowerBook, refer to the AppleCare Knowledge Base document PowerBook: Resetting Power Manager.

Tip - If you get a "port in use" error, try this before zapping PRAM. Sometimes a Restart will eliminate the error. Or use the freeware utilities ResetSerialChannel or PortPeek. A bad printer cable connection may also give you an error saying the port is in use.

Tip - Some newer monitors have internal logic memory that, when corrupted, causes display problems. Use this procedure by a Mac user to reset the monitor memory if your monitor has separate power and video cables. Power everything down, disconnect the video cable from the Mac. Then turn the monitor on, wait 15 to 20 seconds, then turn it off; wait about 10 seconds, then repeat this cycle 3 or 4 times. Then reconnect the video cable and boot back up. Also refer to How to Reset Your Monitor If It's Acting Funny


NVRAM is found on Macs that have a PCI bus. Sometimes it is important to reset the NVRAM so that the computer will redetect devices and cards that are installed, and software settings. NVRAM stores the following information:

  • Patches to the boot code which need to run before disk I/O (Input/Output) starts up. 
  • Settings such as monitor resolution that is stored by drivers or applications.
  • Information necessary to generate the video display.
  • Information for Open Firmware to boot normally into an OS.
  • PRAM settings. (Note - Clearing the PRAM only clears a portion of the NVRAM.)
There are several AppleCare Knowledge Base documents related to zapping NVRAM:
Resetting your Mac's PRAM and NVRAM
"To continue booting, type 'mac-boot' and press return" Message

There are several ways to zap or reset the NVRAM.

CAUTION - Zap/reset of NVRAM on older Macs with G3 and G4 upgrades MAY eliminate a special fix installed (from a floppy) with the upgrade. Check you upgrade documentation.

Tip - Before you reset the NVRAM, try this. Trash the Display Preference file and restart. I had a problem where the brightness of the display looked like a moon-lit night, although the cursor was bright (and the startup screen was bright). Trashing the preference file corrected the problem.

A. The Apple OS steps to zap/reset NVRAM

  • 1. Shutdown the Mac (do not use Restart command)
  • 2. Start up and immediately (before gray screen appears) depress the Command (Apple)+Option+P+R keys.
  • 3. Wait for the Mac to automatically both chime and restart. Hold the keys depressed while it does this at least two times. Now release the keys and let the Mac completely start up.
  • 4. Immediately go to the System/Preferences folder and trash the Display Preferences file.
  • 5. Restart the Mac.
B.Zap/reset NVRAM while keeping scri/INIT/cdev out of the boot process
  • 1. Set all the Extensions Off from the Extension Manager.
  • 2. Shutdown the computer.
  • 3. Remove all the peripheral devices.
  • 4. Start up and immediately depress the Command(Apple)+Option+P+R keys. (Note - If you don't depress the keys "immediately", you'll only reset PRAM.)
  • 5. Wait for the Mac to automatically both chime and restart. Hold the keys depressed while it does this at least two times. Now release the keys and let the Mac completely start up.
  • 6. Immediately go to the System/Preferences folder and trash the Display Preferences file.
  • 7. Restart the Mac.
C.The Apple Open Firmware (OF) steps to zap/reset NVRAM

Most of Apple's Macintosh computers since the first iMac have shipped with Open Firmware, a hardware component level BIOS. [Examples of machines that do NOT use Open Firmware: Apple Power Macs of the earlier generation (NuBus-based), Power Mac clones that are NuBus-based, 68k Macs, and other 68k-based Macs.] Holding down Command(Apple)+Option+O+F keys on startup will present you with a gray screen with a command prompt. At this screen you can enter various open firmware commands to configure or reset your OF settings. From this OF screen, you can "deep reset" your PRAM and NVRAM settings. At the prompt, type, followed by the Return key:

  • 1. Type: reset-nvram 
  • 2. Press Return key (will return to you to the prompt, your nvram settings are now factory default)
  • 3. Then type: reset-all 
  • 4. Press Return key (will reset your logic board and PRAM settings)
  • 5. The reset-all command should cause the computer to restart. If this occurs, you have successfully reset the Open Firmware settings.
Tip - A Mac user fixed a "wake-from-sleep" issue using this OF procedure:
  • 1. Type: set-defaults
  • 2. Press Return key 
  • 3. Then type: reset-all 
  • 4. Press Return key 
  • 5. The reset-all command should cause the computer to restart. 

