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Battery for peak shaving in power distribution units

(Oystein Skarholm) #1

My BR2 is powered through the tether, running on 400volt DC from the PSU topside to the PDU installed in the battery tube. Then the voltage is converted to 14v ( not sure on exact number)
If and when setting gain to 75% and being “hard on the trottles” I will loose communication. I believe this is created by a voltage drop.
I would like to insert a battery that would prevent this instant power drop to occur and thus supply the current and voltage necessary for a short period ( say 2min)

Is there anyone of you brilliant minds in here who can explain how this can be achieved, including a diagram and a part list ?

(Jacob) #2

The simplest way to mitigate this is probably with just a large capacitor across the output of the DC-DC converter (@patrickelectric?). While the capacitor won’t power the ROV on it’s own for 2 minutes, if it has enough capacitance it will reduce the voltage drops.

There are also plans for current limiting in software in the next release which may help with this…

(Patrick José Pereira) #3

Hi @SDI,

Probably you are not able to feed the electronics with your PDU when using this thrust, to solve this, depending of how long you are going to use this amount of thrust, you may want to test with a capacitor, like @jwalser said.

If you are going to use this thrust for a long period, maybe you should aim for:

  • This cut-off system, probably will solve this and maybe future problems that you may have.

  • A 5V battery with a recharge circuit using the PDU output that can deal with the necessary amount of current.

(Etienne Demers) #4


To be quite honest, your question makes me very worried.

If you already venture on supplying surface power then your level of electronics should be sufficient to answer such a basic question.

If you do not have any level of earth leakage protection, 400VDC could kill you.

Are you using the Fathom tether?

If you want any help, you should be more open about how you are setup. Send diagrams and specs so we can see where the fault is. Else, we can only speculate and in this case, it could be fatal.


(Oystein Skarholm) #5

Thank you for your concerns, I appreciate it. I know my limitation in electronics and thus I have not set up the surface supply myself. The system has been purchased as is. And it is using a purposed made tether.
I ask the “stupid questions” so that that I can learn about other peoples experience on the subject.
The manufacturer is working on a gen.2 of the the power supply to sort this out. However, I am investigating on my own to see if I can source up any proven solution and experience on the subject. Prototypes is getting better all the time based on feedback and experience. I am simply trying to collect all the data and experience on the field so that my decision making will be based on knowledge when I choose to purchase a product. I have no plans on making it myself. I am an end user and I make my money from using the products. Spending time making them will be to time consuming and costly. But again, purchasing products that is still being improved also rises challenges as “bugs” gives operational setbacks that costs money. So I like to gain a certain knowledge of my equipment.
Hopefully, you and everybody else will continue to give great input. In my case, I am looking for suggestions and not necessary the solution. Having been provided with two suggestion now, capacitor(s) and battery I have been reading about it and somewhat made up my mind of which I prefer.
Now I can choose to ask the manufacturer if my preferred solution is the best based on the conditions of use compared to cost of a brand new PDU. If adding batteries inside the PDU can give the extra boost for the necessary percentage of the time, then maybe i can save the cost of buying a new PDU (which again will include new PSU ant tether because of higher voltage)


(Etienne Demers) #6

Hello Oystein,

That is reassuring :slight_smile:

I am of a mind that I prefer addressing the problem VS patching it.

Your problem can be caused by:

  1. A underrated power supply
  2. An underrated tether

The more you use your thrusters the more power it needs.

BR specs say their thrusters has maximum power of 350 Watts at 15V. Lets say 325 Watts at 14V.

If you are using all 6 at the same time, you will require 1950 Watts + whatever your control system, cameras etc needs.

Most power supplies nowadays have shunt circuits, so if you are asking it too much current, the voltage will drop to prevent damage.

If this is what is happening, then you will need to limit your thrusters to 50% because the shunt circuit will overheat your power supply. Furthermore, you are not getting more power out of your thrusters as the voltage is dropping anyway.

The more current goes through your tether, the bigger the voltage drop. If this is affecting so much that it drops your 14V supply enough to cause a failure in comms, you risk having a tether overheating and exploding.

Hard to tell exactly what your problem is without knowing anything about it but one thing is sure, patching the problem in my opinion is a mistake.

If the voltage drops, the thruster looses efficiency. You are therefore gaining nothing. Best leave it as is and operate within the limitations of your power supply / tether.

Also, adding a battery or capacitor would require you to isolate the control system from the thrusters or it would not achieve much.


(Oystein Skarholm) #7

Thank you for the answer.
I know its two different worlds but in the world of ships these days (diesel electric propulsion) they are saving lots of fuel by using battery as peak shaving. In stead of using say 2 generators running at outside best efficiency RPM , ready to boost up during wind gusts etc, they can now run on one generator at most efficient RPM, charging the battery at the same time as they run the motor(propeller) Should the motors require more power then the battery will supply that. No need from more generators. I believe something must be possible to do with the ROV as well? The PDU gives sufficient power 90% of the time during a typical pipe, cable inspection going from shallow water at the shore fall, to deep water and then back to shore fall. At the deepest point it would be good to have some more juice stored in the bank. Even if it last for only 10 minutes.

(Etienne Demers) #8

Ok I see where you are coming from.

You can charge a battery when less power is required and when more power is required use some from the battery…

In this case you could use your surface power to power a charger, you would need two battery packs and a circuit like Patrick suggested.

