G4 MDD losing date and time - PRAM battery not flat

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Level 2

199 points

G4 MDD losing date and time - PRAM battery not flat

Recently, I noticed that my G4 MDD was giving the message that normally indicates a dying PRAM battery - something along the lines of "your computer's clock is set to a date before 2001" (I forget the exact wording, but if you have ever seen it, you will know the one). In this case, it is 1970. Once the internet connection is established the date and time pick up the correct settings from the internet time server (which I have set in the Date & Time pane of System Preferences). If I shut down the computer and leave the power turned on at the wall, the settings are retained. If I shut down and turn the power off, the date and time revert to 1970. For some years now I have been turning the power off for 2 reasons - to save power ("trickle" or "standby" current, although small, is drawn by the power supply unit and can hasten the demise of the PSU), and to avoid risk of fire caused by appliances left connected.

I have checked the PRAM battery and it is showing 3.63 volts, which is well within the acceptable range of 3.3 - 3.7 volts, and tried another battery just in case this one was testing good but acting bad once installed - same result. I have also reset the PMU according to Apple's instructions. The problem remains. Normally these two actions should fix this problem. Is there something else I can try? If unfixable, the problem is not serious in the sense that being connected to the internet will automatically make the correct adjustment, but it would be nice to know that it is behaving itself properly all the time.

PowerMac G4 DP 1.25 GHz - sys 10.4.11, Quadra 700 - sys 7.0.1•

Posted on May 21, 2011 3:36 PM

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Answer 1

BDAqua

Level 10

239,213 points

May 21, 2011 3:56 PM in response to Steven Jones

Removing AC power will drastically shorten the life of the PRAM Battery, but did you measure the voltage out of the Mac & with a 1K resistor in series?

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Answer 2

Steven Jones Author

Level 2

199 points

May 21, 2011 4:24 PM in response to BDAqua

Once the battery had been removed from the logic board it was tested with a digital multi-meter. I did not know that a 1K resistor in series was necessary for testing the battery's voltage - in fact, it's the first I've ever heard of that in my 21 years of using Macs. Is there a reason for doing that?

My understanding of PRAM batteries is that they maintain the settings for at least 2-3 years before needing replacement (otherwise, why have them?). Plus, I would rather replace a PRAM battery (for a few dollars) than replace a power supply (for substantially many more dollars). The G4 MDD PSU's are apparently susceptible to failure if constantly left connected to power once the computer itself has been turned off.

Is it possible there is something wrong with the logic board itself?

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Answer 3

May 21, 2011 4:37 PM in response to Steven Jones

Steven Jones wrote:

Once the battery had been removed from the logic board it was tested with a digital multi-meter.

In Apple Service information (shop manuals, so to speak), the usual way to check the battery is to remove the AC power, then check the battery while still in its holder. The voltage should be greater than 3.3 volts.

If you remove the battery, you should place a 150-200 ohm resistance across the battery terminals while reading the voltage with a high impedance voltmeter. If you do not do this, the open circuit voltage can still be 3.6 to 3.7 volts even through the battery is to all purposes exhausted. I have a nearly dead Tadiran from 1999 (probably the original in my venerable B&W G3) that merrily puts 3.7 volts up on the display, until a resistance is dropped across it, at which time it sinks to less than a half volt. (OCG was actually awake in EE school that day oh so many years ago when this very topic was discussed.)

Coincidentally, that same G3, which has sat idle and off mains power for three years, has just been resurrected, with a newer battery, since the incumbent was measuring a smidge under two volts. The date code on the battery that came out was September 2003, though I suspect it went in a year or two later.

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Answer 4

Steven Jones Author

Level 2

199 points

May 21, 2011 4:48 PM in response to old comm guy

Thanks for that interesting information. My car battery displayed symptoms like that years ago - charged up OK, showed 12V when tested, but did not deliver when put under load. I've always taken the PRAM battery out of its holder when testing because of the awkwardness of getting the two multi-meter probes to make contact with the terminals of the battery while still in the holder. I'll test the battery according to what you have described.

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