4.8v last longer than 6.0v? Really?

4.8v last longer than 6.0v? Really?
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4.8v last longer than 6.0v? Really? - 8/3/2005 1:22:07 AM

STG

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I have heard time and time again that a 4.8(4 cell) volt receiver pack will last longer than a 6.0volt (5 cell) pack(with the same cells). This makes senses to me if the servos always were drawing full power(the 6.0volt pack putting out more work), or always at no load but on. But, in the real world, when we fly it we are putting different loads on the servos and it would seem to me that it is the watts that do the work holding the servo where you want it.

My guess is that it is the holding power that significantly draws energy?. For the servo to hold a surface needing 10oz of torque the servo draws more amps than at rest? And at 90oz of torque it draws a lot more watts than at 10oz of torque? Am I wrong here?

I also think that in order for a servo to hold at 80oz of load it is the watts that are supplying the power to do this? So that would mean that the 4.8 volt battery would draw more amps to get the work done than the 6 volt battery. Is this not the way work gets done?

Is this not the way servos work? What am I missing?

Iin the real world what drives these servos is watts (volts x amps = watts) and what really pulls power from the battery is when you put a load on the servos.(fly them around aggressively -- KE puts a lot of work on rudder servo )

4.8volt battery pack x 1400mah = 6,720wh
6.0volt x 1400mah = 8,400wh

As you can see the 6 volt battery carries another cell and so another 1680wh of energy.

Now, if you placed both the batteries under the same load condition so they draw the same amount of watts (flew exactly the same flights--remember it is the watts that drives the servos) do you still think that the 4.8 volt battery with 1680wh less of energy will outlast the 6 volt battery?

Thanks

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RE: 4.8v last longer than 6.0v? Really? - 8/3/2005 1:39:21 AM

olstoney

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Very good question Steve, I'll be interested in the answer.

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RE: 4.8v last longer than 6.0v? Really? - 8/3/2005 3:01:57 AM

smokingcrater

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**disclaimer... I'm not an EE, this is just my stab at explaining it... I'd be curious to do a test, maybe this winter i'll try equal 4 and 5 cell packs in one of my planes with my futaba 9c servo exercise function. **

lets reduce it to the simplest measures... imagine a motor turning at a constant rpm with a constant load (ie an electric motor with a prop) although this isn't exactly what a servo is doing, it is roughly the same. if you increase the voltage on that electric motor, you are also going to increase the wattage. all things being equal, since your battery pack also has higher wattage, then it should be the same. but at higher voltages motors tend to run less efficiently and arc more, so more wasted power and your total required wattage goes up...

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RE: 4.8v last longer than 6.0v? Really? - 8/3/2005 3:38:34 AM

STG

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I have a servo that needs to hold for 30 seconds at 70oz of torque. One is on a 4.8volt battery and one is on a 6.0 volt battery. There is an amp meter in line, will it draw more amps with the 6.0v(5 cell) volt battery at 70oz than with the 4.8v battery?

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RE: 4.8v last longer than 6.0v? Really? - 8/3/2005 3:53:02 AM

huey-pilot

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Another way to think of it is two buckets the same size full of water, one of the buckets has 4 holes in it, the other bucket has 5 holes of the same size. The bucket with 5 holes will empty first. If you want the 5 hole bucket to last as long as the 4 hole you will need a larger bucket.

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RE: 4.8v last longer than 6.0v? Really? - 8/3/2005 4:07:37 AM

STG

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quote:

ORIGINAL: huey-pilot

Another way to think of it is two buckets the same size full of water, one of the buckets has 4 holes in it, the other bucket has 5 holes of the same size. The bucket with 5 holes will empty first. If you want the 5 hole bucket to last as long as the 4 hole you will need a larger bucket.

That is true if it were the case that the servos were allowed to draw full torque all the time, but wont the servos just draw enough power to get the job done?

----------------

I finally found something on this...

http://www.rcgroups.com/links/index.php?id=4592&cat=281&t=articleprint

"Centering Test - 58 oz-in of torque

As expected, as the load increased on the servo, the deflection from the zero-load center position increased. At 4.8V, the median deflection was -10.5 degrees and at 6V, it was -6.8 degrees. The currents drawn at 4.8V and 6V were 438mA and 460 mA respectively."

Ok, here we have the same amount of torque (58oz) and the 4.8v was at 438ma and the 6.0v was at 460ma---very close(within 5%), but the 6.0 volt is accomplishing more work! As it was only allowing for 6.8 degrees of deflection and not 10.5 degrees as with the 4.8v (both these #'s sound like a lot ha?)

It makes sense to me that it can give better centering as it is putting out more watts at 58oz load. 6v x 460ma=2760 4.8 x 438ma=2102.

"With the application of 40 oz-in of torque, it is readily apparent that the servo does not reach the same position as it does under no load. At 4.8V, the servo had a median deflection from the median zero-load position of -4.7 degrees. While this may seem like a lot, it still represents only about 7.8% of the total servo travel, meaning that the control surface deflection is still a very large portion of what the pilot is asking for. At 4.8V under 40 oz-in of torque, the servo was drawing 222 mA.

