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Suggested battery capacity for 30 min flight?

Posted: Jan 06 2021 9:08pm
by Bgt2u
I am building a personal manned VTOL, and am using 8: KDE8218-120 motors, and 3010 props. It should weigh about 400 lbs, with me in it, and l am trying to figure out what capacity lipo packs that l would need to get approximately 30 minutes of flight time. I would ultimately like to use 8 battery packs, one for each motor. I plan on running a 48v (58.8v) system. Using the motor specs below, can anyone give me a suggestion on what l would need for battery power (capacity)?
Motor: KDE1828-120
Constant motor velocity: KV120
Constant Torque: .0796 Nm/A
Constant Km: .4137 Nm/W
Max continuous current: 110A (3 min)
Max continuous power: 5695W (3 min)
Voltage range: 22.2v - 60.9v
Stator poles: 24
Magnetic poles: 28
Prop size: 30.5"-TP (27.5"-TP max on 14s)

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 06 2021 9:32pm
by Chalo
Not knowing what your typical power requirements will be, the safe bet is (max cont current per motor) * (# of motors) * (flight duration). So, 440 Ah or about 23 kWh at 52V nominal. That's nominally how much juice it will take to run your motors at full blast for 30 minutes. In the real world, you won't need 100% of your batteries' current capacity, but also in the real world, 2C discharge rate won't let the cells produce as much as their rating suggests.

23kWh nominal is about 300 pounds worth of Hobbyking 20,000 mAh packs, as one example.

VTOL is notoriously energy intensive. You're closing in on rocket-equation level tyranny here. I think you'll be doing good to demonstrate five minutes of woohoos plus safety margin.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 06 2021 9:57pm
by Bgt2u
Wow, l guess my figures were a bit off. I had calculated that a 48v, 50Ah lipo pack should give me 23.52Kwh, for almost a 30 minute flight time, and weigh about 40 lbs.....was that even close, if not what do you think the flight time would be with that lipo pack? Or how many of them do you think l would need?

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 06 2021 10:52pm
by amberwolf
Bgt2u wrote:
Jan 06 2021 9:57pm
I had calculated that a 48v, 50Ah lipo pack should give me 23.52Kwh, for almost a 30 minute flight time, and weigh about 40 lbs.
I think you lost a decimal place there. ;)

48v x 50Ah = 2340Wh (A x V = W)

The weight sounds about right; I have a 52v 40Ah EIG NMC pack that weighs something under 40lbs (maybe 35-38, cant remember for sure), with no casing, just cells, holders, and busbars and output cable.

You'd need ten times that much battery (500Ah at 48v, not 50Ah) to get the 23kWh, so it would also weigh at least 10 times as much, or about 400lbs. :( So based on the below quote, that will double the weight (at least!) of your system (probably more, because you will need to make it's structure stronger to handle the doubled weight, and you'll probably need more motor to lift the weight, and more battery to supply the power for the motors to lift the weight. You can also run into the problem of needing more motor to lift the extra battery, requiring even more battery to lift all that, requiring more motor to lift more battery to lift more motor and structure, etc. :(
Bgt2u wrote:
Jan 06 2021 9:08pm
It should weigh about 400 lbs, with me in it,

How much battery you actually need depends on the actual flight power requirements, and on the actual cells used and in what configuration.

I wish I knew how to calculate your actual power needs, but I don't know what math to use for it.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 06 2021 11:00pm
by amberwolf
Bgt2u wrote:
Jan 06 2021 9:08pm

Max continuous current: 110A (3 min)
Max continuous power: 5695W (3 min)
If the (3 min) is a time limitation for how long the motors can run at that current / power level, then that is *not* a "continuous" rating, it is a peak rating for a specific time limit. If you need it to run at that power level continously, you would have to find a cooling method sufficient to carry away enough heat to prevent motor damage, which will require experimentation (and probably destruction of at least one motor in the process, and likely some controllers).


Before deciding which motor to use, you first must know how much power you need, continously, to do what you want to do.

After you know that, then you can find a motor that gives an actual continuous (meaning, forever, no time limit, given the correct cooling) power rating suitable to use for the project. ;)

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 12:47am
by Hillhater
We did this before on your other thread.
The estimate was for 40-50kW of power ...and a 20 min flight time.
Assuming you can find motors to do that, you will need 20 kWh of battery minimum, distributed and connected as you desire. but..as AW said, you dont seem to recognise the scale of the battery issue..
basicly That means 100kg of lipo weight and $8-$10,000 of cost. ! :shock:
You need to be realistic. Even a 10kWh pack will weigh 100+ lbs and be marginal on performance and run time.
Remember , with a VTOL you do not want to be running out of battery capacity as you try to land !

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 1:42am
by Solarsail
If your motors need 30kW to hover, then you would need 15kWh.

As I have explained in a previous reply, each kWh weighs 4.4kg or 10lbs if you use 21700 Samsung 50E - 5Ah, 2C (about $3.50 each cell or $200/kWh), and your pack is absolutely minimalist in design, e.g. the BMS stays on the ground with the charger, short CCA cables, no fancy enclosure. These cells are 264 Wh/kg and will beat any other brand or chemistry in energy density. Each cell is 68.5g.

