"Roger Felton" wrote in message
...
In *theory* the "standard" 7.5 AH battery will power your transponder for
8+
hours. Sealed Lead Acid batteries rarely achieve their advertised capacity
in
ideal conditions even when new. Now age that battery for a year or two.
Throw in
some repeated deep discharges. Now cold soak the battery for a few hours
at
altitude. I think you'll find that your 7.5 AH battery has something more
like 4
AH (or less) useable capacity.

Not my experience but then I buy a new battery each season for less than the
price of a tow. Then I treat the new battery carefully so it does put out
the full rated AH. (Recharge it immediately, don't leave it charging all
week and top charge just before the flight.)

Bill Daniels