But this will significantly add to your weight.

You can also try super capacitors which would not need any of the above but would support you for only a short period since it doesn’t store as much energy.


I am working on a surface power solution as well. I will let you know how it turns out. I’ll share it on the forum when I am done.

Can you tell me how many watts your present power supply is?

(Oystein Skarholm) #9

Thanks for sharing. I am currently offshore and will look into the specs when I can get to the ROV,
I saw a smart looking DC/DC on-line UPS that might also be what I`m looking for. Added weight is no concern (within reason) as the ROV is to light weighted for its own power as it is now. Getting some ballast in the basement and more buoyancy on the roof is just what it needs. :slight_smile:

(undersearobotics.com) #10

I ran into this exact issue with an ROV I designed several years ago. The solution was a large capacitor (as Jacob suggested). FYI - Quite a few commercial ROVs use high voltage through the tether to power the system. If I’m not mistaken, Seabotix uses 400 VDC and VideoRay uses (at least in the older systems) 48 VDC. If you want to send power down a small gauge wire raising the voltage or going to AC are really your only options. Yes, there is some added risk (which is one of the benefits of a battery powered ROV) but it’s a very common practice in this industry.

(Todd Sparkes) #11

You are correct Paul it’s a very common practice in this industry. The problem is once you do that for safety and by law on some jobs you need to have Ground Fault Monitoring circuits, GFI, LIMS and emergency stop circuitry. Now your system is not mobile anymore and of course personal safety now becomes a factor especially when handling the tether.

(Etienne Demers) #12

With regards to safety a standard GFI breaker should do the trick. https://www.aliexpress.com/item/10A-2P-RCCB-Circuit-Breaker-CDL7-63-DELXI/32584312780.html?

As long as you put it upstream of the bridge rectifier (it needs AC) it will trip at 30mA current loss for anything downstream AC or DC provided that no transformers are downstream.

Use a current transformer upstream of the breaker with a bypass switch for emergency recovery. The current transformer will give you a means of measuring the current leak even after the breaker has tripped if you are in “bypass” mode.

Note that it doesn’t take many amps to kill you (less than 1) So caution is important if in bypass.

(Luis Gamez) #13

So have anybody else tried to feed the ROV from the surface?? anybody who has developed their own DC converter and its actually working?

(undersearobotics.com) #14

I haven’t tried it with a BlueROV2, but years ago I designed an ROV and powered it with a 48 VDC power supply (topside) and a 48 to 12 VDC DC to DC voltage converter in the ROV. It worked pretty well but because of voltage drop in the tether was limited to 100m maximum tether length.

Here’s a photo of the ROV and some of the components I used. Hope this helps.

(Steve) #15

I had a similar problem when running four T200s, but it was my LiFePO4 lithium batteries hitting their wattage limit. I ended up running my Arduino on a 2200maH NiMH pack and trickle charged it with the main lithium pack. The NiMH pack was cheap and way simpler to charge via a trickle that Li-ion.

(Luis Gamez) #16

hey @paul-unterweiser

Can you share more details about your components??


(Oystein Skarholm) #17

Getting 400Volt surface power and it is working very well when not having to use full power on all thrusters - Sorry if giving the impression that it does not work, bacauce it does work well and I have been operating it for up to 14 hours in one go. I just want more from it, espesially if I am to install two more thrusters.
The challenge the way I see it is: The tether is long (300m) and also kept at smallest minimum diameter to minimize drag (its the tether that is dimensional factor here). That is the reason for looking into having the possibility to add an extra power source that can boost the system when needed. An online DC-DC UPS that allows power through and charging the batteries when spare capacity might be my solution, It primarily when operating from a boat the current drop is noticeble due to drift-off, current and usually working on deeper water. Changing the long tether out with a TMS and a short (100m) tether and a larger capacity cable from the surface is another way of mitigating the problem.

(undersearobotics.com) #18

I built this ROV over 8 years ago so don’t recall exactly what I used, but the topside power supply was something like this and the ROV side converter was something like this.
This ROV used a tether similar to this cable by Outland Technologies.

(Etienne Demers) #19

I live in the Philippines and we have 220VAC from the wall. I plan in fully rectifying it and have a bank of capacitors topside. This will give me about 320VDC.

Shooting this down a tether and going to DC-DC converted that will drop the voltage to 15VDC.

I plan on using a 3inch aluminium enclosure and filling it with enviro friendly liquid coolant.

Might also have a partition to put a rear facing camera.

Once Oystein is back and can tell us how many watts his present power supply is, I’ll be able to figure out how many DC-DC converters I need. (They are very expensive) And what size wires I need in the Tether.

I am planning 2 conductors for power and 2 twisted pairs.

By the way, if people can share the price per meter for the tether they have found it would be great.

The bigger the order the cheaper it costs. Maybe we can get together and buy a large quantity. That will bring the price down.

One manufacturer gave me these prices:
6.50GBP/meter (600m)
8.25GBP/meter (300m)

for this tether:

Another one gave me these prices:

Cable code ECL2023
2 x 1.00mm2
2 x (2 x 26AWG)
Kevlar braid strength member, breaking strain =>45kg
Slightly positively buoyant
PUR outer sheath
Quantity 1000 metres
Price EURO6.63/metre x 1000 metres