I was talking today with a person who deals with placing electronic listening devices in oceans all over the world. Once they are in place he "puts them to sleep" so their batteries don't get depleted. When it is time to find them he sends out an encrypted signal and when the device answers he wakes it up.
The batteries will last a very long time that way. Perhaps all commercial aircraft could use this type of technology in the cockpit and for the black box.