Article 27469 of sci.geo.satellite-nav:
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From: davem@cs.ubc.ca (Dave Martindale)
Newsgroups: sci.geo.satellite-nav
Subject: Significant voltages for GPS 38 and 45XL
Date: 3 Nov 1996 00:58:40 -0800
Organization: Computer Science, University of B.C., Vancouver, B.C., Canada
Lines: 63
Message-ID: <55hms0$c68@redgreen.cs.ubc.ca>
NNTP-Posting-Host: redgreen.cs.ubc.ca

Spurred by another news article, I make some measurements of the external
supply voltages at which several significant events happen in the GPS
38 and 45XL.  They are tabulated below.

All of these are *external* power voltage.  With the exception of the
last one, all are measured with a set of nearly-fresh alkalines installed
in the GPS.

                                                       GPS 38  GPS 45

- min. voltage at which "battery gauge"                 5.5 V   5.65 V
  disappears from status display

- voltage at which GPS just begins drawing some         5.75 V  7.1 V
  power from the external supply

- lowest voltage at which GPS stops using power         6.5 V   8.55 V
  from internal batteries

- lowest voltage at which GPS will operate when         5.12 V  6.25 V
  there are no internal batteries

Some notes:

In both models, the "battery gauge" disappears at voltage where the
GPS is still using 100% battery power.

Both have a range of external voltage where the load is shared between
internal batteries and external supply.

For both models, there is a range of external voltage for which the
GPS will operate perfectly well on external power only, but if
internal batteries are installed then all operating power comes from
them, not the external supply.

Garmin specifies 5-8 V external power for the 38, but you need to use
at least 6.5 V to avoid draining the internal batteries.  Below that,
take the batteries out.

Garmin specifies 10-36 V external power for the 45, and that works fine.
Even down to 8.5 V seems to be good.

However, 8V is not.  If you happen to have a cigarette lighter power
cable for the 38, you can connect it to the 45 as well and it appears
to work fine.  But the cable supplies 8 V regulated, and at 8V the 45
XL is still drawing about 1/3 of its operating power from the
batteries.  To avoid this, you either need to remove the batteries, or
use the 45 cigarette lighter cable that has no voltage regulator.

	Dave

P.S.
This also means that you can't build a "universal base station mount"
that would provide power and computer interface to any of the 38/40/45
by just plugging them in.  If the power supply is less than 8.5 V, you
would have to remove the 45's batteries before hooking it up to avoid
draining them.  But a voltage of 8.5 V is above the 38/40 voltage specs.
It doesn't seem to cause problems for the 38 in the short term, but
might overstress something in the long term.

It would have been *so* convenient if Garmin had designed the
two different power supplies so the useful external voltage ranges
overlapped, even just a bit.  But they don't.