mred

That is true, but the better ESC's have an adjustable cutoff voltage feature and it can be set to kick out at 3.2 or 3.4 volts per cell. I know of one that will not cut out until 2.8 volts and you cannot change that. It just depends on the ESC. I strongly advise anyone to get a decent ESC so that kind of thing does not happen.

I had one land in a corn field and never did find it. I'm sure the combine did, but we never did. I don't guess it hurt the combine very much or we would have heard about it. Can't imagine what it did to the plane though. No LiPos on board either, just Nicads.

Ed

kyleservicetech

Being an old farm boy from the 1950's, one of our smaller models would not have much an effect on a combine. You might find rather pretty confetti coming out of the back of the combine though.

And, our club has had models land in the corn fields when they had radio problems. Trying to find ANYTHING in a 7 foot tall corn field is near impossible. You simply can not see more than a few feet in any direction.

During a fun fly at one of the clubs nearby SE Wisconsin last year, they had warnings for anyone going into a 160 acre cornfield surrounding their field on a high 80 degree day. You can get lost and have real problems with heat stroke or worse.

AEAJR

I would suggest you go back and read the article that is in the first post. This is all explained in great detail with lots of examples.

AEAJR

Actually the risk is minimal.

The dreaded number for lipos is actually 2.5 Volts RESTING voltage. Your ESC cuts off between 2.75 and 3V UNDER LOAD.

Once you remove the load, the voltage bounces back up by anywhere from .2 to .5V.

So, let's assume your ESC cuts at 2.7V under full load. Once the motor stop drawing, the voltage will immediately bounce back up to, say 3.1V, well above the dreaded 2.5V and even above the missunderstood 3.0V.

The only time this can be problematic is where your LVC is based on a % of the starting voltage, as some ESC do. Now, if your battery is partially discharged your starting voltage is lower. Let's say 11.5v instead of 12.6. The LVC might now cut the power at a lower voltage.

You are still not likely to end up below 2.5V resting voltage but you are clearly getting dangerously close.

Also, if you ignore the LVC and keep restarting the motor on lower and lower throttle settings, you can get it to restart becasue the voltage sag will be less under lower throttle settings. Again you could strat to drain the pack down into a problematic range.

My advice is, once your LVC cuts the motor, you should only look to restart in order to make a safe landing. Otherwise, plan to glide in, but don't let the plane drop into the trees or into the ditch. If you can get a short burst at 1/3 or 1/2 throttle, better to do that than damage or lose the plane.

kyleservicetech

Agreed, if your ESC cuts power, really try to just glide in.

Why? It's because these Lipo's or A123 cells might not be completely balanced when you run the battery down to where the ESC cuts off power. If one cell goes down first, it might drop WAY down before the ESC kills the motor. (The other cells can be still putting out rated voltage) This isn't a real problem with a 3S or 4S pack, but it can get serious when you have battery packs of 6S or higher.

I've built up a six channel digital voltmeter for my 6S2P A123 packs that allows watching voltage on all six (series) cells in real time. That voltmeter is accurate to 0.2%. And I've watched one cell go down first while the other cells continue to put out power. And that one cell was at 2.5 VDC and rapidly dropping off before I cut power. And the remaining 5 cells were still above 3 VDC. One reason to keep your cells balanced. But even so, properly balanced cells can still have a "weak link" or weak cell.

That's just the way these batteries work.

PaperAirplane

Going back to the last page when i was figuring mah and ah and amps and volts out i just want to say OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOd iOOOOOOOOOOOOOOHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH HHHH!!!!!!!!!!!!!!!!!!!!!!!!!

I got it now!!!!
Here is how I understand it.

if you have a 1000 mah battery rated at 20c and you have a motor that needs 2000 mah then you could still use the motor, but only for 1/2 hour. and the max mah you could draw from the battery would be 20000 mah (1000mah x 20C).

