Spektrum has had such an animal for surface systems for some time and in answer to repeated questions about them here on this forum when Danny Snyder from Horizon used to answer questions here, he said they were not for use with air systems.
As BarracudaHockey indicated, the introduction of the Quick Connect firmware update pretty much negated any need for such a device and it will tell you on landing if you've had a power system brownout.
The updgrade is free if you have rx's without the Quick Connect firmware.
http://www.spektrumrc.com/Articles/A...ArticleID=1756