Lipo for rx power?

Anyone used a lipo to power an rx - not through an esc? Mentally Working through best power options for a small unpowered glider. Next question - if I couldnt directly power an rx by 7.4v lipo (7.4v is within rx voltage range) - would the BEC circuit on an ESC work if its not connected to a motor?
Basically trying to find longest lasting, lightest and cheapest power options!
Cheers for your help!

Yes, the BEC should work just as normal. As in normal use, the rules and specs would have to be followed for the number and type of servos driven from a given battery voltage

Ensure the motor wires are properly insulated!

You cant run the RX direct off a lipo, even the the Speki RX's will take the voltage the servos wont!

So either as you sugested use a ESC (not sure that will work properly though im sure the ESC side will get hot if not connected to a motor) or use a cheep voltage reg to drop the lipo V, Andy S has some cheep ones, or hobby city/giant cod etc etc

M

Is that what a UBEC is?
Bloody hell things have changed since the good old Nicad days!

No a UBEC is used in line with a ESC that has no BEC, your looking for a Lipo reg

M

If the requirement is for just two servos and to keep it light you could use a single lipo and one of THESE. to boost the voltage.

That could be the answer Allan thanks! Im building a 700mm span lalysee glider so want to keep things light!
You dont know what the unit would do to lipo run times do you? I presume it would drop flight times because of the increased voltage, but not being an electronics genius I dont know! (come on -I cant be an expert at everything!)

Ben, there are a couple of little dinky servos out there that will operate quite safely on the voltage supplied direct from a 2s lipo pack. In fact, I have a couple of them sat on my desk here at work right now They are the ripmax SD100 (also known as the Logic RC LSX110).

They are not particularly cheap, but they are good, and would be perfect in that little wing. You do need to make sure the control surfaces don't bind at all (otherwise the servo will burn itself out sooner rather than later).

However, in all my DLG's up until now, I normally use a 2s lipo that is regulated down to 5v. I use a small regulator that I designed (nice and light, 3g, and easily capable of driving 3 micro servos off a 2s lipo). I then size the 2s lipo pack based on how much noseweight I need, and how much duration I want. As a guide, a 450mAh pack with 3 servos onboard a 1m class DLG, lasts for about 6-7 hours or so.

Oh, and a really neat little product that I have tested in my ELF DLG, is a Smart-lipo from Hyperflight: http://www.hyperflight.co.uk/products.asp?name=smartlipo-240&code=SMART-LIPO-240

Not particularly cheap, but really nice! It's a single cell lipo device that outputs 5v at up to 1A max (perfect for small DLGs) but also has a built in low voltage alarm, and built in charger too.

HTH
Andy

I run most of my gliders on a 2 cell Lipo running through a UBEC but to be honest it's cheaper to get a 4 cell NiMh or NiCd from Component Shop they're less than a fiver. Lots of gliders are designed around a certain weight, some airfoil sections don't like to be loaded to lightly.

Just checked Component Shop a 4 cell 800mAh £4

If it was me I would use a nimh pack. Less possible problems than lipo and the big bonus! - You can fast charge at 1000A for 3 secs direct from your car battery when you find you've left it switched on overnight!!!

(Actually, better to take a sidelight bulb out of its holder and use that to regulate the current...... And for small packs an interior light... La de da)

I would use a 5 cell pack. Especially if its a 300mAh or less size as the voltage can easily dip below the brown out voltage for the receiver with 4 cells, particularly in colder weather. The extra weight is generally insignificant

If I was to use the BEC on an ESC and not connect the ESC to a motor, - wouldnt that just run the same as using the speed controller in a model with the motor not running? Dont see why that would heat the ESC up any more than if the motor was spinning and pulling extra current through the ESC?

Stubbsy wrote:If I was to use the BEC on an ESC and not connect the ESC to a motor, - wouldnt that just run the same as using the speed controller in a model with the motor not running? Dont see why that would heat the ESC up any more than if the motor was spinning and pulling extra current through the ESC?

I think that using an ESC's BEC without a motor running will heat up the ESC really fast.
If you use for example a 3s lipo it will give out around 12.6 volts or so.
The job of the BEC is to then drop the voltage down from 12v to say 5 or 6v to power the servos at a safe operating voltage.It does this by getting rid of the excess voltage through heat,so running a motor flat out helps to dissipate this excess voltage making less work for the BEC to do.
On my Align 450 for example,when running a low headspeed the esc after 5 mins is too hot to touch and goes into thermal shutdown.But when running the headspeed at flatout 100% it stays cool even after 7 mins.
I think thats how it works anyway?
Andy is that the correct way to explain it?

Danny.

Rich wrote:I run most of my gliders on a 2 cell Lipo running through a UBEC but to be honest it's cheaper to get a 4 cell NiMh or NiCd from Component Shop they're less than a fiver. Lots of gliders are designed around a certain weight, some airfoil sections don't like to be loaded to lightly.

Just checked Component Shop a 4 cell 800mAh £4

Cheers for that Rich - I managed to miss your post somehow!

Thanks all for the replies! I think I have so many choices that my best bet is to finish the build off first, and then see what weight I have left to play with. It may be that, with my expert modelling skills, there will be enough weight saving to cram a car battery in there - who knows!

If you do decide to go Lipo then get an SBEC the "S" relates to switching battery eliminator circuit which like the name suggests it switches several times a second to regulate the voltage, not like the UBEC (don't know what the "U" stands for but it regulates the voltage by dissipating heat.

Cheers Rich - Think I read somewhere the "U" stands for Ultimate - Ultimate Battery Eliminating Circuit! - sure someone will correct me if im wrong!

UBEC is a brand name (U = Ultimate), SBEC is the generic term as Rich says.

