Designing a 100ft 10AWG DC Tether for Drone Power Using 24V Batteries and Buck Boost

I have seen several designs for powering a drone  through a tether from a power supply, but would like to explore and perhaps go a different route.
The systems I have seen use an AC power supply (or battery/ inverter) to feed AC through an extension cord to a drone where it is rectified to dc and trimmed to the drone voltage. The rectifier replaces a battery on the drone.
 I want to use a 24v truck battery (or two 12v car batteries) to supply a dc to dc buck boost to kick the voltage up to 32vdc to 36vdc. This is to compensate for the 9.9 v voltage drop along a 100’ tether of 10awg high strand count wire. The goal is to have voltage at the drone between 22vdc and 26vdc (6s) without the added weight of an inverter. This way I could keep all the batteries on the drone.
Another goal is that the tether would supply sufficient power to keep the onboard batteries fully charged so that when the tether is released; it will have a full flight time from the batteries.
The voltage seems easy enough for me to comprehend, but the amperage is what I can’t get my head around.
The drone is 6s (24vdc) and has four 1500w motors. Max continuous current per motor is 70A.
At hover, the drone consumes a total of about 60AH.

Questions:
1. Is this a viable option?
2. What about the amps? My fear is frying the buck boost (or a very expensive drone) when connected to the truck battery.
3 Any (low cost) recommendations for a dc to dc buck boost for this application?
4. Any other components needed?
Thanks for all responses in advance!

Hi Ken-Is it possible?  Yes.What about the amps? 100 feet of 2-conductor #10 wire is going to be really heavy. I think the trick will be to go to a much higher voltage in order to drop the current to a more  reasonable value. Than you can use smaller wire. Don't worry so much about the voltage drop. The thing that concerns me about this approach is safety.  You'd need to implement a way to determine if the cable has been compromised and then quickly shut off the power.Any recommendations for DC/DC converter?  Go to http://www.ti.com/design-tools/webench-power-design/power-designer.html and enter the DC/DC converter design parameters.Any other components needed?  A way to safely shut it down. At minimum, detect that the drone has stopped drawing power and shut off the DC feed.-Rick

Thanks Rick,The weight of the tether is not a problem, It is a very large drone,  and has been calculated in to the all up weight to determine the amp draw.Are you saying much higher DC voltage, or AC? I am thinking the tether power to be more like a charger to keep the onboard batteries charged. Being lipos, the concern is overcharging, so a voltage limiter (regulator) circuit of some type would be required. Maybe a diode to prevent back feed.Could also slip a fuse, breaker or GFCI in the circuit to add some safety margin.I tried the TI Power Design Tool but nothing came up.Looks like the dc/ac/ dc method is the way to go.  I can comprehend the AC transformer calculations easier anyway.  [:0)Thanks for the help!

Hi Dale-Sorry I messed up your name in my response above......don't know what I was thinking.....Regardless of whether it's AC or DC- the higher the voltage you use on the tether the lower the current. This is because if the wattage stays constant you will be using as higher stepdown ratio. I was thinking of DC, though, because you've got to end up with DC anyway, and if you send DC up you won't have to have rectifiers and capacitors in the drone.Concerning using the tether as a charger for on-board batteries- I must be missing something.  If the tether has to supply enough power for the drone to fly, and you need to keep the drone's weight to a minimum, why would you still have onboard batteries?  
-Rick

Hey RickI am guessing the dc from the truck battery would have to be converted to AC (or chopped DC) to run through a transformer, to kick the voltage up right? Might as well go AC inverter and then rectify back to dc at the drone. Power supplies are cheap and plentiful. This was more of a "what if" we went the all DC route.I would like to be able to drop the tether and fly with full batteries.  The weight of the power supply (on the drone) is not an issue during tethered flight, but the idea of carrying the dead weight of an unused power supply during battery flight is not preferred.  Dropping a power supply with the tether from 100' is not preferred either.  :0)This is where the whole idea of boosting the DC began.Thanks

Hi Dale-I take it that the goal here is to have extremely long flight time, but still have the ability to go to high altitudes or great distances upon command. A DC/DC converter that can supply 24 volts at 70 amps is going to be pretty heavy. Unless you need hours of flight time you might come out ahead to just add extra batteries.As for the inverter- remember that inverters usually convert the 12 volt input voltage to 160(ish) volts DC and then convert that to a square wave, stepped sine wave, or pure sine wave. You could connect directly to the 160 volt DC output before it gets converted to AC and feed that up the tether. That would increase the efficiency a bit and eliminate the need for rectifiers and capacitors aboard the drone.  It is EXTREMELY important to be mindful of safety issues.  First, make sure that the connector you put on the end of the tether is designed so that no one can touch the conductors when it lands on the ground.  Second, make sure to add a circuit so that when the drone stops drawing current, the DC source shuts down. It will be very tricky to charge the batteries while the drone is in flight.  It will probably be simpler to design a 160 volt to 24 volt 70 amp DC/DC converter, and arrange it so that when you send the command to drop the tether it switches over to battery power first.   BTW- running 160 volts up the tether means the tether conductors will only be carrying about 11 amps.-Rick 

Hi Rick,"I take it that the goal here is to have extremely long flight time, but still have the ability to go to high altitudes or great distances upon command."Yes! That is the goal.The high voltage DC up the tether was the original idea, except that the dc voltage would be maybe 36vdc at 70a or so. The idea is to keep the heavy converter on the ground and then "tune" the tether voltage to compensate for the 100' length.   The tether then could just be plugged into a battery connector on the drone without any additional components.So if I used such a dc supply on the ground, the question I have concerns if there are any issues with the amperage.Thanks!