Right???!!!!

mred

Almost but not quite. The most you can draw from a 1000mah battery is 1000ma. The rate you are pulling current from the battery is 2000ma, so you can run the motor for 1/2 hour, but it will still only draw 1000ma out of the battery. You can't draw more out of the battery then you can put in, so in this case, it is 1000ma's. The 20C rating is the MAX current you can draw from the battery at a constant rate. So 1000maX20C will be 20amps.

When I was talking about using the throttle to get back to the field, I was talking about landing it, not flying around on it. What I was trying to say was, it's better to use the battery to make the field then to land in a corn field or a group of trees. Running the throttle stick fully back will reset most ESC's so you can use the motor again for a very short time. Normally I try and keep it under half throttle or less while landing or the ESC will cut out again. Also, some planes don't glide worth a hoot without some power, so I was just tell him a way to make it back to the field to land. Like I said before, I never run my battery down to cutoff. I only use 80% or so out of them and then land. That way, I don't find myself in a place where I can't get back to the field even using the motor after cutoff.

Ed

PaperAirplane

Ok ive definitely got it now.

Thanks soo much to everyone!!!

kyleservicetech

That's it! Your 1000 mah battery has 1000 mah/1000 or 1 Ampere Hour. And that 20C rating indicates the maximum current you can pull out of that 1 ah battery is 20 Amps, typical values for a "Foamie" airplane.

So that 1 ampere hour battery is also 60 Ampere minutes (60 minutes in one hour). If you are pulling 9 amperes out of your battery pack, you can fly for 60 ampere minutes/9 amperes or 6.6 minutes at full throttle.

Just backing off the throttle to 80% power can reduce your motor's current draw by 50% depending on the model, so that could double your flying time.

I've been flying Castle Creations ESC's. They have a feature that includes "Soft Power Down", (not certain of its exact name) rather than just shutting the motor off on low battery. I like that better than just killing the motor.

PaperAirplane

Where do the volts come in though?

Do they affect it at all? You need the right amount of volts to get the right amount of watts, correct?

slipstick

Yes. You've spent a lot of time talking about capacity (mAh) and current (mA or A). Voltage is the other important factor.

If your motor is pulling 10A on a 7.4V (2S) battery that's 74 watts.

The same current (10A) on an 11.1V (3S) battery gives you 111 watts (roughly) and so on.

It also ties up with the motor Kv (or RPM/volt) cus that together with the voltage tells you how fast the motor will try to spin the prop.

It's all been covered earlier on in this thread.

Steve

kyleservicetech

The volts issue is covered in the first few pages of my PDF file per below.

http://www.wattflyer.com/forums/showthread.php?t=45173

You are right, the voltage we are exposed to range from a volt or three on a typical battery cell, to 120/240 Volts AC in your house, to 2400 to 14,400 volts or so in the power lines around your home. And that increases to 138,000 to 250,000 volts and higher for those steel transmission towers we all see here and there in in the country side.


For DC (Battery) power, the watts equals the voltage multiplied by the amperes. So a motor pulling 55 amperes at 18 volts is 990 watts. It's 746 watts per horsepower, so 990/746 equals 1.32 horsepower. And the higher quality brushless motors run around 90% efficiency, so that is 1.32 X 0.90 or about 1.18 horsepower on the motor shaft.

(For AC motors you see around your home, things get nasty and confusing in a hurry. That involves voltage, current, phase shift between the two and so on.)

And if you see three wires on the power poles, that's three phase power. It's interesting that our brushless motors are pretty much three phase motors!

PaperAirplane

Wow!
That is a lot of good info!
I read about seven pages of the pdf, very informative.
Once I figure out my power system I will run it by you to double check.



And thanks to "slipstick" too for the reply.

kyleservicetech

Take a look at my following thread:
http://www.wattflyer.com/forums/showthread.php?t=45222


If you are going the Lipo route or smaller models, I'm the wrong guy to ask for double checking, got no experience. But that 45222 thread covers what's involved when electrifying the four stroke 70 glow sized models.

