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Author Topic: Battery/amps conversion 4cell to 5cell?  (Read 968 times)

Offline Dennis Toth

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Battery/amps conversion 4cell to 5cell?
« on: November 04, 2021, 08:29:48 AM »
I am trying to get a rough idea of where the amps will move (assuming same motor, prop and rpm setting) if I go from a 4S pack to a 5S pack. Is there a straight forward formula to calculate how many amps one needs with the 5cell vs. the 4cell? Also how does the total mah capacity change?

Best,  DennisT

Offline Teodorico Terry

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #1 on: November 04, 2021, 08:31:39 AM »
It depends on what motor and prop you intend to use.  Depending on the Kv of the motor chances are that you will have to switch to a smaller prop if you up the voltage. As a rule of thumb you do not want the full throttle amp draw with the prop you intend to use should not exceed the limits of the motor or ESC.  If you are running the ESC in governor mode you have to be careful.  If you measure the current with the motor running you be measuring an average current, not peak. The motor will be actually experience the peak current as the ESC pulses the power on and off (more power - more power on pulses, fewer power off pulses. Less power - fewer power on pulses, more power off pulses).  If you are running it in the inefficient range of the motor/prop combination you will have a motor that gets really hot.

What motor and prop combination are you using?  I can run it through E-calc and make a suggestion based on that.

Offline Dennis Toth

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #2 on: November 04, 2021, 08:40:27 AM »
TT,
The rpm is set by my ESC, motor and prop are the same only the battery pack is changing from a 4S 25C to a 5S 25C. I expect the amps will go down something like the ratio of the voltage change?

Best,   DennisT

Offline John Rist

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #3 on: November 04, 2021, 01:18:41 PM »
TT,
The rpm is set by my ESC, motor and prop are the same only the battery pack is changing from a 4S 25C to a 5S 25C. I expect the amps will go down something like the ratio of the voltage change?

Best,   DennisT
Not necessarily.  The problem is that you are wanting to turn the prop at a constant RPM.  The amount of voltage to achieve this is determined by KV of the motor not the maximum voltage of the battery.  So with the 5 cell pack or the 4 cell pack the ESC will apply the same voltage to the motor for a given fixed RPM.  The 5 cell pack will provide more head room for the ESC to do it's job.  I would think that changing prop size would effect the amp more than changing the cell count.  You can run your motor with no prop and the RPMs will be the same but current will go way down.

The KV of the motor is RPMs per volt.  So if you have a motor with a 980 KV and you want to turn 9500 RPMs, divide RPMs with KV and you get 9.6 volts required achieve this amount of RPMs.  If you are using a 14.4 volt battery (4 cell) you will have 4.8 volts of head room for the ESC to use to maintain the RPMs.  However you get voltage loss in the wiring and loss of voltage as the battery pack discharges.  Hopefully the 4.8 volt head room is enough to cover this loss.  It requires power to turn the prop.  Power is voltage (V) times current (A).  With fixed voltage current goes up and down with changing load.   To make a long story short, that is why you need to match the KV of the motor to the number of cells in the pack.  A lower KV motor requires a higher voltage to achieve a given RPMs.  The higher voltage requires less current to achieve the power needed to turn the prop. So a 5 cell pack will work at a lower current level if you change the KV of the motor to match a 5 cell setup.   n~   D>K
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Offline Dennis Toth

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #4 on: November 04, 2021, 02:51:07 PM »
OK, we seem to be missing the objective, let me explain. What I am trying to achieve is to use a thunder power 5S 1350mah pack with a Bad Ass 2320  820Kv motor and have it run for 4min and 50 sec (this is what I need to complete the Old Time pattern). This pack save about 1 3/4oz over the next option which is a thunder power 4S 2200mah (I have looked at the available 1800 mah packs from other suppliers and they weight the same as the thunder power 2200 mah pack. Ihunder power does not offer a pro lite 1800 mah). So, the rpm is set at 9800 in the ESC (I use fixed rpm mode and use the timer just to time the flight). I have a test stand that is setup for the 4S pack and I am trying to determine if I can get amp data from the 4S pack and covert it to what the 5S would roughly run in the ship. I'm trying to save a lot of trial and error at the field by being close to start with in the A/C of my shop.

