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Old 08-09-2011, 04:44 AM   #1
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Default Question about ESC and LiPo battery

Hi,

I am looking to buy some ESC and LiPo batteries and have a couple of questions:

1)
I have 4 brushless motors which have a current rating of 13A and max current rating of 18A. For a quadcopter, all 4 motors would be supplied by a single battery. I think 4 of this ESC : (hobbyking.com/hobbyking/store/__6457__Hobbyking_SS_Series_18_20A_ESC.html)would work well along with this battery pack : (hobbyking.com/hobbyking/store/__8935__Turnigy_2200mAh_2S_25C_Lipo_Pack.html) What do you guys think?

2)
I also have a question about connectors. The LiPo battery link above mentions that the battery discharge connector is a 'XT60'. Since this end connects to the ESC, do all ESCs come with the corresponding connector? The ESC in the link above doesn't mention anything. What do you guys think?

Thanks a lot!
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Old 08-09-2011, 08:09 AM   #2
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That stuff should work. But if it were me I'd consider going for a higher 'C' rated and/or a larger battery. With four motors 18A you could easily be pulling close to 70 amps out of your battery which is near it's burst limit and would give a big voltage drop, and would damage the battery if you sustained such power levels for any length of time. Also running the battery at those sorts of amp draw would give very short flight times, around 2 minutes duration.

On the other hand....If you think that you wont be running the motors anywhere near their full rating then all should be ok.


The ESC's wont come with XT60 (or any other) connector. You will need to purchase some XT60 connectors and solder then to the ESC's. you will either have to make a 4 into 1 XT60 harness or solder all four ESC's into one XT60. If making the harness you will need some wire. While you are at it also buy some heatshrink insulation to protect the solder joints.
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Old 08-10-2011, 05:10 AM   #3
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It would really depend on the props. The motors may be rated for 13A continuously, but if you have really small props on them they'll draw much less. Overpropping will mean that they draw more.

The battery you've picked is good for 55A continuously. You don't really want your total setup to pull any more than that except maybe during very intense periods of throttle use.

Assuming you prop each motor for 13A at full throttle (their rated max), you'd be at a total of 52A which is just within the pack's limit. However, you're not gonna be flying a quad at full throttle very much, so it should be just fine...
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Old 08-12-2011, 09:58 AM   #4
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Hi,

Thanks a lot for the replies. To make things more clear, the motors I have are the SuperTigre Brushless 400 from here : http://www.supertigre.com/electric-power/motors.html. I have them combined with 7x5 props (due to size issues, even though they are not among the recommended sizes). So hopefully, I will be able to lift, say 1.5-2Kg (3.3 - 4.4 pounds) with these. According to the motor specs, they can take in 13A constant current and 18A is the surge current.

Assuming the motors-props config can lift 2Kg at say full throttle, that would mean 4 motors would draw 52A. The question then is, will the motor-prop combo be able to do that? What do you guys say?

or should I go a little more conservative and get a battery that can supply more current, say these ones -

http://www.hobbyking.com/hobbyking/s..._2S1P_45C.html

or

http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html

Finally, what difference does a 3-Series battery pack make over a 2-Series battery pack except that the voltage is going to be 7.4v instead of 11.1v. How important is the voltage setting?

Thanks a lot for the replies!
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Old 08-12-2011, 10:13 AM   #5
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The amount of power the motor will use is determined by the voltage and the size of the prop it's using.

On a 2S battery and that tiny 7x5 prop those motors are going to have very little power. At a guess they will be drawing less than 5A so power around 30-40W. No chance of lifting 2Kg.

On a 3S battery (50% more voltage) you will get a lot more power because that's the way electric motors work. The higher the voltage the faster they try to turn and with the same prop if you turn it 50% faster it uses a lot more current. Even then you'll probably only be up around 10A and still not much chance of direct lift of 2Kg.

The recommended prop sizes are not just for fun, they're what you NEED to use if you want to get anywhere near the recommended power levels. Smaller props with higher battery voltage, larger props with lower voltages.

If you want to lift that weight with a 2S battery and only a 7" prop you'll need motors with a much higher Kv.

Steve
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Old 08-12-2011, 01:02 PM   #6
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+1 on Steve's comments.. On 7" props and 2 cell batteries you will hardly have enough thrust to lift the weight of the motors!.. ok this may be slightly overplaying it but you certainly wont have the preformance you are looking for unless you use the right size props.

