BlueROV2 total draw current (from T200's and BasicESCs etc)

Blue Robotics Products Thrusters and Motors
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Hi BR team and community,

I am curious to know when the BlueROV2 is operated with maximum thrust (on all 6 T200 thrusters) (e.g. moving forward and up simulatenously); is the total thruster draw 150A? (i.e. the BasicESCs are all 25A each, so 6x 25A = 150A; at the battery nominal 14.8V voltage; I’m guessing - not sure if this is stepped up or down, or just regulated (to 14.8V) at the ESCs?).

If so (as in if 150A max continuous draw can occur), I don’t understand how the recommended (10AH, 10C) battery can handle this current draw (The max continuous battery current supply is governed by 10(AH) x 10(C) = 100A, I think?).

Is there simply a (‘current-limiting’) gain function applied (via code /firmware /software) when all 6 thrusters are engaged to maximum thrust simultaneously? e.g. so they can’t actually all be on (at 100% thrust) simultaneously?

Thanks for your help: I’m greatly looking forward to some insight on this!

Further, for an alternative battery; what is the maximum current I do need to plan for to be able to use the BlueROV2? (e.g. as above: with all 6 thrusters operating at once (and their ESCs, plus marginal amounts for the electronics etc).

i.e. how do I go about calculating wiring /connector requirements (e.g. for battery leads) and battery specs? (min discharge required to meet demand).
(I (loosely) understand the AWG system and designing wiring for current carrying requirements, voltage drop etc; its more just figuring out the actual BlueROV2 peak demands (both real; software limited?, and potential; with 100% of all thrusters enabled/driving); especially for calculating requirement per extra T200 thruster (and its BasicESC); for adding in another thruster to the system.)

I know there’s a lot of questions in there! - Thanks a lot for any help/insight into this!

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Mick,

There is no current limiting in software at the moment. The pilot inputs are limited by a gain setting that is set to 50% by default, which effectively limits the outputs to 50%. In most cases, all 6 thrusters are not running at the same time, when they are, it isn’t for very long. Most batteries have a ‘burst’ current rating for supplying higher currents for brief periods.

@adam can help here, but my understanding is that if the thrusters are drawing more current than the battery can supply, the battery voltage drops, which in turns lowers the power consumption of the thrusters. So the current rating on the battery is what it is capable of supplying (with some headroom), and you simply can’t draw more current than the battery can physically produce at a given voltage. This is pushing the battery to it’s limits and shouldn’t be done excessively, but won’t break the battery right away.

Current limiting in software will happen at some point.

-Jacob

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Hi Jacob,

Thanks for the reply. (I’m looking forward to hearing from Adam too for more insight).

I understand the user-controlled (25%-incrementally-stepped) gain function (great idea btw! - like a ‘novice’ vs ‘expert’ mode; or also great for more sublte movements/control) - I was just wondering if there is anything gain-wise going on ‘in the background’ (code) as well? - Sounds like not (yet*).

: perhaps this is reasonably easily done in the code? Similarly to how the gain is currently implemented for the controller; limiting the pwm signal to the fraction of 400 microseconds (400 _U_s = 1900 _U_s (max) -1500 _U_s (stationary)); that is desired per motor /ESC?
e.g. to limit it to 75% thrust give it the signal 1500+0.75400 = 1800 (If I understand this correctly - but, I guess that the thrust(or pwm signal)-to-currentdraw relationship is non-linear in which case the pwm signal would have to be modified based on the desired current-limiting amount; for say 20A vs 22.5A vs 25A loads per thruster; although I’m sure you’ve got the function for this already…that’s a bit of an aside from this post anyway).

And like you say; perhaps this ~150+A ‘burst’ current is ok anyway without need for extra (reduction) gain in the code. (Not so if anyone wanted to race these ROVs though! (in 3D; using all 6 thrusters continuously!) Haha).

However, as has been mentioned explicitly or otherwise, some Hobby batteries may not actually achieve their 10C rating anyway; so perhaps deliver less than even their deduced 100A max current…

If the (6) thrusters draw more than 100A total and cause the voltage to drop as you say, then the thrusters will produce significantly less thrust than is possible (based on the BR T200 thrust-voltage spec charts).

So technically a 15C+ 10AH battery would be better…

Regarding the BR battery: seems to be a similar-ish situation: the Samsung 30Q’s 18650 are rated for 15A each. In a 4S6P configuration thats 90A max continuous current (6Pcells x 15A/Pcell =90A’s total: which is what is stated on the battery: 5C x 18AH = 90A). (However I’ve seen it stated that Samsung actually ‘under-rate’ their cells significantly, rather than over-rate them (as some less scrupulous manufacturers might do); so say you got 20A off each cell, thats 120A max continous current (still not quite 150A to supply all 6 thrusters at ‘full-tit’; although perhaps they can even supply 25A each for ‘burst’ currents? - I note that the battery is stated with a 10s burst current of 132A (7C): 22A/cell; so thats pretty close and is likely completely fine for normal use.).

With regards to the wiring:
Both the Turnigy battery leads, and the BlueROV2 use 12AWG wiring from the battery to the electronics housing.
If someone can help provide insight as to why this is sufficient (?) I’d greatly appreciate it!