It seems like I need roughly 13 amps static for the 5S 1350 to have 20% head room at the end. Granted the inflight amps are lower and I have taken that into account. If I test props of different diameters on the 4S test rig and get the amps I would like to do a rough conversion to what it would be with the 5S so I can see it I will get to the level I need. In short I am trying to save some weight by trying the higher voltage but lower amps of the 5S and hoping I can find a prop that will pull the ship and give 20% ish battery head room.

Best,  DennsT


Online Ken Culbertson

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #5 on: November 04, 2021, 03:39:23 PM »
I have done full patterns (PA not OT) on a 2820 970kv motor using a 4s 2200 battery.  5:40 run time.  Draw down was to around 15%  and a couple of times on a dead calm day around 10% which is really low but apparently not too damaging as I had 50  flights on them and no swelling.   I would think with the lower KV motor, less run time and higher voltage that you can get what you want with the 5s 1350 but you might be closer to 15% than 20% when you land.  This might reduce the live of the battery a bit but how much does a 5s 1350 cost, maybe $40?  Wait for TP's Black Friday sale and get 2!

Ken

 
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Offline John Rist

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #6 on: November 04, 2021, 04:01:25 PM »
What Ken said.  820 KV really is a 5 cell motor.  I am running 970 KV motors on 4 cells.   I am not sure your 820 KV motor would have enough head room to work on 4 cells.
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Offline Dennis Toth

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #7 on: November 04, 2021, 04:37:55 PM »
Wait a minute, not using 4 cell packs for the flight pack. Will be using the 5S, what I'm trying to do is get a idea of how much I need to trim the prop to get the amps down enough to use the 5S pack and still have a reasonable amount of head room. Just using the  4 cell on a test rig to compare amp draw from different diameter props to get an idea of what will work for on the 5 cell setup. Does it not just go by the ratio of the voltages?

Best,    DennisT

Offline John Rist

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #8 on: November 04, 2021, 04:52:53 PM »
Wait a minute, not using 4 cell packs for the flight pack. Will be using the 5S, what I'm trying to do is get a idea of how much I need to trim the prop to get the amps down enough to use the 5S pack and still have a reasonable amount of head room. Just using the  4 cell on a test rig to compare amp draw from different diameter props to get an idea of what will work for on the 5 cell setup. Does it not just go by the ratio of the voltages?

Best,    DennisT
Short answer is no.  This is because you are not applying the full voltage of the battery to the motor.   The ESC, set in the constant RPM mode, acts as a voltage regulator to step down the voltage to arrive at the set RPMs.  As long as the LiPo battery voltage is higher than needed it all works.  A little higher or a lot higher makes no difference.  The voltage and current at the motor should be about the same.   This is my take on it.   I may be wrong but I don't think so.
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Offline Fred Underwood

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #9 on: November 04, 2021, 05:26:07 PM »
If I understand correctly, Watts fly the plane.  I didn't see how much battery you use with 4s, but unless you have a fair amount of battery left with the 4S, you won't have enough with that 5S.  Your total Watts available are quite a bit less.  I'm under the impression that ESC's don't really drop or adjust voltage much.  The apparent voltage change is more an adjustment of the length of time to apply the voltage vs off time.  A pulse width function, not a resistor function.  There may bae a better explanation from the engineers, but then I may not understand it.
Fred
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Offline John Rist

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #10 on: November 05, 2021, 01:16:03 PM »
It's not simple. That's fore sure.  You have high speed switching into a combination of  inductance, capacitance, and resistance.  But as I see it for a given motor swinging a given prop at a fixed RPMs it will require a given amount of RMS voltage to achieve those RPMs.  The ESC through some magic will supply this amount of RSM voltage at a fixed value to maintain this study rate of RPMs regardless of cell count, (as long as the battery voltage is higher than the required RMS value).  Current will vary up and down as the load changes.  At a fixed load the current will be at a level that yields the watts required to support the load.  So from this I conclude that for a given motor swinging a given prop at a fixed RPMs the current will not change much if you change the cell count.  There will be some changes due to changes in efficiencies.   My thinking may be flawed but that how I see it.  Wouldn't be the first time I was all wet.   D>K
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Offline Dennis Toth