Given that you have the motors already and you probably dont want to buy new ones.... If you go with a 3 cell battery and use a 3 blade 7" prop then you should get the motor somewhere near it's optimum performance.
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Old 08-12-2011, 03:25 PM   #7
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On two cell batteries you're only going to get around 700 grams of total thrust with any 7" prop, only 660 with a 7x5 APC E prop. With a 1500 gram (minimum) plane that's a huge problem. At that point it doesn't matter if it will fly or not, you'll have great problems getting off the ground.

With the 7x5 prop you're only going to draw 17.2 watts per motor for a total of 68.8 watts, let's call it 70 and be optimistic. If we figure 100 watts per pound is adequate (you're not going to 3D a four engine whatever, are you?), your 3.3 lb plane at 70 watts is.........(the envelope please).......21.2 watts per pound. Not acceptable for Del Monte.

There's not enough pitch available on a 7" prop to pull you out of the quagmire. If there were your pitch speed would be a hundred miles per hour. Your thrust would be even less than with the 5" pitch prop. So there's no way to make 7" props work with 2 cell batteries. I suspect the same is true of 3 cell batteries. There's just too much gap between capability and requirement.

You need to work backward. First choose the prop you want to use, then attempt to match up available motors to see if you can make something work. The SuperTigres aren't going to cut it. You're going to end up with very high KV motors to have any chance at all. You need scads of RPMs and a prop designed to deal with that kind of stress.

Denny will now be by to hawk his beloved Hacker motors. You'd better listen because you are talking a high stress environment here. My crystal ball sees your wallet shrinking! But your long term most economical decision will be to spend what you have to. The Supertigres won't Hack it. (had to say that)
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Old 08-12-2011, 06:59 PM   #8
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RR,

I still think it might work ok with a 3 cell battery and a 7" three blade prop. A 3 blade 7" prop is about equivalent to a 2 blade 8", which is the recommended prop for three cells and so should pull close to 13A.. lets call it 11A.

11A x 10.5V = 115W per motor.. thats 460W total. Thrust should be around 450-500g per motor, so about 1800 - 2000g range total.


if an 8" prop could be shoe-horned in then a 8" fine pitch 3 blade should give even better figures..... A fine pitch prop would be much more efficient for a model that's designed to hover so the 3 blade GWS slow fly props are worth looking at.

Steve
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Old 08-12-2011, 10:25 PM   #9
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You mean I blew the chance to say the Supertigres won't Hack it? Let me go down to my dark and dismal laboratory and see what comes out of the occasion........3 blade.......3 cells.......Supertigres Hacking it. I'll be back.
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Old 08-12-2011, 10:41 PM   #10
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Hmmmm. Lack of 3 blade props in Drive Calculator, so first i checked your statement that a two blade 8" is about equivalent to a 3 blade 7". Checked out a 7x4 MAS electric 3 blade against a 8x4 MAS GF and they were very very close. So I began checking out pitch to see what works.

Figuring from that analogy, a 3 blade 7x5.5 with a 3 cell battery with a large C rating should yield 1,508 grams of thrust. That gets the plane off the ground nicely at 9.9A per motor, 107 watts per motor. These guys are only going to be between 65% and 70% efficient there, so cooling might be an issue. Be a bit careful there. Pitch speed is 60 km/h, 37 and a fraction mph. At 10 A per motor your battery can hack it.

If you went to 8" pitch (7x8 3 blade) you'd get 438 g per motor static thrust, 1,752 g total, not too shabby, brings pitch speed up to 54 mph, maybe more appropriate, each motor drawing ehhhhhh....9.8 A. Another doable combination. Different models and manufacturers of the same pitch prop will give somewhat different results but none should be a deal breaker. You have plenty of slop there as you are not near the limits of motor or battery performance.

OH! Flight duration is projected to be about 4 minutes and a bit less than a quarter with a 2200 mAh battery. You might want to think about more capacity.

Where's Denny hawking his Hackers? (that's almost as good as saying the Supertigres can't hack it, isn't it?) This is a golden opportunity to talk about the advantages of getting a more efficient motor. I won't steal his thunder. Listen closely when he speaks.......
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Old 08-12-2011, 11:17 PM   #11
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You don't need any pitch speed (well almost none)... it's a helicopter... 'static thrust' is all that matters.
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Old 08-13-2011, 01:14 AM   #12
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Originally Posted by Rockin Robbins View Post
Denny will now be by to hawk his beloved Hacker motors. You'd better listen because you are talking a high stress environment here. My crystal ball sees your wallet shrinking! But your long term most economical decision will be to spend what you have to. The Supertigres won't Hack it. (had to say that)
LOL or

Problem is all my experience is with models that fly like an airplane, and have ZERO experience with models that go straight up and down like a chopper can.