Hi Dale-How much will 100 feet of 2-conductor #10 wire weigh? That's one reason I was focused on lowering the current by raising the voltage so you could use a smaller wire.
If you plan to use the power from the tether to charge the batteries, you've got several things to consider.  First, you've got to provide a greater voltage then the no-load voltage of the batteries, and second, you've got to monitor the state of charge of the batteries and regulate the charging voltage and current, which will be tough since you will be charging and discharging at the same time.  If you don't charge the batteries from the tether then you've got to provide a way to seamlessly switch form tether power to battery power.If you want to compensate for cable loss totally from the bottom end, you'll need to adjust the voltage according to the amount of current being drawn.  Remember too that using #10 wire to carry 70 amps means you will have a significant amount of heating, and since the resistance of copper changes with temperature you may need to sense temperature and factor that in too.Another possibility would be to add two tiny conductors to the tether and tie them to the two main conductors at the top. Now you've got a way to measure the voltage at the top of the tether from the bottom of the tether.  This is known as a Kelvin connection. Advantage:  Accurate, easy to implement the power supply at the bottom. Disadvantage: The tether just got heavier and more complicated.You could also use a radio link from the drone to get feedback about the supply voltage.  You may also want to look at locomotive DC-DC converters.  You can get them in a 75 volt input voltage which would cut the current in the tether by more than half.-Rick

I just stumbled across this: Blue Vigil.  I should have done a search earlier. I found several how-to's and several ready-to-go kits.  Hope you're not re-inventing the wheel.  
-Rick

Hey Rick,200' of 10awg high strand count (1050), tinned silicone wire weighs 7.8lbs. This isn't the problem...Yes, There are other commercial systems out there.  Most have a converter on the drone as I had mentioned in my OP.There is even a DIY  here: https://diydrones.com/forum/topics/successful-tethered-flightOr this one:http://ardupilot.org/copter/docs/common-power-tether.html
But here is the deal...My drone is much larger than these. It weighs 30lbs with all the batteries and can carry a payload of an additional 25 lbs. So the 7.8 pounds of tether is nothing for this drone to carry.From what I have seen, none of these systems allow the tether to be dropped to allow the drone to fly on batteries alone, which is the whole point.It just seems that there should be some way to take the 24V at 48AH (aproximately) from a truck battery on the ground and be able to run it up a 100' tether to the drone. The batteries could handle any spikes in demand, so the tether could have lower amps equal to the hover amperage required. (Approx 60A) This amperage draw includes the extra weight of the tether. Thanks,Dale

By my calculation, if you're running 60 amps over 10 gauge wire, 100 feet (2 conductors = 200 feet of wire), you will have to feed 50 volts from the bottom to get 25 volts at the drone.  You will dissipate 1.479KW of power in the wire.  It will get pretty hot!
I could be wrong.  Please check me. I used ElectroDroid on my smartphone for the calculation.-Rick

You doubled the run length to 200' but it's only 100' from the source to the load.Silicone wire as used in RC applications is different than a typical wire .So you will see different results depending on the calculator used. 10awg high strand count, tinned wire is typically rated at 180A DC.This calculator shows an input of 36vdc will have an output of 24vdc over 100' of wire.This equates to only 720w dissipated in the wire.http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=35&phase=dc&noofconductor=1&distance=100&distanceunit=feet&amperes=60&x=94&y=13

How are you going to power the drone with a single conductor?

I apologize- you are correct.  The calculator in ElectroDroid seems to **IMPLY** a single wire drop, but comparing it against other calculators, it does appear that it is the round trip drop.
Sorry for the error- but now that I've got that out of my system I should be 100% correct for at least the next year (I wish!!!).
-Rick 

What an epic project! Do you have any video? I'd really like to see it haul up all its cable along with its own weight?

Hi Rick,

My name is Tharun. I am thinking to construct a tethered drone. I have a drone frame of type S550, which is hexa frame and weighs about 800 grams. The total weight of the drone comes around 1500 grams. I am thinking to use a tethered solution to my drone. Here is my method to do that.
1. In India we have AC power supplies which is 230V 50Hz.
2. Feed AC 230V to a 16 AWG wire of 100 feet in length.
3. Use SMPS to convert 230V to 12V 60A DC on the tether end side and feed the drone.

I use a 10x4.5 propeller and then A2212 13T 1000kv motor's.
Do you think the tether system will work on my drone? What modification should I have to do?

Hi Dale! Did you ever get this project working? And could I pick your brain on this as I am trying to make a identical system on a huge octocopter
Thank you in advance
Kind regards