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #11 on: November 05, 2021, 01:36:22 PM »
John,
The ESC works like a pulse width control on the voltage, it sends out pulses of whatever voltage the pack has,  it is just in shorter or longer duration to get the rpm it is set for. The current draw is the pressure that the electrons are pushed at to turn the load (prop) at the set rpm. So if the load is reduced the amps go down, to reduce prop load you either reduce diameter, blade thickness or pitch. Diameter has a major t impact (it is a squared function) then the other two. The trick is when you reduce diameter you also reduce some pulling power, the ship slows down, now you need to increase pitch a bit (could increase rpm but this is like adding back the diameter), its a balance game sometimes you can hit the right combination an get the benefits other times it just won't.

Best,    DennisT

Online Ken Culbertson

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #12 on: November 05, 2021, 02:51:31 PM »
This has so many moving parts that it is difficult to predict what prop you should use.  I would just slap on a 11-5 or 10-5 three blade, fly the thing and see what you get.  Wind alone can make a 20% difference in how much battery you use.  The prop charts that the MFG use are useless for CL.  They test in a high RPM RC environment.  Cutting holes in the clouds is not the same as trotting around a sphere.

Good luck - Ken
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Offline Mark wood

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #13 on: November 05, 2021, 04:28:21 PM »
John pretty much explained it correctly. The propeller dictates the power required at a specific RPM. The motor will dictate the voltage and consequently the required current to run the propeller. To utilize a higher voltage battery to reduce the line loss power (P=I^2 R) the motor must be changed such that it runs the propeller at that speed at higher voltage (lower Kv rating). What you are after is a lighter battery for a flight which is the same as saying you want the highest energy density battery you can get and not many battery maker tell you that directly. Your stated mission is a battery to fly the flight as light as can be and have a remaining energy margin of 20%. Here's the kicker, most batteries have similar energy densities so you're really after a package that holds just enough energy to get the flight in and not damage itself.

What you really need to know is the power required to run the propeller. If it's an APC, that can be looked up on their website. A number 13 amps doesn't tell one much any more than stating a motor is a 1,200 Kv motor.. In isolation neither means much. Static power required is a good starting point and it can be measured on your power meter if you have one. You can calculate the total energy of the flight to be static power in watts times flight duration in seconds, with the in flight power reduction sizing a battery to that works fairly well for battery sizing when you  consider the static running time and flight time and all combined energy required. So with that, I can assume that the power you're thinking is 13A x 18.5 V = 240 Watts. That's sounds like a 10x5 propeller at around 9800 rpm to me.

So the total energy required for a flight is 240 W x 290 s = 69,600 joules. Now calculate the total energy capacity of the battery for a sanity check. Your first battery has 4S (14.4 V x 2.2 AH) x 3600 s/H  = 117,216 Joules of energy capacity. The battery you want to go to has 5S (18.5 V x 1.35 Ah) x 3600 s/H = 89,910 Joules of storage. Take the flight required 69,600 divided by the capacity 89,910 = 77% which tells me the smaller 5S battery should work for your installation but may be marginal for a PA pattern. For a PA flight, 240 W x 330s = 79,200 J or 79,200 / 89,910 = 88%.

 As a sanity check I fly a Cobra 2820 840 Kv motor using a 4S 1800 pack and it works great. I have a similar plane using a Cobra 2820 1170 Kv and a 3S 2200 pack. Both power packs fly the airplanes equivalently well. And weigh basically the same. The small difference is the 3S pack runs at a slightly higher power consumption consistent with the line loss calculation. Static power check using the in line power meter is around 240 W using an APC 10x5 prop.

For the 4s 14.4 V x 1.8 AH x 3600 = 93,312 J and the 3s 11.1 V x 2.2 AH x 3600 = 87,912 and both packs weigh about the same. Admittedly the advantage in  this example goes to the 4S pack.

Oh here's an interesting observation and you can prove it out with the math. For an average flight which is roughly 0.1 hours the required battery size will be approximately the static running current times ten.
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Offline Mark wood

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #14 on: November 05, 2021, 04:33:04 PM »
If I understand correctly, Watts fly the plane. 