Nice thing about those "Hacker" motors, they have a number of different winds on the same motor "Shell" or frame, so you can pick and choose which winding gives the most amount of thrust with the least amount of current. And those Hackers can run at their indicated maximum power levels without overheating or failing.

What will help here is one of those computer programs such as www.motocalc.com, free for 30 days. You can simply put in a motor, propeller and a given battery and it will provide the thrust that's available.

The results will be about 15% high for the Hacker motors for some reason. Could be the motocalc program does not allow for the prop wash hitting the fuselage or wings. Other brand (el cheapo) motors might have specifications that are not even close to their actual values.

Be a little wary of any motor that suggests it will put out over 100 watts per ounce of motor weight. Quite often, those motors putting out 150 or 200 watts per ounce of motor weight are going to run HOT, and at the same time their efficiency drops like a lead brick.

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Old 08-13-2011, 03:21 PM   #13
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Denny, you're not giving motocalc enough credit here. Neither Motocalc nor Drive Calculator cares one bit about what the published performance claims for a motor are. Their results are based on an evolving and continuously updated database of actual tests. Now that we have data recording ESCs these programs will be even more accurate in the future.

Missed the word quadcopter in the first post. I was envisioning a four engined bomber or something. No clue how I missed the obvious.

Okay, that means that even 8" pitch props are VERY marginal, developing 1700 some odd grams of thrust on a vehicle that will weight between 1500 and 2000 grams. Maybe the Supertigres really can't Hack it.....

Typically you have some higher KV motors in quadcopters to get more RPMs out of the smaller props. The faster you turn a motor, the more proper bearing support is important. Hacker motors have a bearing design that eliminates the possibility of the armature striking the magnets while rotating at 10k rpm. And they are much more efficient, turning your watts into torque instead of heat.

Remember, a quadcopter or helicopter has the glide angle of a buffalo turd. They're just smoking craters looking for a place to happen. Your primary goal has to be reliability.

I could care less if the motor in my Slow Stick takes a defecation break. I'll just fly in my preferred mode, gliding to a landing. My first love is sailplanes anyway!

If you lose a motor on the quadcopter, you are going to have a bad day. That's especially true on a heavy model such as you are contemplating. It looks to me that you're going to have to run the Supertigres pretty wide open just to get off the ground. At 65% to 70% efficiency, up to 35% of your watts are going to produce heat. Your quadcopter with SuperTigres will be a 400 watts times 35% equals a 140 watt heater! That's more than any soldering iron you'd use in wiring your plane. Your motors won't probably be enclosed, but they also don't have a lot of air moving past them. The inside part of a prop doesn't produce that much thrust. At the least you'll have to determine that you aren't melting down motors!
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Old 08-13-2011, 04:21 PM   #14
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RR,

Not sure where you get 1500 - 2000g from?.. A quadcopter would I'm assuming consist basically of a few carbon rods with the electronics screwed on.

I'm getting the motors, battery, ESc's, props, R/C etc to about 600g or so.. So a total flying wight under 1kg should be easily achievable.
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Old 08-13-2011, 04:27 PM   #15
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That really puzzled me too. But in his second post he says "So hopefully, I will be able to lift, say 1.5-2Kg (3.3 - 4.4 pounds) with these." I couldn't think of a quadcopter weighing anywhere near these numbers and he didn't explain.
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Old 08-13-2011, 04:38 PM   #16
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I assumed that maybe he intended to use the copter to lift some 'cargo' of some sort?.. i'm sure the basic flying weight of the model would be much less than 1.5-2kg
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Old 08-13-2011, 05:02 PM   #17
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Originally Posted by Rockin Robbins View Post
Denny, you're not giving motocalc enough credit here. Neither Motocalc nor Drive Calculator cares one bit about what the published performance claims for a motor are. Their results are based on an evolving and continuously updated database of actual tests. Now that we have data recording ESCs these programs will be even more accurate in the future.
Agreed, but some of those real cheap motors have different real life ratings, depending on which production run they were built on.

I even ran across a name brand motor (Not Hacker) whose KV specs were off by some 30%. The motor had to go from a 6S to an 8S A123 battery to get sufficient power out of it.

As for those Hacker motors, one of my A50-12S motors is four years old, another was purchased last year. They are identical in performance with a 6S2P A123 pack and a 14X10 APC-E prop, both turning at 7850 RPM.

A lot of bad press has been put out there on those Rimfire motors, but one club member has one that does the job just fine. Same old story, don't make much sense to put a $60 motor in a $30 model, or put a $30 motor into a $500 model.

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Old 08-13-2011, 05:28 PM   #18
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Hi all,

Thanks a lot for the replies. I am basically a newbie at this, so please bear with my (probably) stupid questions/comments.