My basic calcs (using this online DC calculator); indicate that if we use the nominal voltage of 14.8V, and a 150A peak current flow over 0.5m, with 3% allowable voltage drop (recommended) this requires an 8AWG wire. Even if we used the fully charged voltage (16.8V: = 4.2V/cell x 4S); this calculator indicates we still need a 9AWG wire (not a 12AWG) (smaller AWG number is thicker wire & higher current-carrying ability - for those interested /learning like me). Granted the insulation on the wires is rated to 200’C; so do these cables just run really hot at max thrust?! - which is not generally ideal in the insulated parts of an ROV (WTEs).

(Note that these calcs above do not have any factor of safety (FoS) built in; the 0.5m wire distance is about the actual distance the wires run from battery (cells) to the power terminal block in the Electronics WTE.)

With regards to the connectors:
The XT90 connectors are rated for max continuous 90A (although they specify “90+Amps” on hobbyking; which is a terrible way to rate something electrical! IMHO); so these also seem to be under-rated if we want to pull the full 150A to get the BlueROV2 really moving! (full thrust on all props). Also, these XT90’s are rated at 90+A for up to 80’C; so if the wiring is running ‘hot’; (close to its 200’C insulation spec) with the 150A running through; then perhaps these connectors are doubly insufficient in this application? (i.e. the 200’C wire connected to the XT90 connector could heat the connector up over its 80’C spec, resulting in failure/metling of the nylon casing etc…hmm. Which does seem unlikely though, given the nylon melting temp (>200’C); so the wire insulation might melt first!. I wonder what FoS they have built in to both the wire and connectors: either current or temp wise…)

Thanks for any help /insight from an engineer!
(This is all new to me so I’m sure I’m missing something(s!); and it’s based on the assumption that there ‘can’ be a 150+A current draw in the first place (from 6 thrusters)…).

This would be really helpful information to anyone doing any mods on their BlueROV2 or designing an ROV from scratch around the T200 thrusters.

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This is an old thread, but I’ve got exactly the same questions as Mick (thanks for a great post btw!)

Since the BlueROV batteries can’t be shipped to our location, we had a local company manufacture something similar using the same Samsung cells. After the first few tests, when admittedly I did run 4 motors at full thrust with 100% gain for about 30-60 seconds, the interconnecting tags in the battery overheated and melted the battery jacket and eventually cracked.

The manufacturer is now rebuilding the battery for us using thicker tags, but when I started looking into the wiring, I found that 12AWG wire used for the battery in BlueROV2 is only rated to 34A, while the battery is supposed to handle 90A, and in theory the BlueROV Heavy config could draw almost 200A, if all motors ran at full power. How is this possible, and does this mean that our ROV has some “undiscovered” potential? e.g if we ran 2 batteries in parallel - but this would require 8AWG wiring, as Mick mentioned, correct?

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Hi @vozalexander,

The reason we use 12 AWG power cable is quite simple. That’s the largest gauge cable we could stuff through our penetrators, we tested thoroughly, and it seemed to work fine. After years of sales and customer use, this amount of copper has never caused a problem.

The long answer is current rating is not so simple as single number, in fact its more of a religious question where you ask 10 different people and you may get 10 different answers, all valid in their own way. Going one amp over a current rating will not necessarily result in things exploding, it will just result in more heating and voltage drop in the cable than what the rater deemed acceptable. What is acceptable may be very different in different circumstances. Also of note is that most current ratings are constant, and flying an ROV is far from a constant load. If the ROV was capable of drawing 200 A in a spike, using 200 A constant rated components everywhere would be total overkill, as the actual sustained average is much lower. As long as everything can handle the actual average and the occasional high spike, all is well. Its also worth noting that most of the cable is exposed to water, which cools it far more effectively than air.

On a recent practical ROV dive, we logged current draw over time, and found the average was about 20 A. Sure, there were short spikes up into the 60-90 A range, but these last no more than a second or two. In synthetic (tank) testing with all thrusters at max throttle 100% gain, a BlueROV2 heavy will draw around 110-120 A in an instantaneous spike, but this will quickly drop. There are couple reasons for this:

  1. Under heavy load, the thruster will not see the 16.8 V of a max charged battery, but probably closer to 15 V due to voltage drop. Actual current draw will be closer to 18-20 A per thruster at full throttle, and this will of course decrease as the battery discharges and voltage drops more.

  2. At full throttle, Ardusub does not run does not run all thrusters at full, the rear horizontal thrusters will always run a bit lower to increase vehicle stability.

  3. All our thruster thrust and current draw numbers are all at static conditions, which means the absolute velocity of the water entering the thruster is zero before it passes through. As soon as this number is non zero, either because the ROV has nonzero velocity through the water or the water in a small test tank is circulating, both of these drop.

  4. As current is drawn through the power system, things will heat up a bit and resistance will increase, leading to greater voltage drop and thus lower current draw.

In your battery you should be using solid nickel strips at least 0.12 mm thick and 7 mm wide. Bigger is better, and between the battery halves where current flows you should be using at least 3 strips in parallel on each side.

-Adam

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Hi Adam,

Thank you very much for this very detailed and helpful response, it is much appreciated!

-Alex

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A post was merged into an existing topic: Power Sense Module questions

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"On a recent practical ROV dive, we logged current draw over time, and found the average was about 20 A. Sure, there were short spikes up into the 60-90 A range, but these last no more than a second or two. In synthetic (tank) testing with all thrusters at max throttle 100% gain, a BlueROV2 heavy will draw around 110-120 A in an instantaneous spike, but this will quickly drop. "
I would like to know how to measure the current consumption described above.

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