That is perfectly correct. It takes X number of watts to counter act the drag of the airplane at velocity and it takes those X watts plus some to turn the propeller and then again plus some to run the motor to run the prop to pull the airplane. It's all simple conservation of energy. What's goes out must come in. I tell my students, the goesintas must equal the goesoutas.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Tim Wescott

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #15 on: November 05, 2021, 05:42:55 PM »
Gonna wade in here (sigh).

So, before I start -- I've designed switching motor drives before.  That's basically what an ESC is, except that the ones I've designed have been fancier and much larger for the amount of power handled.  Which is the long way of saying -- yes, I know what I'm talking about.

John, Fred, and Mark seem to be on track here.

I am trying to get a rough idea of where the amps will move (assuming same motor, prop and rpm setting) if I go from a 4S pack to a 5S pack. Is there a straight forward formula to calculate how many amps one needs with the 5cell vs. the 4cell?

To a first-order approximation, yes.  If you know the current for four cells, multiply it by 4/5 -- that's close to the current for five cells if you don't change anything else.

Also how does the total mah capacity change?

The same factor of 4/5.

About all the battery voltage, Kv, RMS voltage, current at the motor, change your prop, freak out, tear off all your clothes and go dancing through the streets, etc., etc. --

An ESC is a switching amplifier, and a motor is a honkin' big inductor.  To the extent that the efficiency remains the same (this is important), if all you do is increase the cell count by one you'll take the same power out of the batteries to drive the motor at the same speed.

In general, if the system works with four cells, it'll work with slightly diminished efficiency with five cells -- you'll lose a few percentage points efficiency, your motor and ESC will run marginally hotter, the current reduction won't be quite a factor of 4/5 -- but unless something is really wrong, it'll be within 5%.

I'm even sure that it'd be worth the experiment to try slapping six cells into a "4 cell" system.  I suspect that at that point the motor would start getting perceptibly warmer, and you'd have to go to a more expensive ESC, which might also run warmer.  All things being equal, efficiency would be down even more than with the four to five cell jump.  I'd expect that if you tried it, you'd say "yes! a win for science!", then you'd want to go shopping for a motor with a lower Kv rating.

...  Depending on the Kv of the motor chances are that you will have to switch to a smaller prop if you up the voltage. ...

Nope.  Not for CL stunt.  Yes, that's absolutely solid advice for someone flying RC without a speed-regulated ESC, but it's not how electric CL stunt works.  Control line stunt is just about the opposite of that.  With "normal" RC, the voltage to the motor is always limited by the voltage at the battery, and any time you nail the throttle that's what the motor sees.  With control line stunt you never ever want the ESC to be at "full throttle" -- when that happens, it means that the motor speed has fallen out of regulation, and unpredictable things (like sagging in the overhead maneuvers) will happen.

What does happen is that the system regulates the speed of the motor.  The motor, being pretty efficient, needs the power it supplies to the prop plus 10 to 20 percent more.  The ESC, also being pretty efficient, needs the power it supplies to the motor plus 5 to 10 percent more.  Matching the motor Kv to the "full throttle" output of the ESC does make everything more efficient, but not terribly so -- as long as you're not overheating anything, running out of voltage overhead when the plane's overhead, or drawing the batteries down too much each flight, you're fine.
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Offline Mark wood

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #16 on: November 05, 2021, 06:30:46 PM »

An ESC is a switching amplifier, and a motor is a honkin' big inductor.  To the extent that the efficiency remains the same (this is important), if all you do is increase the cell count by one you'll take the same power out of the batteries to drive the motor at the same speed.

In general, if the system works with four cells, it'll work with slightly diminished efficiency with five cells -- you'll lose a few percentage points efficiency, your motor and ESC will run marginally hotter, the current reduction won't be quite a factor of 4/5 -- but unless something is really wrong, it'll be within 5%.

I'm even sure that it'd be worth the experiment to try slapping six cells into a "4 cell" system.  I suspect that at that point the motor would start getting perceptibly warmer, and you'd have to go to a more expensive ESC, which might also run warmer.  All things being equal, efficiency would be down even more than with the four to five cell jump.  I'd expect that if you tried it, you'd say "yes! a win for science!", then you'd want to go shopping for a motor with a lower Kv rating.


