# Power/Dive time for 2017

(TonyNJ) #1

Rusty,

Figured I’d start a new thread on this discussing the future plans of power and how it impacts power/dive time.

From previous posts, you had said you were looking to send 48v but now more than 90w down the tether…so that puts us at about 1.8 amps down the tether line. If we have that option, coupled with the higher capacity batteries you are looking into, how much bottom time would you estimate with 6 – 8 lights on a BROV2 under medium usage?

medium usage = dive 80 ft in ocean surge, with stabilization on.

If I could get 3-4 hours bottom time, that would be really great, any thoughts on where you estimate dive time being with both of those added power options?

-Tony

(Rusty) #2

Hi Tony,

The main thing to consider with power-over-tether is the voltage drop that will occur in the tether. I really like this calculator:

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=133.9&voltage=48&phase=dc&noofconductor=4&distance=100&distanceunit=meters&amperes=3.2&x=50&y=14

You can see that for a given input voltage, you end up with a much lower voltage at the ROV-end of the tether. This will fluctuate based on the load. The rest of the power is burned off as heat in the tether cable.

With 6 lights running at 50% power you’d have a power draw of about 45 Watts for the lights, plus roughly 10 Watts for the computer, camera, Pixhawk, and ESCs, and then the power needed to control the ROV. You’ll see that those 90 Watts get used up rather quickly.

Since you only want to go down 80 ft you could get by with a shorter tether and less power loss.

What sort of current speed conditions do you expect?

-Rusty

(TonyNJ) #3

Rusty,

I hadn’t thought about how much the voltage drop would be, that calculator is useful.

So I see that the voltage drop is quite different from say a 100 meter tether to a 25 meter tether. Is that difference in power something you would be able to automatically accommodate for on the ROV when using different tethers?

If I needed to go deeper, long term you could potentially market a heavier gauge tether, though I’m not sure the implications of ROV handling,etc…

Giving more thought on the tether, most of my intended target wrecks for the 2017 season are between 50 ft and 150 ft. I probably need some extra tether for underwater roaming, so I’m thinking either 50 or 75 meter tethers. (I don’t want to the tether snug either, as to prevent the boat rocking from tugging the ROV by the tether.)

As far as underwater conditions: The Atlantic off of NJ is tougher dive conditions that many areas. It does vary quite a bit with tide, weather,etc., but for the most part the current is light .5 - 1 knot, but the issue is mostly surges and swells that the ROV will need to constantly compensate for to remain steady. Visibility can be anywhere from 3ft to 25ft depending on a variety of known and unknown factors. Lighting is very important, as sometimes silt, plankton, and algae block much of the light on the upper layers, but down at bottom it can be much better vis.

I know I’m looking for a lot on the power front, though I think the BROV2 is the perfect size (no boat crane needed) yet big enough to mount accessories. Not sure how long it will take to surface and dive to my intended depths, but if I could get dive times to 3 - 4 hours between a battery change, that would be perfect.

Thx

-Tony