Electronics Forum / Basics / April 2006

Hi, John.  Although there aren't any technical specs available on the
Radio Shack web site, I'd guess your FM transmiter doesn't use more
than a watt or two of power.  If you can remember to turn off your
Sirius radio, it should use less than a watt when it's off.

Modern cars do have some current flow from the battery when the car's
off, but it's small enough that it normally doesn't make a difference.
Usually it's around 1/8A to 1/4A.  It seems like you might have doubled
that.  It's still less than leaving a glove compartment light on.  As
long as you use your car every couple of days, and you've got a good
battery, it shouldn't be a problem.  Again, assuming you remember to
turn off the radio.

I would be a little concerned about your wiring, though.  You didn't
mention where you hard-wired the radio and transmitter.  I would hope
it's to the original radio circuit, or the cigarette lighter circuit.
But you should at least make sure your radio and transmitter are on a
fused circuit.  If they aren't, you could end up melting wiring
harnesses and giving yourself a major headache if a wire gets nicked or
something goes wrong.

Good luck
Chris