Interesting info Tim. Thanks. Not being an EE type guy, it's nice to get an insight to the internal workings of the black envelope. It makes sense that the larger components have lower resistance and in turn better efficiency. For us electricity mere mortals looking at the wire losses is impordant but it seems likely the benefit of going higher voltage and lower line loss is, in part, offset by the losses in the switching components. From what I can tell by observation of A-B tests the impact is nearly negligible. For me, I look to find the battery size based on it's energy density for the mission. Then the size (cell count) sets the rest. The integration process, airplane, propeller, power plant, energy storage, check, optimize repeat. What I see a lot is people going to higher cell count batteries in search of the efficiency and incurring a significant weight penalty which in turn results in not real gain but it must be better... Right? Of course it is, wouldn't have done it and spent the money if it weren't. Therefore, it is better.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Tim Wescott

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #17 on: November 05, 2021, 08:18:22 PM »
The weight per watt-hour of LiPo packs are pretty constant -- if you get the watt-hours right.  So a 5-cell 3000mAh pack will weigh about the same as a 4-cell 3750mAh pack and so again a 6-cell 2500mAh pack, and they all pencil out to having the same energy capacity (around 55 Watt-hours).

Where folks run into trouble, is either thinking that because they need a 3000mAh 5-cell pack then they need a 3000mAh 6-cell pack (so it's 20% heavier), or they can't find a decent 2500mAh 6-cell pack.

(Note that the above capacities are chosen because they make nice round numbers when I do the math, not because they're all in anyone's catalog).

I think a big part of the reason that you see the "big boys" going to higher cell counts is because the active timers (Burger and Fioretti, mostly) ask for more speed from the motor in the overheads, and they tend to use more energy per flight.

So if you have a 5-cell setup with a matching Kv on the motor for a constant-speed application, then when you go to an active timer the setup may well run out of poop in the overhead maneuvers because you have this perfect storm of more speed being demanded from batteries that are running more flat -- putting another cell on the pack easily fixes both of those problems, and if the limiting dimension in your battery tray is across the width of a cell, then you don't change that dimension in your new heavier and thicker (but not longer or wider) pack.

There's no reason, if you have a 5-cell system that's running out of steam in the overheads, that you couldn't go to a motor with a higher Kv, or a larger cell size, or both.  I suspect that with most setups, as long as the ESC can stand another cell, it's easier to just bump the cell count up by one.
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Offline Dennis Toth

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #18 on: November 06, 2021, 06:51:45 AM »
Tim,
Thanks for jumping in here, I was hoping you would as someone who knows EE in detail. My limited basic electrical courses are way in the past but I did remember some. So for my purpose I can get a rough approximation of how the current will change going from the 4 cell to the 5 cell as I play with the prop load.

For me I am not looking to emulate a 4-2-4 Fox35 type run in my electric setup but more of a pipe run. I simply set the rpm in the ESC, play with the gain a little then work with the prop to get acceptable results. One thing about electric is since the motor does not "unload" on launch and holds the rpm in the wind we can use a more efficient diameter/pitch prop that will pull through maneuvers as the load increases. The current IC setups use poor prop diameter/pitch to keep the engine from running away as the wind starts to push the plane, allowing the engine to unload and you go faster and faster.

I look at battery size first to have the capacity to cover the flight time matched to the KV of the motor then I try to get that in the lightest weight possible by looking at the different packs available that deliver the needed power/capacity. This is like playing with fuel tank load on an IC where you reduce the fuel load to get in the flight without an overrun. For me fitting battery size to a fixed ship design (like OTS or Classic) where you can't change the design to carry the load is the challenge but doable.

Best,   DennisT   

Offline John Rist

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #19 on: November 06, 2021, 10:05:06 AM »
When you’re wrong you’re wrong.  Before I was guessing what happens to the current when you change the cell count from 4 to 5 cells (everything else staying the same).  So I decided to run an experiment and let the science dictate my thinking on the matter.  I do not have any 5 cell LiPo batteries in hand so I could not run 4 to 5 cell test.  However I do have a Ringmaster that runs on 3 cells and I have lots of 4 cell batteries.  So I ran the following test between a 3 cell and a 4 cell LiPo battery plugged in to my Ringmaster.

