External Power via Tether

Hi everyone,

We’ve begun building up our BlueROV2 and we are definitely excited about the build. I know that there had been a previous discussion back in May 2016 about power over tether, but I was hoping to re-open that discussion.

We aren’t looking for particularly long tether power…I’m just hoping we can work supply power town the tether on the one of the spare twisted pairs so that we can work longer with the vehicle and possibly push a longer tether length of about 200-meters or so.

My thoughts were to supply between 48 VDC and 120 VDC down a twisted pair and then use DC-DC converters on the bottom end to bring the supply to 15 VDC. There are concerns about this:

  1. Spec on the subsea wattage necessary for the system when it is running 6 thrusters and 4 LEDs.
  2. Heat in the tether.
  3. EMI in the tether or in the subsea end.
  4. Heat at the subsea DC-DC converters and managing that.

We can manage #4 by replacing the battery can with an aluminum housing and properly heat-sinking the converter there. And, based on what we’ve seen with other systems, their shouldn’t be a noise issue in the tether caused by the power riding near the Fathom-X twisted pair. I am worried about the subsea wattage, the possible heat in the tether (to supply that wattage) and the EMI from the DC-DC converter.

Does anyone have any thoughts on this? Or should we be leaning more towards the approach that Patrick Rowe took, to which @tciii referred, using 3-phase AC power?



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I believe that Rusty has mentioned that BR is working on extending the bottom of the battery by supplying supplemental power over the tether.
I do not know that any details are available yet so maybe Rusty will chime in here about what he has in mind.

Hi @jsnyder, this is something that we’re actively working on. We’re fairly far out from releasing a product though. There’s a lot of things to think about for something like this.

First of all, the tether only has 26 AWG wires, so you can’t do anything useful with a single twisted pair. You can get useful power through all of the wires (4 positive, 4 negative). Since the Fathom-X is based on an Ethernet over powerlines module, we can send DC power through the tether using all of the wires and use those same wires for Ethernet data.

As far as voltage goes, you can use a voltage drop calculator to estimate how much power you’ll get at the end of the tether. The maximum power output is achieved when the voltage drop is exactly 50% of the input voltage at the topside. For example, if you send 4 amps at 48V and the voltage drops to 24V at the end of the tether, the ROV will receive 24*4 = 96 Watts of power. That’s enough to keep things running and do some gentle maneuvering, but not enough to do anything substantial.

You can use much higher voltages, but I’d caution you against the safety issues with that. Most commercial ROVs use high voltages such as 360 VDC but they also include line insulation monitors (LIM) that protect against current leaking into the water. LIMs are not cheap.

Our approach will be as safe as possible while increasing the ROVs endurance to nearly unlimited. It’s still in the early stages so that could all fall through still.


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Thanks guys. Rusty - that tether gauge was part of the concern I had…I didn’t want to turn the tether into a toaster element by trying to drive too much down the line. We have another system that utilizes a LIM module so we are familiar with those (and the pitfalls associated with them if they are overly sensitive).

I like the news about the approach you are developing. It will be exciting to see that power solution as it comes together.

  • Jeff

Hi Rusty! Any update on this? :slight_smile:


Thanks for checking in! We’ve made a lot of technical progress and things are working as we hoped. Still working on transitioning that to a product but that’s happening.


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Hi @rjehangir

Any updates on this?


Hi Luis,

Our internal testing is going well but nothing to report yet! :slight_smile:


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Hi Rusty,

Any update?

Hi, Rusty

I am a middle school teacher and very new to ROV’s and you products. I have my student participating in the MATE ROV compitetion.
My question is: Is your tether sufficient enough to carry the current(amps) needed to supply the power to the T100 thrusters? We have a 5 thruster vectored setup and were looking to power (topside marine cycle 12v) two thrusters per teather and the two remaining wires for manipulator motors over a 45 ft tether. Is your 26awg/8 conductor sufficient enough ? We have tested and it seems we are losing to much power over that distance and thst guage. I am wondering if i may have a power leak or short. Motor seems to be trying to spin but not moving. I am thinking that a thicker guauge (maybe 18awg/8 conductor) will be needed. Can you advise please. The students are excited and ready to get this thing in the pool. Thanks

You will not be able to power 5 thrusters on 12V through any significant length of tether.

You may be able to manage if you limit it to 2 thrusters operating at a time, and keep the throttle commands under 20% of the maximum.

Generally, powering a vehicle through the tether is a difficult and potentially dangerous problem. This is why we usually recommend batteries.

Hi @Mac,

I actually wrote an article about using Blue Robotics components on competition ROVs that answers exactly this question and more! In the thruster section, I talk about voltage drop and how to select the power power cable for a vehicle depending on your current requirements. Our voltage drop calculator I mention in the article is an extremely valuable tool that I think will make this much clearer for you:

I had a great time competing in 2008-2011!


  1. There is connection between two Fathom-x tether interface boards,the single twisted pair wire is used for both power and data transmission between these Fathom-x boards.What will happen when 12V DC transferred through Fathom-x tether wire, the Raspberry Pi and Pixhawk only need 5V DC power for working? 12V power from tether wire make any problem to raspberry pi and Pixhawk?

  2. By convention, the pairs used for Ethernet communications are the green and orange pairs. The Fathom-x converts this two pair connection to a one pair connection, at the same time extending the range significantly how?


I can help answer these questions.

  1. The terminals of the Fathom-X tether input are isolated from everything else so if you apply DC power, such as 12V, that power will not be used on the board at all. The signals are capacitively coupled to the wires and won’t be affected by a DC voltage. To use that voltage for something useful, you will have to tap into it separately.

  2. The Fathom-X uses radio frequency communication through the tether. That means that, much like your cell phone, data is being transmitted on a number of difference frequencies. That’s a fundamentally different way to transmit data than the digital communication used through Ethernet pairs.


Thank you for your response