ROV Peak and Constant Current Draw


#1

Hello! I think it best to give a little background first before I ask any questions. I am currently working on my MSc project, which is to design and build my own custom ROV system. The system was originally going to be based off the 2.8, however as the project has developed, it has departed further from the openrov to a point now where only a couple of components are shared between the two (Motors, ESC, Propellers & battery layout are the only shared features now), and I have come up with my own solutions based on my own knowledge and skills. The system uses a raspberry pi 2 running ubuntu 14.04 and ROS, using a usb webcam for video capture, and an arduino mega for low level motor control, IMU interface, etc. Connected via ethernet cable to ROS laptop on the topside, allowing for distributed computing. I decided to follow this path as, hey its my MSc, where is the merit in just cloning someone else’s work, plus I know ROS and Arduino well enough to challenge myself at this point. I am happy to report that after a couple of weeks I have my software down, and can control all my basic functions with a 360 controller with live video stream, and am now in the fabrication and hardware phase.
My system uses a different hull design, but the same 3 thruster layout as the openrov, except I can change the angle of the two rear thrusters (vertically) independently via 2 servo motors (they’re in sealed chambers and magnetically linked to the thruster pods, so no issues with water). I know my power budget for most of the system, but I am still waiting on some components to arrive, so can’t properly test. My main issue is the batteries which I will probably have to order from the US (UK here), and with shipping times and budget I can make only 1 order, so I need to be sure they’ll work. So from what I understand the OpenROV uses 2 banks of 3x 3.2V LiFe batteries in parallel so they’re able to provide 19.8A continuous current to the system (9.9A each), although I can’t find any information about instantaneous current. So my question is how close to the current limit does the whole system run in a worst case scenario situation (All systems on and switching all motors on simultaneously)? As my own system will have a heavier current draw given the 2 additional servos constantly operating. I am trying to assess whether to use the same batteries (I use the same 2x bank of 3 layout), or instead use these batteries from A123 systems which sacrifice capacity (3300mAh down to 2500mAh) for much higher current limit (70A each, with instantaneous 120A). http://www.batteryspace.com/A123-System-Nanophosphate-LiFePO4-26650-Rechargeable-Cell-3.2V-2500-mAh.aspx
Obviously if there is sufficient overhead in the OpenROV batteries I’d much rather use those due to the higher capacity and lower cost. If anyone is interested I’ll try to make a post about the project after it’s complete, as currently I have 3 months left before it has to be complete, along with a 40000 word report, so I don’t have much time to do so right now. Thanks for taking the time to read this far, and sorry for a wall of text.


#2

An OpenROV 2.8 will draw about 3 amps average on a mission, thus our claim to a 2 hour battery life using 2 3300mAH battery packs in series.

Each thruster will draw about 7 amps running full throttle under bollard pull conditions. So with both horizontal motors running full throttle, the ROV will be drawing something like 15 amps.

-W


#3

Hello there,

I need the same information but what i’m trying to do is to power it using DC power supply.

So if each motor draw about 7 amps, do I need a power supply that can supply at least 21A? At what voltage? I only have the ESCs and brushless motor from openrov store.


#4

The current battery configuration provides about 9.6V at full charge, and the previous config provided about 11V. The battery setup has 2 banks of 3 batteries in series. Each battery provides 3.2V @ 9.9A constant and 19.8A peak. What length tether do you plan to use? If your tether is too long and the conductors are not thick enough you will have a problem with voltage drop at end of the tether. You can expect to lose about half your power through the cable and this will vary with load. If you use a big chunky cable it will create drag and the ROV will not maneuver well. There are other problems associated with power over tether. You will need a PSU that tracks voltage and current. But I recommend that you use the standard battery config. You will not get a power supply for $60 plus shipping that performs as well as the batteries. There is more info in this post Thinking of sending DC voltage down cable


#5

@Brendan

I see this is getting rather too complicated and expensive, now I don’t know what tether cable and power supply to use with that much current and voltage, it’s become too thick and way to expensive.

My actual problem is I can’t get the same battery as with in the openrov, would it be possible to replace them with NiMH batteries (1,2V, 2800mah, other details unknown) instead of Li-FePO4 (3.3V 3300mah)? If I could, how much of it would be sufficient and in what configuration for around 1 hour of operation?

I noticed openrov is claimed to be able to run for around two hours with 2 banks of 3xLi-FePO4 = 9,6V (6x3300mah) = 9.6 V 18600mah for two hours correct?
Can I get by with 8 NiMH batteries in series?
That would give the same 9.6V 22400mah isn’t it?
Since I don’t know how much current it could provide (known as the C rating I believe, I have zero experience in this field) and I’m sure it’s way less than a Li-FePO4, would it be a good idea if I parallel another 8 NiMH batteries to boost possible supply current?

Or regular 18650 batteries would be good?


#6

You can replace the 26650 lithium batteries with NiMH, but it’s going to take a bit of work on your part.

A good quality “C” NiMH cell is about 5000mAh at 1.2V. If you put 8 of them in series, you’ll have a battery pack that is 9.6V at 5000mAh, compared to 9.6V at 6600 mAh for the stock lithium batteries. 4 “C” cells will fit into each battery tube, though they might be a little tight- you should check fit this first before doing all the other work. You might need to get new battery tubes that are slightly longer.

The battery tubes need to be connected in series, so you’re going to have to change the external wiring of the battery tubes. Since the full current draw of the ROV will be going through each battery tube, you should consider omitting the polyfuses in each battery tube, as they will limit you to part-throttle operation if you don’t. You should also bring the power into the ROV electronics through both sets of power pins on the DB-25, since the blocking diode on each individual set of power pins is not rated for full-throttle operation of the ROV.

None of this should be difficult for somebody who has experience building electronic stuff. If that’s not a description for you, I’d find a friend who does have experience with building electronics to help you with this.

If you decide to rewire your ROV for NiMH batteries, let us know how it goes, and please post pictures to the forums.

-W


#7

Awesome. I decided to use 8 of NiMH then, and going to use my own electronics and battery compartment. I do know a little bit about electronic but what’s completely new and I have no idea of is all these RC stuff (ESC, types of motor, types of battery, C rating, etc.), but will the current draw will be sufficient though from NiMH batteries compared to lithium? Burning my ESC and motor is the last thing I want, they’re expensive, and hard to get.

That, or car/motorcycle battery over the tether. Cheaper but harder to do :worried:


#8

From doing some quick research, most of the NiMH “C” cells have a Max discharge rating of between 6-7A. You should be okay with one bank of 8 cells on the lower thrust settings, but if you want to run things flat out then I would suggest a second bank of 8 cells. I had a quick look at some 18650 datasheets and the max discharge rates on can vary from 4.5A to 30A, so performance could be better or worse depending on the battery. Good luck with your project. :slight_smile:


Powering with DC power supply? How much current and voltages?
#9

Thanks! I actually have tried it with 8 x NiMH, and can run all three thrusters at medium speed just fine, but batteries are depleted real quick, like, ~4 minutes quick. Will try to experiment with car battery then 18650 if car battery failed too.


#10

That’s great you were able to get it up and running :+1: You should also keep in mind that the thrusters will use more power in the water than when they are running unloaded in air.