Thus 15kWh can weigh as little as 150lbs, and 10kWh as little as 100lbs.

So let's say you want a module that can deliver 2.5kW to one motor. These cells are 2C. So you need 1.25kWh per module. You would need 1250 / (3.65 * 5) = 70 cells per module. That is a 10s7p or 12s6p or 14s5p.

One more thing -- you need a way to turn the power off at 10 modules. One way is to install heavy duty 100A switches for each module. But bringing 10 heavy switches and associated cabling to the cockpit will add a lot of weight. You can have 10x 100A contactors. Again weight is a problem and contactors consume power during flight. The best solution in my opinion is to use the IRF7530 MOSFET instead of a contactor - lightweight and no power consumption.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 5:26am
by Hillhater
Solarsail,... as i said, we have to be realistic, not idealistic.
30kW to hover ..OK so presumeably more to climb and manouver etc ?..maybe 40kW as you previously suggested ?
Cells... Samsung 50E are one of the best cells, but at 2C (10A) thay can only provide 4.5Ah,..or 16Wh.
https://lygte-info.dk/review/batteries2 ... %20UK.html
So for eack Kwh (at 10A) you would need 63 cells.
And for those 1250 Wh module ..78 cells...(4.3kg )
That is 11% more cells , weight, and cost.
But , most importantly, by the time you have used 80% (4.0Ah) of cell capacity , the voltage is down to 3.1v...that a 25% voltage (and power) loss from fully charged .
So being “realistic”..and safe,.. you should plan to only use 80% of any pack capacity.
Thus that 15 kWh pack is realisticly a 19kWh pack of 1200 cells (19 x 63) and weighs 82 kg (182lbs) ..WEIGHT OF CELLs only .!
connections and packaging etc would add a few more pounds
Would you let me know where to buy 50E cells for under $6.0 each ?
Unless someone is skilled, well practiced, and confident, ..building multiple 1-2 kWh packs of 21700 cells , for use in a human flight project,......is not a task to take lightly .
Professionally assembled packs are recommended...at significant cost. !

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 12:07pm
by Grantmac
Consider perhaps doing an EV conversion on an existing ultralight helicopter.
Power requirements are significantly lower, ground effect is better utilized and battery exhaustion/power failure is less dangerous.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 12:08pm
by Stealth_Chopper
100 KW Hours :banana:

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 12:28pm
by Bgt2u
Wow, all of thsee comments have me thinking that l chose the incorrect motors.
All of the figures below are with a 30.5" prop on the KDE8218-120 motor.
# of motors - throttle - thrust/Lbs
12 - 50% - 239.64
12 - 62.5% - 323.28
12 - 75% - 420.6
14 - 50% - 279.58
14 - 62.5% - 377.16
14 - 75% - 490.7
Since 100% throttle is only rated for 180 seconds, l am not sure what throttle percentage would give me a usable flight time....180 seconds isn't going to do it. According to my above calculations, (and previous help from Solarsail), l don't even know if these motors would work for me. Does anyone have any suggestions of a motor that would give me 45-50 LBS of thrust per rotor, with a continuous run time longer than 180 seconds? At this point, l would even settle for a 20 minute flight duration. I don't have to stick with a 48v system. 96v+, is not out of the question. I just need to get things right before l spend any more $$$$.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 5:25pm
by Hillhater
Where/how did you obtain those thrust figures ?
Are there “power” (kW) figures associated with the “% throttle” data ?

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 5:33pm
by Bgt2u
All of the information is found on this page under 'performance data'.
https://www.kdedirect.com/collections/u ... 8218xf-120
I then took the 50%-85% thrust isted (under 59.9v) and multiplied that by 12 and 14 motors, and then converted Kg's to Lbs.

I would like the members on here to please understand, that as of 5 days ago, l had absolutely no knowledge of anything to do with motors, esc's, propellers, etc. I have however, built a few small custom made 72v electric cars, but, this is obviously very different, and challenging. Even though l am an A.S.E certified mechanic, and have a Bachelor's degree in Electro/Mechanical/Strucujral Engineering, none of this pertains to this project, that l am attempting to create. So it is new to me. Although l am a quick learner, l still require some extra help, guidance, and patience. As always, any help is greatly appreciated.

Thank you for all the help!