It was interesting to note that at higher voltage, the performance of the servo was markedly improved. The angular deflection at 40 oz-in decreased to approximately -4.0 degrees and the servo only drew about 8% more current (239 mA). This makes a good case for using 5 cell receiver packs if you have the option and your servos are rated for it. "

As you can see again 8% more, but increased performance!

I think I am on to something here.

< Message edited by STG -- 8/3/2005 4:18:39 AM >

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RE: 4.8v last longer than 6.0v? Really? - 8/3/2005 9:33:48 AM

Red Scholefield

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Why don't you simply measure the capacity taken per minute of flight time.

1. Charge/dishcarge each pack and record the capacity.
2. Charge each pack and go fly both in similar flights for a recorded period of time.
3. Immediately after flying discharge each pack and find out how much capacity is remaining by subtrcting 3 from 1.
4. Divide answer 3 by time 2 and you have mAh/minute which will give you the answer to "Does a 5 cell pack last as long as a 4 cell pack of equal capacity?

The difference will be small but never the less measurable.

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RE: 4.8v last longer than 6.0v? Really? - 8/3/2005 11:25:06 AM

JNorton

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Watts is a figure of energy used. P=IXE or power equals current times voltage. The servo motor has the same DC resistance or AC impedance regardless of the voltage pack used. This being so there will be more current draw if more voltage is applied. More current is more wattage used on the same servo with a 6 volt pack as compared to a 4.8 volt pack. The wattage does not stay the same from 4.8 to 6 volt packs, but increases.

It has been the experience of most users with 6 volt packs verses 4.8 volt packs (of the same cell size) to have less flight time. I totally agree that you do as Red suggests and you actually measure the remaining capacity of your packs.

Some of this boils down to user application. Most of the guys use 6 volt packs for the higher torque rating they get from using the higher voltage pack. They need the higher torque because they fly 3D or really ring out their aircraft with stunts. Their servo's are used more hence more battery drain.

Just my 2 cents,
John

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RE: 4.8v last longer than 6.0v? Really? - 8/3/2005 1:53:53 PM

STG

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Thanks guys. I see by the test I quoted in the post above that at the 2 different torque values that the 6.0 volt pack is drawing from 5% to 8% more amps, but at the same time holding the servo closer to its correct position. 5 to 8% does not sound like a lot to me if the performance of the servo in increased. I could see 2 battery packs of the same Ma rating and voltage to be different by 5 to 8%. It looks like the real penalty that is paid by going with 5 cells is the weight of the extra cell more than shorter flight times.

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RE: 4.8v last longer than 6.0v? Really? - 8/5/2005 8:16:06 PM

IGOS R/C

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STG

I would like to add a few points. As we know from electricity law the power will be the same for a given motor because drained power is its particular characterists that resides in wire diameter and its extension (winding), rotor weight, friction etc. If the Amps go up the Volts must go down just to verify the law that is around for a long time. But what we see is a servo specification that is underestimated. The servo's motor can handle and is design for 6V with a safe margin or it will burn out but is related as to 4,8V. When we plug a 6V pack on it there is a gain in speed and torque. When we increase the Volts the speed (not flow) of electrons goes up increasing the servo's speed and that's all. But why a gain in torque ? Who's increase it ? Speed is not torque. As per electricity law the power must be the same or Volts up current down. Here is the point. The wire's motor can yet bear and suck a little bit more current for 6V. Of course the power will be bigger and obviously will be the torque. Assuming that the maximum and safe current is that one at 4,8V we only could get a increase in servo's speed.
Hope I help anyway.

IGOS R/C

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RE: 4.8v last longer than 6.0v? Really? - 8/5/2005 9:07:48 PM

JNorton

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quote:

When we increase the Volts the speed (not flow) of electrons goes up increasing the servo's speed and that's all.

When you increase the voltage the current draw also increases - so the current flow also increases as well as the energy level (what you call speed).

It doesn't stay the same. All in all IGOS/RC you do a pretty go explanation considering that English is your second language.

quote:

But why a gain in torque?

When you increase the magnetic field (from increasing the voltage) the torque of the motor will increases. The field pushes the armature which rotates the servo arm.

Enough everybody is probably falling asleep!
John

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RE: 4.8v last longer than 6.0v? Really? - 8/5/2005 10:01:50 PM

IGOS R/C

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Hi JNorton !

Seemed to be confusing to explain anything without establish a fixed point or variables and surpass the language barrier. You're right. But I go a little bit ahead and say (establish) that our motor's servo is at its maximum current limit, this means that the battery pack has much more current to supply but due to its design specifications (winding wire diameter mainly) there is no way to get it. Better understand with a motor whose nominal current is 20A and the power supply can deliver just 10A. What will happen ? The wires from supply get hot may be melt. The motor will try all the time to get what it needs. Any motor need a current in excess from the supply to run properly. And it takes just the amount of current it needs. Not more not minus. That's the law.
Thanks for your comments.

IGOS R/C

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RE: 4.8v last longer than 6.0v? Really? - 8/5/2005 10:20:53 PM

JNorton

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Just agreeing.

Like I said everyones probably falling asleep. The quickest way to end a meeting with my boss is to explain in detail what I'm doing. It is a shame the dumbing down of society, at least RCers want to know how something works.

We design them, build them, paint them, fly or drive them and crash them. All in all RC people are great. Nice meeting you IGOS R/C.

John

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