I recently posted on the subject of using power over Ethernet (PoE) thru the tether as a reply in the Open ROV electronics discussion. Since this topic will need research and discussion on it's own, separate from the workings of the on-board electronics, I will start a topic here. For convenience I will repeat the posting below:
At the June 9, meet up of OpenROV at Noisebridge in SF we discussed alternatives for sending power over the tether. Someone (forgot name – sorry) mentioned power over Ethernet (PoE) and we investigated that approach. See reference article here ( http://www.bb-elec.com/tech_articles/Power_Over_Ethernet.asp and http://en.wikipedia.org/wiki/Power_over_Ethernet
Basically you can inject 48VDC into one end of the Ethernet cable and split out regulated DC (12V, 9V or 5V) drawing 12Watts or 30Watts depending on which version of the 802.3 standard your PoE devices support (802.3af or 802.3at).
It would also eliminate the need for the Baluns (twisted pair adaptors - http://en.wikipedia.org/wiki/Balun ) and possibly one of the voltage regulators if this works out.
I purchased an inexpensive ($25) PoE kit (injector and splitter pair) that delivers 12W and supports 10/100Mbps (TP-Link PoE Adapter Kit Model No.TL-POE200).
I did a simple test and powered the BeagleBone with the 5V output from the splitter.
There is more work to do here. PoE likes to have 2 pair of UTP wires. I haven’t yet experimented with choices of cable (there are CAT 5 rated 2PR cables from Belden – 72001E Indus Cat5e 2PR AWG24 PVC).
Also I want to see if this can work with a lower voltage at the injector side (say 24V – from 2 small 12V batteries in series).
On the Splitter side, we may be able to adapt a standard unit (or buy the required center tap transformer) and step the 24V down to 12V (DC-DC converter) for the motors and draw a 5V charge for the electronics. We also discussed using a large capacitor (MegaCap) vs a 9V battery for reserve power for the electronics.
The assumption is that batteries are still needed for the motors since surge power would be more than 30W (12V@ 2.5A), but average power may be less than 30W. If we start with charged batteries on the ROV and add a trickle charge circuit from the PoE source, we could have extended missions for the ROV based on the amount of energy available on the surface.
Will keep folks posted as progress is made and if any one wants to pursue these ideas I would appreciate any help.