Posted: Jan 07 2021 6:11pm
by Bgt2u
I would like the members on here to please understand, that as of 5 days ago, l had absolutely no knowledge of anything to do with motors, esc's, propellers, etc. I have however, built a few small custom made 72v electric cars, but, this is obviously very different, and challenging. Even though l am an A.S.E certified mechanic, and have a Bachelor's degree in Electro/Mechanical/Strucujral Engineering, none of this pertains to this project, that l am attempting to create. So it is new to me. Although l am a quick learner, l still require some extra help, guidance, and patience. As always, any help is greatly appreciated.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 6:50pm
by Hillhater
Ok.. so from that..
490lb thrust needs 14 motors at 3.1 kW each (75% throttle,..55% output)
For 30 mins run time (max..20 mins + safety ),.. each motor would need 1.55kWh od battery...22kWh total.
At 10+ lbs /kWh, that is 220 + lbs of battery , leaving <270 lbs for everything else, including pilot.
Until you can establish what the “continuous” rating is for the motors, we are just guessing.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 7:02pm
by Bgt2u
Thank you that helps a lot. Yes, we are just guessing on what 'continuous' is. I would assume that the 180 seconds of use would go up dramatically if run between 50-65%, however, only real time testing will tell. I guess anything below 100% throttle would have add to the pathetic 180 seconds. On the plus side, l only weigh 130 pounds, so that helps too! One question, how did you calculate the battery weight of 220+ pounds?

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 8:25pm
by Hillhater
how did you calculate the battery weight of 220+ pounds?
It will vary somewhat depending on the choice of pack chemistry and build method..
But at best you can expect 260 W per kg theoretically at cell level with low current draw.
But as i pointed out above to solarsail, once you put a load on a cell (say 10 A) the “available” energy reduces.
So allowing for that, and the additional weight of pack construction and packaging ....then a figure of 200-220 W/kg is more realistic.. That gives 4.5kg per kWh (10 lbs). ...and hence 220lbs for a 22kWh pack capacity
Anything less than the best 21700 or 18650 ncr type cells , will weigh more per kWh

....we are all waiting for the long promissed improvement in cell Energy Density...but no sign yet ! :roll:

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 07 2021 8:54pm
by Bgt2u
Great explanation. NOW, at least l know where to start, on the battery issue. Thank you. That really helped. Yes, battery technology needs to vastly improve.....but for now, one must use what is available. So far l have found battery packs that are 60v (71.4v) @ 50Ah, (that should be 3.57 kwh per pack). Would 6 of these put me in the 'ballpark'? Also, the total weight for them is 226 pounds.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 08 2021 9:07am
by Hillhater
Battery packs are a big gamble unless it is a known reputable vendor, using known brand quality cells and components.
There is a lot of fake product in the market.
Whilst the capacity is ok, that is not the full story for pack performance.
You need to know full cell specs

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 08 2021 1:18pm
by Bgt2u
I will do some searching for reputable manufactures, as l do not want crap cells/packs, especially when my life could depend on their reliability. What l am going to do is power each motor with a seperate batpack, that way if one fails, l won't crash as fast....lol. l figure a 25ah-30ah batpack to each motor should serfice. Feel free to correct me if that is wrong. Do you know where l can get some decent batpacks, like from who? Maybe a RC hobby site?

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 08 2021 4:54pm
by Hillhater
As you are discovering, battery packs are a whole area of potential problems..
I hav never bought a 1 kWh pack of any type,..only smaller individual RC lipo packs or 48v packs for EBikes etc
So my financial risk is not great, and using multiple smaller RC packs allows me to replace a single failed pack easily and cheaply.
But with unitary 1+kWh packs a single cell failure becomes a bigger problem.
For Lipo, many of us on ES have used Hobbyking with good success. They make “ Multirotor” packs that are easily assembled into higher capacity / voltage packs
https://hobbyking.com/en_us/batteries-c ... 132&is_v=1
I have never dealt with these vendors , but others here have, and they list pre assembled packs of those 21700 samsung 50E cells,....worth investigating ?
http://queenbattery.com.cn/our-products ... ooter.html
EM3EV.. are also a reliable supplier and custom build packs, but can be difficult to communicate with..
https://em3ev.com/shop/50v-14s7p-rectan ... tery-pack/
There are many others, depending on where you are located as supplies are becoming irratic in some countries
Suggest you buy samples and test for yourself to confirm suitability and service etc

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 08 2021 6:50pm
by Dauntless
Well, let's see. 30 minutes of flight time meaning this is an ultralight/fly for fun out in the middle of nowhere? Or you take off from an airport after taxiing, make whatever departure and fly a half hour before picking up the pattern and flying in for a landing, taxi back, etc.?

I assume you don't glide to land even if this is just an ultralight. in the U.S. you like to land with 30 minutes of fuel left, etc. Once you figure out how much battery for 30 minutes, you might be looking for several times that for this 30 minute flight.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 08 2021 7:33pm
by Bgt2u
Dauntless, this is actually a VTOL, so, none of that would apply to the 30 min. flight time :) .
Thank you Hillhater, l will look into those sites. Once again, much appreciated.

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 08 2021 8:34pm
by Hillhater
VTOL is certainly a very different ball game to winged craft, but you wont have the back up of a unpowered “glide” down if you run out of power.
Even if only hovering a few feet up in my back yard,.. I would certainly want a very noticeable alert at 25% capacity level. :shock:

Re: Suggested battery capacity for 30 min flight?

Posted: Jan 08 2021 10:43pm
by Bgt2u
That is correct. I do realize that if l lost power, that unlike an airplane, l would drop like a rock, so yes, l will have battery percentage available to view, during flight.