[aprssig] GPS/Puck or Other Wise

James Jefferson Jarvis jj at aprsworld.net
Sun Jul 10 08:55:47 CDT 2005

> On Jul 9, 2005, at 4:14 PM, Stephen H. Smith wrote:
>  4)  Why worry about switching the GPS on and off at all?   A device
> like the Garmin GPS-18 powered by a decent switching mode regulator
> (not a 7805 linear device) only draws 50-60 mA at 12VDC which
> translates to about 1 Amp-Hour a day.   You can leave this device on
> for a week or more without affecting the car battery significantly. 
> The advantage is that the  GPS is always locked  and has a currrent
> ephemeris and almanac ready-to-go.

It's doesn't work quite so well in cold climates. Here is the midwest it gets 
to -20 degrees F for a week at a time and that will kill your battery if it 
has any loads on it. 

"The standard rating for batteries is at room temperature - 25 degrees C 
(about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity 
drops to 50%."

The above information is from "The Deep Cycle Battery FAQ" found at:

Besides the decreased capacity the fluids in the engine are more viscous at 
lower temperatures which requires the starter to work longer and harder to 
get the engine started.

Two winters ago the GPS (7805 powered Garmin 25) on my truck would kill the 
battery if left for more then a few days without driving. Even in winter time 
in Iowa I still do most of my traveling on bicycle, so when ever I would go 
to use my truck the battery would be dead. I thought the problem was a weak 
battery, so I replaced the battery with a bigger battery. Still had the same 
problem. As an experiment, I disconnected the GPS and after a week the truck 
started up fine in sub-zero temperatures. 

Blatant plug: From that experience I developed my APO circuit which turns off 
a GPS or other load when the vehicle battery voltage drops below a a 
threshold. After installing the first APO prototype two winters ago I've 
never had my vehicle not start. You can read about the new APO circuit at:

There's another possibility for people using the TM-D700 and a GPS. When the 
radio is one there is 8 volts going to the microphone. Put a reed relay in 
parallel with this line. Use the contacts on the reed relay to turn the GPS 
on and off. Now whenever the radio is on the GPS is on. I can draw up a 
schematic if anybody is interested in this.



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