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Battery size and type recommendations?


(Paul) #1

I’m considering building a variant of the BlueROV and am trying to calculate how big a battery bank I will need. The charts in the thruster docs are very helpful, but I’m curious what batteries are you using in the R1 for testing? What are other folks using? Is anyone making provisions for larger battery banks than will fit in the 4" housing?


(Harold Scadden) #2

Paul,

The big question would be how many thrusters will you be using … what is the average RPM range that you will use them in. What are you going to have for power loads from other items.

I would take the chart for the thrusters and assume that you are at least full power the whole time. The rating on a T100 thruster is 135 watts. Ok, now what?

If you are looking at your typical batteries you see the “milliAmpere Hour” ratings. So lets play math.

Your battery is a lithium pack that outputs 11.1 volts with a mAh rating of 4400 mAh. Now you convert this to see how many “Watts” this can output for an hour. You want your “mAh” value in “Ah” so you are dealing with amperage in a whole value vice on a 1000 scale factor. So 4400 mAh / 1000 = Ah. We end up with 4.4 Ah.

Now you take the voltage x Ah and this will equal Watt-hours. In this case it is 11.1 volts x 4.4 Ah would equal 48.8 Watt-hours.

If your burn rate with the thrusters is at maximum … 135 Watts … then you can run it at that speed, assuming this is the ONLY load on the battery for, ( 48.8 Watt/hours divided by 135 Watts) = 0.3615 hours or 21.69 minutes.

You need to total up what the power consumption requirements are for all of your electrical loads and then I would add some slop to it because somethings like the ESC are going to burn “X” amount of power but I don’t see a consumption rate for the circuitry itself. You would have to measure the input current and assume a certain output from the thruster charts and see what the difference is. That would give you your power consumption for the ESC doing its’ job.

Once you have the total power requirements … determine how long you want your run time to be for a “dive” and do the math for how large a battery you would need. Just so we can have a practice scenario, lets say you are using four thrusters theoretically at full thrust (I love to assume worse case then I don’t loose sleep at night) and the power of your camera, control board and say some LED lights adds up to 15 Watts. So four thrusters (135 Watts each + 15 Watts) = 555 Watts. Lets assume you want one hour of run time for worse case power consumption. You will need 555 Watt-hours of battery life.

Here is a link to a 148 Wh battery with a dimension that would allow you to pack some of these puppies into a single enclosure. http://www.batteryspace.com/High-Power-Polymer-Li-Ion-Battery-14.8v-10-Ah-148-Wh-with-PCM-at-25-Amp.aspx

The other thing you have to deal with is the discharge rate on the batteries. You will need to wire your batteries in parallel configuration to get the discharge rate. Two of these packs in parallel would give you a possible run time of 30 minutes give or take a few minutes. We want one hour. So now you need four packs wired in parallel to meet or exceed your power requirements for your desired run time.

Since those batteries price at about $241 each that is well over $1000 with shipping etc.

I didn’t want to make this a War & Peace post but I think you get the scope of the answer now. Lots of questions to ask … and lots of calculations to look at.

 


(Harold Scadden) #3

Forgot one thing. The design that I have been working on has housings JUST for batteries. Depending on the discharge rate the batteries are going to get pretty stinking hot. Just because the ROV is surrounded by water doesn’t mean that the batteries are going to be cooled.

The other nasty fact of life is buoyancy. It can really make your day suck. You will need to calculate the buoyancy value of the enclosure that you are using. (NOTE to Rusty … you might want to add this value to the specification for each of the enclosures as a theoretical value for fresh and saltwater as an “empty” container. It will help people get an idea of how much stuff they can pack into one.)

Once you have the buoyancy value, that will tell you how much weight you can add until you end up going negative for the value. Ideally it would be nice to hover around Zero so your thrusters don’t have to drag dead weight around or have to keep the craft submerged because it wants to float all the time.

So the moral of the story, batteries are only one of your concerns. There are a lot of other variables that are going to go into play. Now the nice part is you can use buoyancy foam, which they sell in the store on here, to compensate for heavy battery packs etc., but you still need to figure out everything or your ROV might end up being name the Seaview and it goes to the bottom of the ocean.

 


(Paul) #4

@Harold - Yup, worked on the numbers. Now I’m asking for real world, hands on, experience. FWIW, I spoke with Rusty and he reports getting about 30 minutes of fairly high RPMs using a single 5,000 mAh LiPo battery in their prototype BlueROV, so roughly 10 amp/hour power draw using 6x T100 thrusters. That’s less than half the amp draw I would have predicted.


(Harold Scadden) #5

@Paul … one answer would be a coulomb counter on the batteries with it reporting the remaining life to the people on the surface or at least maybe controlling throttle rate to keep you from getting crazy.

The thruster draw seems weird to me. If you have six thrusters going at say 75% the numbers don’t jive. Now if you are running in one direction at a time (assuming 10 amp draw) then I could see you squeezing 1/2 hour of total run time.

I still like the idea of a coulomb counter circuit phoning home with the burn rate. Life is much better then.

 

 


(Paul) #6

@Harold - You’re right, the calculated vs real world numbers aren’t even close. But I half expected that based on the service times I’m seeing with my Deep Trekker ROV (also LiPo powered). What I find, in real working conditions, is that I most often use the thrusters at their lowest gain setting and for very brief periods of time. The only time I use full gain is during dives, ascents and showing off during a demo. :wink:


(JL Picard) #7

Hi,

Here are some thoughts on the topic of batteries.