D.Resetting CUDA and removing the battery will also zap/reset NVRAM (see below).


Most PCI-based Power Macs have a small button switch (red or gray/silver), called the CUDA, on the logic board. Pressing the CUDA switch (see figure) resets all data (PRAM, NVRAM, clock and other) stored in the microcontroller/ memory chip. The function of the Cuda chip is to:

  • Turn system power On and Off
  • Manage system resets from various commands
  • Maintain PRAM
  • Manage the real-time clock
Pressing the CUDA switch does the same function as removing the battery, only faster. [CUDA stands for "Capacitive Unit Discharge ASIC". "ASIC", in turn, stands for "Application Specific Integrated Circuit".]

There are several ways to determine if your Mac has a CUDA switch and where it is located. Its location varies from model to model, but a visual scan of the logic board should reveal it. Go to MacGuru's Power Mac Mother Boards and look at the logic board picture for your Mac and the marked CUDA location.

  • 1. The 61, 71 and 8100s do not have a CUDA.
  • 2. On the 73, 75, 7600s it is directly under the back of the processor. You may have to pull the processor to see it.
  • 3. On Beige G3's it is directly left of the PCI slots near the edge of the logic board. 
  • 4. On the B&W G3s, you will see two buttons behind the PCI slots marked "Power On." The left one (as you face the front of the Mac) is the normal Power button. The right one is the CUDA button.
  • 5. On G4s it is in the right rear of the logic board near the battery.
  • 6. On the 6400 and 6500, look at this site
  • 7. On the iMac it is located near the RAM modules.
Newer Macs have a similar reset switch called the Power Management Unit (PMU). Refer to these procedures for resetting the PMU. Some Macs, especially G4s, will not boot properly after a power failure or an inadvertent turn-off of power while the Mac is operating or at sleep. A PMU reset will usually fix this problem. Caution - Follow the procedures per Apple's instructions, or the memory battery can be quickly discharged.

Apple's Service Manual procedure for resetting CUDA after you remove/gain access* to the logic board (See Note 1 below) is:

  • 1. Remove or disconnect the memory battery. Leave the battery disconnected for 5-10* minutes.
  • 2. Reinstall or reconnect the battery.
  • 3. Depress the CUDA button (for 5 seconds) with a non-metallic (plastic, wood, etc.) device.
  • 4. Reinstall the logic board.

  • *(Other sites say 10 minutes minimum.)
*If your Mac allows access to the CUDA switch without removing the logic board, disconnect the AC power and perform the above procedure.

Tip - Sometime a Mac will not start up after experiencing an AC input power failure. Usually a CUDA or PMU reset will fix the problem.

Tip - Other sources say just depress the CUDA button (with no mention of disabling the battery). This would be quicker and OK for a first try at troubleshooting.

Tip - I found this on a Mac site. "Sometimes computers lose their mind. Don't ask me why or how it happens but it does. Sometimes you simply have to totally discharge the computer. One thing I have learned about the Blue and White G3 and the Gray G4 computers is that sometimes pushing the CUDA (PMU) is not enough. If your machine still is dead, then try the following. 

*    Unplug the Unit
*    Remove the Battery
*    Push the CUDA (PMU) Switch (count to 5 slow)
*    Push the Start button on the front of the computer (count to 5 slow)
*    Let the unit sit for 15-30 minutes
*    Replace the Battery
*    Plug it back in, and push the start button

If it doesn't work I usually repeat the steps above, pull the ram, and let it sit longer. If it doesn't work after this, it is take it to the shop time."

Tip - Another great tip that many times works - Resetting the Logic Board

Resetting the logic board can resolve many system problems. Whenever you have a unit that fails to power up, you should follow this procedure before replacing any modules.

