On Jul 2, 9:21*am, Chris Nicholas wrote:
Can anybody tell me what battery life you get in practice, when
running a transponder from a dedicated lead-acid battery, and what
size battery that is? And what model of transponder is it?

Has anybody measured the actual current drawn during operation, if so
what is it?

I am particularly interested in flights which include some high
altitude, cold ambient operation such as in parts of North America.

I am only interested in modern, solid state transponder operation, not
the older Mode C with a heater for the height encoder.

I have been given figures which suggest that a 7-amp-hour battery
would run one for 12 hours at surface ambient conditions, and I wonder
if anything like that is achievable in practice.

Thansk - Chris N.

Chris N. (In UK, but collecting data from anywhere that has it!)


Many "modern" transponder installs in the USA will still have a heater
for the encoder. In the USA the Becker ATC-4401-1-175 Mode-C is pretty
common with an external encoder and the most popular encoder is an ACK
A-30 and that has a heater.

There is no single number that is meaningful to answer your question.
e.g. AGM batteries have significant temperature related effects. What
will work fine on one flight won't on another if the electrolyte
freezes. Most glider pilots however don't fly in those cold conditions
(but those who do need to be aware).

What problem are you trying to solve? Do you need to add a battery to
a glider and want to work out how big? For what flight profile? Are
you specifically worried about cold soaking?

This sort of question, prompted me a few years ago to give a talk on
glider batteries, the slides are at http://www.darrylramm.com/glider-batteries/
Hard to follow without speaker notes but you'll see some relevant
things. Do your own power budget, factor in your own battery temp and
aging factors and if nothing else take the manufacturers numbers for
the transponder (and encoder if separate). Or worse case use the
numbers for the Becker and encoder I have in the slides - those
manufacturer numbers are pretty good in practice. More recent
transponders will do better. I'd like to see actual measured numbers
for the Trig TT21. BTW the slides don't talk about circuit breaker of
fuses losses, I just did not have time and have no updated them. Just
don't choose really low value circuit breakers and you'll be fine.
Technical folks can look up the breaker actual specs and add those
into the budget.

So some wild ass numbers... Even a Becker ATC-4401-1-175 with ACK
A-30 encoder (with a nominal power draw around 0.49A) can give around
12 hours off a 7Ah battery at nominal conditions. A typical AGM
battery at 0C might be 15% down on it's spec (measured at 20C). Things
get "interesting" around -20 degrees where the electrolyte in an AGM
battery starts to freeze. The thermal mass and the somewhat natural
insulation of an AGM battery means the internal temp will take a while
to soak down. I had plans of inserting a thermometer into an AGM
battery but never did that. If that is a real concern on say long cold
wave flights you need to look at solar panels, battery insulation, pre-
heating batteries or different battery technology.



Darryl