JamesCam,
I read you, but I'd almost say... don't bother.
Most "passive" equipment now lasts longer than the shelf life of the battery.
Look at your kitchen timer, or your watch.
The only time they drain the battery is when they go "beep"" for some reason, which uses a few hundred
milliwatts rather than
microwatts.
With current technology, a receiver that listens and only answers to an interrogator (rather than to every whale passing by) will, as I said, die only when the battery dies. Always assuming the battery is designed to continue functioning at the temperatures 4000m down.
Just thought of this while typing, actually
What
is the temperature 4000m down? And what does
that do to the currently used batteries?
CJ