  • 1 Unplug the computer.
  • 2 Press the Power On button on the front of the unit.
  • 3 Open the side access panel.
  • 4 Remove the battery from the logic board.
  • 5 Wait at least 10 minutes before replacing the battery.
  • 6 Make sure the battery is installed in the correct +/- direction.
  • 7 Reassemble the computer and test the unit.

This procedure resets the computer’s PRAM. Be sure to check the computer’s time/date and other system parameter settings

After you reset CUDA/PMU, you may have to reset the Control Panels as described above for PRAM, and reset the date and time. If you saved the TechTool Lite PRAM information, use TechTool to reset the Control Panels.

Here's some additional AppleCare Knowledge Base documents related to startup problems.

Sometimes a CUDA/PMU reset (and/or battery replacement) does not solve the problem of the Mac appearing to not having power to the computer. In this case, it could be a failed power supply. Some Mac power supplies have an internal fuse, however, the supply must be removed and the supply case opened to examine if the supply has a fuse. If you have a volt/ohm multimeter, power supplies can be tested for voltage output.

If you determine that your power supply has failed, here's some power supply web sources.
You can also check eBay and my favorite, LEM-Swap for buying & selling Mac stuff.


The memory battery is a very important part of your computer, and all Macs have one. This battery keeps a small amount of memory active to store important information when you turn off your computer, or the computer is unplugged from AC power. The information actually stored varies from Mac to Mac. Some of it deals with the computer's PRAM and NVRAM, but one of the most easily apparent piece of information it maintains is the date and time. The battery also powers the SoftPower (Power key) circuit and ADB control.

There are several Apple Care Knowledge Base documents related to batteries:

When the battery dies or you take out the battery, the clock stops and resets to a default date and time. Many older Macs reset to August 27, 1956. This is the birth date of one of the designers (Ray Montagne) of the CUDA microcontroller. The original Mac clock was designed to work until 2040 (The only computer with no Y2K problem!!!). Newer Macs reset to a different default date - the date that the clock shows when the clock registers are filled with zeros. The date is (midnight) January 1, 1904, defined as time zero for the clock. Year 1904 was selected as the start date because the simplest rule for leap-years can be used (every fourth year has an extra day), which simplifies day and date calculations. Year 1900 was not a leap year, and so would have complicated matters. Apple has since redesigned its date utilities to handle dates until 29,940 (Do you think your current Mac will last that long?).

Macs (and clones) use one of two different batteries - a 3.6 volt lithium, 1/2 AA size or a 4.5 volt alkaline. The tables below show which Macs use which battery.

A low voltage battery can cause the memory information to become corrupted. You can check the battery voltage, while installed on the logic board (Mac power Off), with a DC voltmeter. If below 3.0 volts for the 3.6 volt lithium or below 3.8 volts for the 4.5 volt alkaline, it's a weak, unreliable battery. Depending on the length of time that you Mac is shutdown or unplugged, a lithium battery may last 3-10 years and a alkaline battery may last 2-7 years. 

Tip - If your battery life  is not very long, one possibility is that there are external peripherals attached to the system which remain On when the system is turned Off.  It is possible that this situation produces a negative bias on the system board that drains the battery.  Try turning off external peripherals like modems, printers, etc. when you turn your system off via the keyboard and see if this has any effect. 

Tip - If you do not have a voltmeter, remove or disconnect the battery and unplug the Mac's AC power cord. Wait at least 30 minutes for the residual power, from a capacitor, on the memory chip to dissipate. Do not install or reconnect the battery. Start up the computer. If it starts normally and finishes the boot process, the battery is defective.

Also, a useful freeway utility is PRAM Battery Checker - Checks if your PRAM battery has died.

There are three different 3.6 volt Tadiran model number lithium batteries used in Macs:

  •  TL5101/S (950 mAh)
  •  TL5151/S (750 mAh)
  •  TL2150/S (850 mAh, high drain)
If you are not sure which one is used in your Mac, get the TL2150/S since it will work in any Mac. Similar batteries are made by Maxell (model ER3STC), Saft (model LS 14250BA), Tekcell and Radio Shack (model 23-026).

