For you guys looking for a cable solution for the PNA's..........Make
one out of a simle car cell phone charger. *It steps 12V down to 5V
for most cell phones. For club ships we have a small gel cell comitted
to the PNA that get's moved between gliders.

Ipaq 31x chargers output 3.3vdc on pins 2 and 3 in addition to 5v and
neg on pins 1 and 5. Failure to feed it that will allow it to continue
draining internal power while in use. USB pins are limited to 500mA
and the 310 eats 700-800mA on full brightness so a standard single pin
5v arrangement is not adequate. The extra voltage kicks it into 'fast
charge mode' and allows it to drink external power while preserving
it's internal battery...

Having a loose gelcell sounds like a potential missile or control
jamming hazard (assuming the club ships do not have a proper battery
box to mount it in...). There are slick/small/cheap and readily
available 5-5.5vdc lithium-ion backup batteries around that even come
with their own chargers and work quite well (get one that can
discharge at 700mA or better for a 31x...) for this purpose. Commonly
the 3500mAh model is used for this, which is smaller than a deck of
cards but provides some 5 hours of power for a 31x on full brightness.
I have a 7200mAh (almost exactly the size of a card deck) unit for
using it outside my own ship which is equipped with a 12Ah 12v SLA
batt to a 5v converter.

All the 31x external battery solutions still require a cord with pins
2 and 3 jumped to the 5v pin, either bridged with the appropriate
resistors (my preference) or resistor network, or even just straight
bridged so they all have 5v (others have told me this is OK for power,
but not communicating data). The stock car charger/cig lighter cord
for the 31x already has this built in of course, should one want to
use 12v batteries and keep life simple (which goes completely against
being a glider pilot , although not as elegant or safe of a solution
for portability as the 5v options using a modded cord...

-Paul