[Ham-Mac] Using external battery to power a PowerBook

[Dick Rucker]

I want to power my G3 Pismo PowerBook from external batteries for an
extended period (say, an ARES outing of 1 to 2 days).  What I have
determined so far is:

The external power adapter for the G3 is rated at 24 vdc at 1.875 amps

Catalog listings for replacement internal batteries for this PB seem to
range from 10 to 12 volts, and are quite expensive even for modest
capacities, relative to the gel cell batteries available at hamfests.

Since I have a pair of 12v, 38 AmpHr batteries that aren't being used for
anything else, I thought if I could rig a connector that would fit the power
adapter jack on the back of the PB, then I could supply it with either 12v
or 24v dc that way.

I obtained a burned-out yo-yo power adapter for a Pismo, cut off its plug to
the G3, and then tried to find out how it was connected.  Since the yo-yo
power supply was good, it showed around 24 vdc across the white center
conductor and the shield, with the center conductor positive.

The plug was another matter.  Since it had shorted and burned out right
where the cable entered the plug, I cut material away until I found two
small wires potted in a sleeve and leading to what amounts to an 1/8" stereo
audio plug inside a cylindrical sleeve.  The sleeve connected to the shield
on the cable.  The white covered wire was connected to the inner-most ring
of the plug, but I couldn't tell whether the black was connected to either
the middle ring or the tip.

So I took the simple-minded approach and made up a new cable with a
connector for either one battery, or two batteries in series, and applied
the positive side to the inner-most ring on the plug and the negative side
to the cylindrical sleeve.

I booted the G3 from its internal battery, then plugged in my external
batteries, and monitored the current drawn.  Regardless of whether I had one
battery, or two batteries in series, the current drawn by the G3 when the
external power cable was attached was about 25 mA.  So obviously, the G3 was
still using the internal battery and ignoring the external supply.

That makes me think that the black lead was connected to either the tip or
the middle ring and has a role to play in getting the G3 to accept power
from the external power jack.

Ideas anyone?

Dick, KM4ML