The 4.5 volt alkaline battery is a Rayovac model number 840. The Rayovac models 841 and 844 are substitutes.

Replacement batteries may be obtained from most computer stores or Radio Shack (about $13 + tax for the 3.6V Lithium). Some camera stores have the 3.6 volt lithium batteries. Local Batteries Plus stores have Apple/Mac memory batteries. Here's the part numbers and prices.

1/2AA 3.6V Lithium, P/N LITHS14250BA, $8.99 + local sales tax
4.5V Alkaline, P/N COMP3LF22BP, $14.99 + local sales tax

For web purchases, go to Sources for Logic Board Batteries , or use the table below that provides some sources and prices (along with web links) for the 3.6V and 4.5V batteries.

Other World Computing
Mega Macs


Here are some creative ways that Mac users have made substitute batteries.

Changing the battery is not too difficult once you gain access to the logic board.  If you have an older iMac, additional information is provided in iMac Battery Replacement Manual, or How to Change the Battery (in tray loading 233mhz, 266mhz & 333mhz iMacs), or How to Replace iMac Battery  If you have a G4, here's a QuickTime movie of battery replacement.

For the 3.6 volt lithium battery, pry off the battery retainer (see figure) and pull out the battery. Insert the new battery, making sure of the +/- orientation, and snap on the retainer. For the 4.5 volt alkaline battery, unplug the connector and pull the battery upward off of the Velcro strip. Install the new battery on the Velcro and plug in the polarized connector. Before installing the new battery, wait at least 30 minutes for the residual power, from a capacitor, on the memory chip to dissipate.

After you replace the battery (See Note 2 below), you will have to reset the Control Panels as described above for PRAM, and reset the date and time. If you saved the TechTool Lite PRAM information, use TechTool to reset the Control Panels.


Note 1. CAUTION - Resetting the CUDA/PMU or replacing the battery requires that you have access to the logic board. Electro-static discharge may damage sensitive logic board circuitry, or you may come in contact with AC power. Ensure that your Mac is turned Off, but remains plugged in to the grounded AC power. Attach a grounding strap to the Mac's metal case and to yourself, or as a minimum, touch the metal case.

Note 2. Safely Disposing Of Batteries - Used lithium batteries should be recycled. Many Radio Shack locations take them back. If you do decide to trash a lithium battery, place it in an empty 35mm film container and tape the container lid. Alkaline batteries are mercury free and you can trash them. 

Note 3. There are several maintenance things that Mac users can do to make their computer operation more reliable and usage more enjoyable.

  • As a minimum, interface the power to your Mac from a surge protector and route your dial-up/broadband through a surge protector. 
  • If you can afford one, get an uninterruptable power supply (UPS) that provides battery back-up power during power failures and can shut the Mac down automatically during a prolonged power outage.
  • Do not unplug the Mac from AC power or turn Off power strips for prolonged periods since the internal memory battery will discharge and may fail. Memory batteries are not rechargeable.
  • Backup you files frequently, especially important files, to another hard drive, to Zip or floppy disks, to CDs/DVDs, to flash drives or to an offsite storage location.
  • Keep about 10% to 15% free space on small hard drives/partitions.
  • Defragment your hard drive periodically. Not required for OS X.
  • Periodically run a disk utility like Disk First Aid, Norton Disk Doctor, Tech Tool or DiskWarrior to keep the file structure in order.
  • Rebuild the desktop periodically.
  • Examine the files listed in the Trash BEFORE you empty the trash.
  • Keep your Mac clean, inside and outside.
  • Here's some links related to maintenance on your Mac.

  • Preventive Maintenance
  • Maintenance Tips
  • Basic Maintenance
  • Maintenance
  • Defragmenting your Mac's hard disk For OS 9 and below.
  • Disk First Aid: Purpose
  • Cleaning Your Computer

Tip - You may want to print this page so you can reference the information and procedures when you have problems.

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