That 3.6 volt PRAM battery question

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That 3.6 volt PRAM battery question

I have been reading a lot of these forums lately due to my 3rd G5 dual 2.7 that is not starting up at all. It just gets up to the grey apple screen and stops there, And of course the wind turbine that follows.The pram battery is mentioned often that it must be changed every 4 years or so or when the computer starts to act up.I have taken out the battery, the one that is purple and green Tadiran 3.6 volts, and I have checked the voltage with a very expensive digital voltmeter that reads exactly 3.6 volts on it.This battery has been in the computer since it was bought new in 2005. Is the battery good ? can it be possible that even though it reads 3.6 volts, the internal chemistry inside will make it act up once installed?
I know the answer will be ..Just pop in a new one and see..
Isn't the voltage supposed to be less after 5 years of use ?

G5 dual 2.7 tower, Mac OS X (10.5.8), 8 gb ram, pioneer 111, stock 5200 vid card

Posted on Sep 1, 2010 7:40 PM

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Answer 1

Sep 1, 2010 8:34 PM in response to pat hughes

The no-load voltage will stay nearly 3.6 volts well into discharge. I have an 11-year-old Tadiran TL-5151 battery that just showed 3.69 volts open-circuit on my Fluke 189 DMM (a really nice unit, BTW, that has given wonderful service). Put a 75-ohm resistor across that battery and the voltage drops to under 0.7 volts.

A brand-new SAFT or Tadiran will drop to about 3.35 volts with that 75-ohm resistor across it, which corresponds to a load current of about 45 ma. The Thevenin equivalent internal resistance (or Norton for those of a current mind) in the good battery is on the order of 6-7 ohms, as opposed to three or four hundred for the very old, discharged battery (which was probably shipped with my B&W G3, judging by the date). That is why the battery I describe has three letters put on it in magic marker.

So, the point is that *open-circuit voltage readings are not helpful*, especially with older batteries that have been in a machine for a while.

I also have a "May-03" Tadiran that is similarly magic marked, which kind of suggests that I placed it in the G3 at about four years. That's seven years ago, so I don't remember if I had problems or just replaced it preemptively.

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