Might be you can find something useful in there!

PaperAirplane

Okay then.
I will still post it here for anyone else who wants to double check it.

AEAJR

PaperA,

What typically happens is you try to work these things through in your head and it can be a bit confusing if you have no experience working with electricty. Then, one day, it clicks.

The tool that REALLY helps you understand AND will save you a ton of money in burned up motors, batteries and ESC is the wattmeter. This tells you what is actually happening in your power system.

> WATTMETERS - WHO NEEDS THEM
> http://www.wattflyer.com/forums/showthread.php?t=11863

This discussion will help you understand their value and give you examples of wattmeters. I use the Watts UP meter. Simple to use, easy to understand.
http://www3.towerhobbies.com/cgi-bin...&I=LXLMV0&P=ML

I have seen people burn up all kinds of things because they used the wrong combo of motor/prop/esc/battery. It is a good investment for anyone who is going to pick their own power system components.

PaperAirplane

I am out of money right now, but I may still be able to get the wattmeter if there isnt anything cheaper.

Thanks.

PaperAirplane

Question:
If my motor draws 14A and has 175 input watts the voltage would have to be 12.5V.
I know this is somehow not correct because I have never seen a 12.5V battery and a 4S battery would not be necessary for a 175 watt motor.

Can someone explain what I did wrong?

Thanks

kyleservicetech

Which battery?

175 Watts/14 Amps = 12.5 Volts. Divide 12.5 by 3 cells equals 4.16 volts, I think that is a little high for a Lipo??

But if its four A123 cells, they have a lower voltage, and that could be right.

Or, you've got 4 Lipos running at 3.1 volts each, which might be overloading them.

PaperAirplane

This is the motor:
http://www.hobbypartz.com/alpha-450-...ess-motor.html

and this is the battery:
http://www.hobbypartz.com/alpha-450-...ess-motor.html

kyleservicetech

Confused?

Looks like both web pages show the motor?

PaperAirplane

this is the battery:
http://hobbyking.com/hobbycity/store...00mAh_3S1P_20C
this it the motor
http://www.hobbypartz.com/alpha-450-...ess-motor.html

reubin

It's good article in the first post. It's explained in great detail with lots of examples.
very thank you ,AEAJR.

AEAJR

reubin,

Glad you found it helpful.

AEAJR

Your math is right but your approach is wrong.

If you want to know how many watts your current system is producing you have to measure it. A watt meter is good for that.

OR

You can estimate it based on published information from the motor mfg for that battery/prop combo.


If you have done your watts/pound calculation and detemine you want a power system that produces 175 watts then you work from that.

What voltage battery do you plan to use?

2 cell lip0 = 7.4V
3 cell lipo = 11.1V


To get to 175 watts you need a motor/prop combo that will produce 175 watts on that voltage

175/7.4 = 23.6 amps
175/11.1 = 15.7 amps

So, you look for a motor/prop combo that will draw 23.6 amps on a 2 cell lipo, or 15.7 amps from a 3 cell lipo.

From here you look at things like desired performance. Is this a sailplane where climb is king and top speed does not matter? Or is this a pylon racer where top level speed is important and climb is less of a concern. That will help you decide what kind of prop you want.

You can have two motor/prop combos, both of which produce 175 watts, but produce very different performance. This is where kV comes in to play.



I have been trying to get you to read the first post. This is copied from the first post:

If you are going to design your own systems you will need some help and some
knowledge. What I have compiled is a list of links that take you to resources
to help you. First I recommend you read this article for background
information. It will help you understand and use the rest of the resources
listed below.

Sizing Electric Power Systems -
http://www.ampaviators.com/index.php?option=com_content&task=view&id=41&Itemid=27

Remember that the system operates as a whole. If you have the right motor
with the right battery and put on the wrong prop, things can get pretty ugly
pretty fast