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[aprssig] Backpack Portable Battery Specs

Keith VE7GDH ve7gdh at rac.ca
Tue Feb 26 19:39:20 UTC 2008


Bob WB4APR wrote...

> The GPS and VHF Receiver are the biggest power hogs by a
> huge factor. The transmitter for APRS is usually insignificant..
> And for APRStracking, at a hiking pace, you usually don't need
> either the GPS or Receiver most of the time.

It depends on what Ev W2EV had in mind. He said "with a full-blown TNC"
so I'm assuming that he wants / needs receive capability. Of course, he
could use something like a Tracker 2 as well.

> So rather than lug a battery that is 10 times larger than needed
> just to power the receiver and GPS which are not needed most of
> the time, you might consider leaving them off except for when
> you need a fix.

That may or may not suit Ev's needs, but I can't answer for him, but if
he doesn't need to know exactly where he is all of the time, that may be
practical.

> In fact, leave the GPS at home too and carry a 1 oz map. Most
> hiking maps have lat/long coordinates, and most hikers know
> where they are. So, just key in your lat/long into your D7  at
> each rest stop, TX a few times, and then turn everything off.
> In fact, you don't even need a map.  Before the trip, just jot
> down an approximate LAT/LONG for key landmarks say, every few
> hours along the trail.  Then with this tiny list, you can
> interpolate where you are and enter in pretty good estimates of
> position all day long.

Here is where you assume that most people hike the way you do (just
going by your postings over the years) in good visibility, in non-rugged
terrain, and on well defined trails that you are probably familiar with.
The map and D7 may very well suit your needs.

First, all hikers should carry a good map and a compass if they are
going to venture off the beaten track. If they "know where they are" and
visibility is good enough to see a couple of landmarks (i.e. not
night-time, not cloudy, not snowing, there are actual landmarks and they
haven't been dropped into a situation in the middle of the night in a
snow storm helping out another SAR team in terrain they aren't familiar
with where every mountain around them looks the same, etc.) then a
couple of compass bearings can yield a position within +/- a few hundred
metres, or a bit more if the landmarks aren't separated by 90 degrees or
of the landmarks are ill-defined.. e.g. broad hilltops rater than a
prominent "Matterhorn style" peak. For hiking on a well defined trail,
just "following the trail" will get you there if there aren't any
left-right decisions needed at forks in the trail.

I probably think more like a SAR type because I did that for 15 years. I
used to hike and ski all of the same kinds of places where people used
to get themselves hurt or lost. I was very familiar with the terrain. In
the "early days" GPS receivers didn't exist, but I always had a radio
along as well as a map and compass. In the later years I was involved
with SAR, I didn't go anywhere without a GPS receiver. I still don't but
I haven't been in a SAR situation for years now. Even without APRS, I
was always interessted in exactly where I had hiked and skiied and
downloaded the track when I got home, so the GPS receiver stayed on. For
the last several years I was involved with SAR, I dreamed of APRS as it
is today with relatively small equipment that could go out with a team
for 24 hours and expect it to still be working at the end of that
period. Of course, they would have spare batteries too! For SAR
managers, it is very important that they have an accurate track of where
a team has been. Knowing (exactly) where they have been can be a major
part of making decisions on where to move resources to. Downloading a
track when the team gets back to the SAR base was great. Having that
available in real time would have been invaluable.

Having a GPS receiver removes a lot of uncertainty. Most of the time you
can count on it working. Most people start a trip with new or freshly
charged batteries. Most people carry spares. Besides, a GPS receiver
makes a good paper-weight for maps if the batteries fail or if you drop
it and break it! It's all part of "being prepared".

> If you really want continuous tracking, consider what we are
> doing for our Buoys, that is, turning on the GPS for 30 seconds
> only every N minutes. Then turning on the TNC for the last 5
> seconds of that time. Then waiting 5 more seconds and turning
> on the Radio just in time to TX the posit and then shutting
> everything down until the next cycle.
>
> GPS takes typically 50 mA?
> RX takes typically  50 mA?
> TX takes 500 mA for 1% of time averaging 5 mA
>
> So you see, the GPS and receiver are the biggest power hogs by a
> factor of 10 in battery consumption.

What you are describing is probably practical for new GPS receivers that
can get a lock really quickly. Ev didn't really ask about GPS receivers.
He was asking about batteries. Wes gave a really good breakdown of
current consumption. Gerheim indicated he could put out position reports
(terrestrial and via satellites) with a D7 and a GPS receiver. You
indicated you could do it with just a map and a D7. Perhaps Ev could
kick in with a few more details of what he hopes to achieve and for how
long. If he wants "cheap" he can carry a (heavy) lead-acid battery with
him. If he wants "easily available" he could go with battery packs that
take readily available AA cells. If he wants something with good shelf
life, he could go with non-rechargeable Lithium cells. If he has means
of charging, he could go with NiCad or NiMH. For the absolute lightest
battery, go with the older Li-ION or a newer Li-PO and a switching
regulator. The manufacturers seem to be promising that big breakthroughs
in battery capacity are within sight. For extended trips, solar panels, 
hand generators and wind generators can be used as well. It just depends 
on what your power needs are and whether you need that capacity 24/7 for 
the duration, or if you can turn everything off sometimes for charging.

73 es cul - Keith VE7GDH
--
"I may be lost, but I know exactly where I am!"





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