- Well, I was thinking of the worst case situation with the weight of the quadcopter. Basically, all I am going to have is 4 motors-prop-ESC, 1 battery and some electronics held together by a balsa wood frame with maybe carbon fibre rods to provide more stiffness. I am not really sure that the weight would end up being 2Kg, but now that I think of it, it would be around 1.2-1.4 Kg.

- I have a few questions about the terminology used here. The 'kV' rating refers to the rpm/Volt. Does this indicate the rpm of the motor per volt of applied voltage under zero load (no prop) condition? If that's correct, then in the case of SuperTigre motors I have, 950 kV means 950 rpm/V * 11.1 (in case of a 3-series battery pack) ~ 10500 rpm under no load. So, If I attach a prop now, the rpm of the motor would drop down depending on the size of the prop. Also, when the manufacturer says Max constant power : 145W, does that mean, for some combination of motor-prop, the product of the current and the voltage applied would be maximum, and that combo will result in the max thrust generated by the motor. So basically, you would maintain full throttle and try out different props and log the currents drawn and find that some props result in the motor developing close to max power, which they then specify as the recommended props. Is my line of thinking ok, or is it totally nonsense?

- Finally, given that I have the motors now, I am thinking it would be a good idea to try out one of the recommended props and see how it goes..

Thanks a lot for your patience and help!
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Old 08-13-2011, 07:52 PM   #19
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I think you're way over on the weight of this thing. If it's so small that you can only use 7" props, I don't see how you could get over half a kg unless a brick is part of the structure. Assuming you're 750g or less you could easily live with the 3 blade 7x8" props with the 3 cell battery (I'd go larger than the 2200 mAh you said at first if I wanted to fly more than four minutes) I think you have a combination you can live with. Even a 1 kg quad would fly okay with that power system, but I just don't see how you could get to that much weight.
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Old 08-13-2011, 07:57 PM   #20
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+1 on what RR has just posted... Plus in answer to your question on kv, power and prop size... yes, you have it all right
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Old 08-14-2011, 08:14 AM   #21
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I tried on 9x7 and 10x5 props and they were noticeably producing more thrust at the same power setting than the 7x5 prop. Btw, how do you guys convert from electric power (W) to mechanical force (grams of thrust)? Is it through a formula or more by experience?
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Old 08-14-2011, 09:47 AM   #22
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It's the prop size that's critical because it's the prop that creates the thrust. There are formulae available but they're quite complex involving motor losses, propeller efficiency etc so to get a good estimate of the thrust available most of us simply use one of the many motor calculator programs available, some online some not, some free, some not. See http://www.rcgroups.com/forums/showthread.php?t=606703 for a comprehensive listing of them.

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Old 08-14-2011, 12:41 PM   #23
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Boeing - everyone has been giving some great advice, I hope it works well for you. I'm still trying to figure out things with wings and 1 motor. Do keep in mind though with such a small prop you would do better with a much higher kv rated motor.
As for all the hacker hawkers - supertigre actually puts out a decent little electric motor. I have a handful of them and they are beefier than they look. And I've had zero issues with any of them.

That's not flying, that's just falling with style.
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Old 08-14-2011, 05:26 PM   #24
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Originally Posted by slipstick View Post
It's the prop size that's critical because it's the prop that creates the thrust. There are formulae available but they're quite complex involving motor losses, propeller efficiency etc so to get a good estimate of the thrust available most of us simply use one of the many motor calculator programs available, some online some not, some free, some not. See http://www.rcgroups.com/forums/showthread.php?t=606703 for a comprehensive listing of them.

Steve
Since you're attempting to fly a model with practically zero forward speed, seems you'd need a motor with low KV, low RPM and a large diameter prop for maximum thrust. Something like the rotor of a helicopter.

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Old 08-14-2011, 05:31 PM   #25
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Originally Posted by bbqflyer View Post
Boeing - everyone has been giving some great advice, I hope it works well for you. I'm still trying to figure out things with wings and 1 motor. Do keep in mind though with such a small prop you would do better with a much higher kv rated motor.
As for all the hacker hawkers - supertigre actually puts out a decent little electric motor. I have a handful of them and they are beefier than they look. And I've had zero issues with any of them.
LOL
As for motors, I just ran across a motor whose mfg gives it a very high power rating. Until you look at the watts per ounce of motor weight. It was 215 watts/ounce.

Motocalc suggests that motor will have 500 degrees F in its windings while running at its maximum power level during a flight.

IMHO, any motor running much over 100 watts per ounce of motor weight might be pushing it a little.

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