Danny, the reason most ESCs heat up at low throttle, is down to the switching losses in the FETs on the ESC. Brushless ESCs have two lots of switching on and off to do. The first lot are to switch the right motor phases on and off, so that the motor can turn round and round. The second lot (which are usually a much higher frequency) are to regulate the power going to the coils, and act as a "throttle". at full throttle, the second lot of switching becomes none existant and they are on all the time. At part throttle, you have the two lots of switching superimposed. Now FETs are most efficient when off, very efficient when on, and not particularly efficient when half on and half off. Unfortunately the time between on and off is not zero, so the more switching that happens, the more time spent in the most inefficient bit of their operating boundaries.

Oh, and inefficient = dissipates power as heat.

The BEC bit of ESC's is a seperate chunk of circuitry, and should operate independantly. It shouldn't make any difference if the motor is connected or not (as long as you insulate the motor connection leads).

HTH
Andy

Thanks for that Andy.
So basically does that mean he can just use the bec part of the esc without it getting hot?

Danny.

Yep, although depending on the load presented by the servos, and the supply voltage to the Esc, it may still get toasty warm (eg if a 3s pack is used, and many servos are being powered). You can try it on your little helicopter. Plug the batt in, and wiggle the servos around for a minute our two. Notice how hot the esc gets. Leave it idle, and it should cool down again (all without the motor turning btw).

An esc with a switching Bec should stay relatively cool doing that test too....

Andy

Thats intresting, I was just going off memory when i was setting my first electric heli up, no motor conected and setting up ccpm the ESC was getting very hot.

Must have just been my aplication

M

If you have no motor connected, and you open the throttle, that might cause a bit of heating, otherwise the only other reason for that, would be a linear BEC doing its job. If you were waggling the servos about a fair bit, off a 3s pack, that can cause a fair bit of heating.

Also, don't forget, in normal operation, you normally have a cooling airflow over the ESC....

Andy

Looks like I have opened a can of worms here!

Here we go......... Basic physics........... Current doesnt flow in an incomplete circuit, (fact) so if there is no motor connected to an ESC and the motor wires are insulated, doesnt that mean there will be no flow through that part of the esc as it is an incomplete circuit? Wouldnt that then suggest that an ESC wouldnt get hot, regardless of where the throttle stick is positioned, because that part of the circuit is inactive?
As I understand it, the motor draws the current it requires through the ESC. On Tx command, the ESC limits the amount it can draw, which in turn limits the motors speed - the heat caused by resistance in the ESC is actually then what makes it hot.

Not really a can of worms, there's no black magic going on inside the ESC, just a lot of switching things on and off. Quickly!

Right, the first bit, you are right. There is one other thing that sometimes rears its head if the circuit design is a bit shoddy, and that is shoot through. Each phase of the motor (3 of them in total, 3 wires remember) is controlled by half of a H bridge. At any one time, two phases are active (making a full H bridge circuit) and one is inactive. The H bridge that is active can switch current through the motor in one of two directions. (positive and negative). It needs to do this to commutate the motor correctly. Now, FETs take a finite amount of time to switch on and off. In the H bridge, two FETs need to be on, and two off. If the circuit design is a bit shoddy, and there is not enough time left between switching one pair on, and the other pair off, you basically get a short circuit for a small fraction of a second. This is known as shoot through, and obviously causes things to get a bit warm. On decent ESCs, the controlling logic is well tested, but on cheap ones, it sometimes isn't. (it needs to be tested, because once you hook up a motor, which is basically a series of coils and magnets and can have a wide range of inductance, capacitance and resistance, it can affect the FET switching time a fair bit).

The second part is correct. The heat is cause by the resistance in the current path, mainly in the FET and associated circuitry, plus the switching losses. The current is limited by switching the FET on and off quickly. Because the motor is basically a coil with inductance, this smooths the switched voltage/current out a bit......

HTH
Andy (who has a strange desire to uncover the blackboard and get the coloured chalk out again....)

I feel sick

Andy,I have a quick question.
Whats the deal with the Kontronik ESCs.Every one I use runs incredibly cold all the time regardless of what the throttle curve is set at.
Once you bring the headspeed up you hear it switch to a closed loop circuit.(or that's what it says it does)
How does that work?

Danny.

Barnes: Man up.

Danny, Kontronik ESCs will fall into the "incredibly well deisgned" bracket, which means that they use the best available FETs (Most efficient, fastest switching) and they design the switching circuits properly too. The BEC's are also switching type too, which means they don't heat up much due to the high efficiency.

Regarding the closed loop bit, then it is just the logic in the controller being a bit clever and holding the motor at a set RPM (or RPM band rather). Normal ESC's operate in a fashion where they are not bothered about the actual RPM, but instead just control the power going to the motor in a linear (or sometimes exponential, or curved) fashion. Operating in a closed loop mode (Or governor mode as it is sometimes known as) is just a posh way of saying that the ESC brain is monitoring the RPM, and doing it's bestest to hold that RPM within a certain band around the commanded value, by automatically varying the power going to the motor through the FETs.

The ESC is basically saying to the motor - Right, get going,come on chop chop. OKay, you are moving now, what speed are you spinning at? Ah, that much. That's not enough according to mr Pilot, so I'll give you some more power. Now what speed are you at? Oh bugger, too much, I'll reduce the power. Now what speed? Ah, bingo, just what we need, I'll keep the power as it is, and check again in a moment to make sure nothings changed.... And so on....

It sounds complicated, but in reality is dead simple. That said, it can be quite tricky to get the gains/PID settings right in the ESC software, but again, a decent manufacturer like Kontronic will be good at that.

Andy

PS. All ESC's need to monitor the speed of the motor attached, otherwise they can't turn the FET's on and off in the right order at the right time to make sure the motor keeps going round. Adding a governor mode, is just an extra bit of software