Battery Module
My most core design requirement is for the power system to be able to broken down (battery removed from the AUV and battery broken into sub 100Wh elements)
I considered a few different configurations but came back to standard 3” tube (in part as I have a couple of non air transportable batteries that I can use in the system on local projects).
After saying I need to be able to remove the battery for air transport, after much working out of very small boats (islander canoes) and having lost more electrical equipment to letting out the magic smoke in those boats, it is also important that components only need to be opened when needed (ideally not on the boats). Hence also looking at both a battery isolation system and a battery charging in situ answer.
I looked into a few threads but space for a SSR will be an issue and just breaking the power cables with a subconn style plug is not really on the cards given the amperage.
I fell into 2 thoughts either using 4 by 2S4P with integral BMS to allow charging without balance leads or using 4 by 8S battery’s run in both series and parallel in the Enclosure (series for AUV operation and parallel for charging)
Link to the Fusion 3D file Fusion
I feel that the 4 by 8S battery configuration will be an easier implementation although it will likely mean I will have to build up the batteries myself (I also have access to a battery welder) but has some limitations. (build would only be a couple of spacers to set the shape of the battery pack, a couple of Epoxy Insulation Board 0.5mm sheets, a couple of Nickel 0.2mm and some 100mm Heatshrink PVC and some 11AWG Silicone wire)
By running 4 batteries in 8S configuration full current draw is coming from each pack (ie 80-100 amp) so I should be using nominally XT90’s but this is especially tight. I have already a 70 amp limitation in the electronics Terminal Strip Jumper Bar so am looking at using XT60’s (I did look around at “short burst” over current). Under normal operation it should be a more consistent 5amps or less (peak around maybe 10 amp)
I thinking I am settling for 4 connections (+ve wire / -ve wire / PRV (or vent) / a 3 or 4 wire lead to a subconn plug or alike mounted external to the AUV)
I think using the Battery Switch Kit for BlueROV2 from UnderseaROV may be the way to go as it fits into the end cap (note Fieldwerx Mosfet system seem to have disappeared)
I see that I would not use a standard Bluerov switch inside the AUV skin but run a cable to the outside skin of the AUV, this cable would be (at least) +ve and -ve nominal 3.7V 4 by 8 Series (i.e. 32 series) for battery charging (say 10-20 Amp) and then a dummy plug set up to be a switch (plugged in turns on the unit). I am still tossing up if on the 3.7V or the 14.4V side [leaning to 4 wire 14.4V as its what’s standardly wired in the unit from UnderseaROV].
Endurance
Given this power (basically a 4S8P 18650 battery around 5200mAhr)
THIS IS ALL THEORETICAL and Not Correct (it is just used to give a guess of what the unit could do as a starting point)
Given Eliot’s data in this thread and working backward
For a given pumping flowrate of a thruster (sort of equal to speed of the AUV (Only if you ignore friction etc))
For a T200 @ 14V
| m/s | knots | RPM | PWM (µs) | Current (A) | Power (W) | Force (Kg f) | Efficiency (g/W) | Endurance (hr) | Endurance@66% (hr) |
|---|---|---|---|---|---|---|---|---|---|
| 1.0 | 1.9 | 826.4 | 1571.0 | 0.2 | 2.8 | 0.3 | 90.0 | 69.3 | 46.2 |
| 1.5 | 2.9 | 1239.6 | 1608.0 | 0.8 | 11.2 | 0.6 | 51.4 | 17.3 | 11.5 |
| 2.0 | 3.9 | 1652.8 | 1655.0 | 2.0 | 28.0 | 1.0 | 36.4 | 6.9 | 4.6 |
| 2.5 | 4.9 | 2066.0 | 1707.0 | 4.2 | 58.8 | 1.6 | 27.6 | 3.3 | 2.2 |
| 3.0 | 5.8 | 2479.2 | 1762.0 | 7.6 | 106.4 | 2.4 | 22.5 | 1.8 | 1.2 |
For a T500 @ 14V
| m/s | knots | RPM | PWM (µs) | Current (A) | Power (W) | Force (Kg f) | Efficiency (g/W) | Endurance (hr) | Endurance@66% (hr) |
|---|---|---|---|---|---|---|---|---|---|
| 1.0 | 1.9 | 439.9 | 1553.0 | 0.3 | 4.7 | 0.3 | 56.9 | 40.8 | 27.2 |
| 1.5 | 2.9 | 659.9 | 1588.0 | 0.9 | 13.2 | 0.7 | 56.3 | 14.8 | 9.8 |
| 2.0 | 3.9 | 879.9 | 1628.0 | 1.9 | 26.5 | 1.3 | 49.4 | 7.3 | 4.9 |
| 2.5 | 4.9 | 1099.9 | 1668.0 | 3.4 | 47.0 | 2.0 | 42.6 | 4.1 | 2.7 |
| 3.0 | 5.8 | 1319.8 | 1712.0 | 5.2 | 72.6 | 2.8 | 39.1 | 2.7 | 1.8 |
From this it can be seen there is an efficiency swap over between the T200 and the T500 at around say 1.7m/s (hence the decision for the T500)
Thus with the proposed battery (and the posabiltiy to add other battery modules as well) this gives scanning areas of approximatly X meters by X meters at the following speeds and lane spacing (eg a 2m/s and a 4m lane spacing you could do a 264m by 264m box area)
| Speed | m/s | 1 | 1.5 | 2 | 2.5 | 3 |
|---|---|---|---|---|---|---|
| Endurance @66% | Hrs | 27.2 | 9.8 | 4.9 | 2.7 | 1.8 |
| Range | km | 97.8 | 35.4 | 17.5 | 9.9 | 6.4 |
| Photogrammetry | 8 | 884.5 | 531.9 | 374.1 | 281.4 | 226.4 |
| 4 | 625.4 | 376.1 | 264.6 | 198.9 | 160.1 | |
| 2 | 442.2 | 266.0 | 187.1 | 140.7 | 113.2 | |
| Sidescan | 150 | 3,830 | 2,303 | 1,620 | 1,218 | 980 |
| 60 | 2,422 | 1,457 | 1,025 | 771 | 620 |
I looked at 21700 batteries and the additional length was difficult, and 26650 work but would not fit back into the flange so overall length was also an issue, I also understand this is theoretical and does not account greatly for drag (other than derating the power to 2/3 of its capacity)
So where am I wrong and what do I need to fix what can be done better