Several points to consider for a given application:

  • Maximum voltage (in V) <-- this depends on the components you are using
  • Energy in W.h <-- the more energy you have, the longer your ROV will be able to operate
  • Maximum current drawn (in A) <-- this depends on your components and how your software drives your hardware
  • Dimensions, weight...
Important characteristics when selecting a battery pack
  • Chemistry (LiFePo, Li-Ion,...)
  • Rated voltage (in V)
  • Charge (in A.h)
  • Maximum current and/or "C" rating (which is NOT expressed in Coulombs)
Maximum discharge current & "C" rating

Your application should not draw more current than what the battery can safely deliver. Otherwise: excessive voltage drop + overheat (and in turn, malfunction, fire, dead ROV, etc. …)

Lithium-xxx batteries and battery packs have a given “C rating” (<span style=“text-decoration: underline;”>/!&lt;/span> different from the Coulomb unit). Typically 1C, 2C, 5C, 10C, 20C, 40C, etc…

Maximum Discharge Current = Charge (A.h) * “C rating”

Example for a typical LiFePo battery pack (4 LiFePo cells in series)
5,000 mA.h ; 12.8V; 3C
Total energy = 5 A.h x 12.8 V = 64W.h
Maximum current = 5 A.h * 3 “C” = 15A
Maximum power (*) = 15 A * 12.8 V = 192W

(*): in practice, voltage will drop at such amperage, so available power will be less

 

Considerations for LiFePo batteries

The higher “C” rating, the better. The pack will run cooler, with a lower voltage drop, and will last longer.

Avoid over-charges and particularly over-discharges; those kill you battery.

It is recommended not to use more than 80% of the available energy, to avoid over-discharge.

Example

You want:

  • Enough power to drive 2 x T100 Thrusters at max power at the same time ("Full speed ahead!" :-) )
  • 30 minutes autonomy using 2 thrusters at 50% thrust continuously
You need:
  1. Chemistry and Voltage I would go with LiFePo because: safe (low risk of fire), energy density is high, easy to charge, widely used. Confirmed with Rusty, it is NOT safe to use 14V+ to drive a T100. Those are rated for 12V. So, in short: 12.8V LiFePo pack (4 LiFePo cells in series, each cell with a 3.2V rated voltage)
  2. Energy
    1. How much current do we need to develop 50% thrust? Check the documentation of the T100. There we learn that:
      • Max power is 135W
      • Max current is 12.5A - to play it safe, we will limit it at 135W/12.8V = 10.5A
      • Maximum thrust (@135W) is 2.36kgf (5.2lbf -- lbf? who still uses such units from another age, nowadays? :P). Hence, 50% thrust is 1.18kgf / 2.6lbf
      • Electrical power/current required to develop 50% thrust (2.6lbf) : 44W / 3.7A
    2. Energy required?
      • Time: 30 minutes = 0.5 h
      • Power: 44W
      • Energy = 44 * 0.5 = 22W.h
    3. Battery charge required?
      • Total charge required = 22W.h / 12.8V = 1,719 mA.h
      • Battery charge recommended = 1719 / 80% = 2,150 mA.h
  3. Maximum current
    1. T100 max current = 135W / 12.8V = 10.5A
    2. Application requires 2 T100 to run at full speed simultaneously. Max current = 2 x 10.5 = 21A In practice, you may want to apply a healthy safety factor, say 1.5x or 2x. Recommended max current: 30 to 40A
    3. For a 2200mA.h pack: Minimum C rating = 21A / 2.2A.h = ~10C Recommended C rating: 15 or 20C
Conclusion:
  • Harolds' battery is probably a no-go, because the nominal voltage is too high (14.8V)
  • But the website that Harold gave has a lot of choice! And this battery should be ok, given the requirements specified above.
If a large battery is required (plenty of energy for long dives), other points will need to be considered:
  • Weight (Buoyancy) (**)
  • Volume / form factory (can it feet inside the enclosure of the BlueROV ?)
 

 

We are not yet sure what we will do with our project (ready-made battery pack, custom-made, “DIY”). For those interested, some volume/weight/performance considerations in case we go the DIY route here.

JL

PS: (**): Someone from Bluerobotics should provide those data on http://docs.bluerobotics.com/bluerov/#specifications :slight_smile:


(Paul) #8

Excellent overview. I’ll have to study this in more depth as you’ve covered the topic very thoroughly.


(Harold Scadden) #9

@JL Picard … my battery is just fine.

The motor itself has one voltage rating and the Basic & BlueESC both have input voltages that will accept the battery that I pointed out. I am assuming from the specifications on the T100 (being 12v) and the T200 (6-20V) that the input voltage range should be geared toward the ESC module that you are using and not the thruster itself.

That is at least my take on life. I do find it weird about the T200 input range … interesting and if anyone knows the design info behind it, please post!

 

 


(JL Picard) #10

@harold

I had the same “feeling” as you, and asked Rusty if using voltages higher than 12V would be ok. He advised me not to exceed the specs (we didn’t specifically talk about 14.8V Li-Ion, but that was the general idea). That said, I don’t know if the limiting element is the ESC or the motor.

The T100 with BlueESC only has 2 connectors for power supply (ground and positive, shared by the motor and the BlueESC). The additional 4 connectors on the ESC cable are : 1) additional ground, 2) PWM input, 3&4) I2C input (SCL and SDA). It is not possible to supply the motor and BlueESC with different voltages.