SET UP:
Motor = Cobra 2221/10 KV 1500
Prop  =  APC 10x6 EP
ESC  =  Trunigy Plush 30A
Timer  = KR set at 9000 RPMs
Amp/Volt meter = Astro Digital Meter

TEST RESULTS:
3 Cell LiPo Arrouind 3300 20C
No load voltage =  12.2 Volts
Full load voltage = 11.5 Volts
Motor running current = 20.8 Amps

4 Cell LiPo TP 2800 25C
 No load voltage =  16.2 Volts
Full load voltage = 14.0Volts
Motor running current = 17.3 Amps

Power being consumed P=Voltage X Current.
3 Cell 11.5 V x 20.8 A = 239 Watts
4 Cell 14.0 V  x 17.3 A – 242 Watts

From this it can be seen that the power in both cases is about the same.

So to answer the original question: Calculate the power being consumed in the 4 cell setup by multiplying the current times the voltage applied.  To predict 5 cell current take the power obtained in the 4 cell setup and divide it by voltage of the 5 cell battery.  If you use 14.4 volts for the 4 cell battery and 18 volts for the 5 cell battery you should come close to a good answer.

Moral of the story if you are guessing at an answer you may well be wrong.

Attached is a photo of my test setup
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Offline Dennis Toth

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #20 on: November 06, 2021, 10:37:10 AM »
John,
Cross multiply and divide by the bottom number works close enough for starting point got 17.08 amps.

What I am working on is an S1 Ringmaster that is an original Sterling kit with a new fuse (saved 3oz over the original kit fuse with motor mounts, 1/8 ply doubler). Mine is coming out with the 5S pack at 32oz. This is why I am working to get the 5S to work. Doing the simple math I need to get to around 12 ish static amps which might need cutting the prop diameter to 8 1/2" diameter with a 6.5 pitch would give a 4.9 ish lap time on 60' C to C.

What is your total ready to fly weight, line length and lap time?

Best,    DennisT

Offline John Rist

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #21 on: November 06, 2021, 11:04:34 AM »
John,
Cross multiply and divide by the bottom number works close enough for starting point got 17.08 amps.

What I am working on is an S1 Ringmaster that is an original Sterling kit with a new fuse (saved 3oz over the original kit fuse with motor mounts, 1/8 ply doubler). Mine is coming out with the 5S pack at 32oz. This is why I am working to get the 5S to work. Doing the simple math I need to get to around 12 ish static amps which might need cutting the prop diameter to 8 1/2" diameter with a 6.5 pitch would give a 4.9 ish lap time on 60' C to C.

What is your total ready to fly weight, line length and lap time?

Best,    DennisT
Dennis see
 https://stunthanger.com/smf/gettin-all-amp'ed-up!/e-ringmaster-as-built-by-tom's-building-service/ 
For info on my Ringmaster.   Line length 60',  27-1/2 oz ready to launch  don't know lap time.
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Offline Tim Wescott

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #22 on: November 06, 2021, 11:44:19 AM »

Power being consumed P=Voltage X Current.
3 Cell 11.5 V x 20.8 A = 239 Watts
4 Cell 14.0 V  x 17.3 A – 242 Watts

From this it can be seen that the power in both cases is about the same.


Hey John -- thank you for taking the time to do the experiment, and posting your results!  I was feeling a bit exposed to just be pontificating from theory; it's good to see someone taking actual measurements on this.
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Offline Dennis Toth

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #23 on: November 06, 2021, 12:08:46 PM »
John,
I did a quick search on the Hyperion 3S 1600 at 5.1oz, on the same site the 3S 1800 pack was only 4.6 oz, for $2 more you save 1/2oz on a Ringmaster that's pretty good deal. One of the things I am considering is moving the battery pack from the wing pocket to the front fuse side. This gets the CG at around 1 1/2" back from the leading edge without adding extra weight. The 5S pack is shorter then the 4S packs and fits well, problem is once I make this change I need to cut some slots in the fuse for the hold-down straps so it is a commitment and that's why I'm trying to get a feel for if the 5S pack will work before cutting into the fuse.

In the setup thread you indicated you used a 9x6 EP prop (Tom's recommendation) in the test you listed a 10x6 EP, which prop draws the 20 amps? With your setup can you get through the OTS pattern (particularly the vertical 8's)?

Best,    DennisT

Offline John Rist

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #24 on: November 06, 2021, 05:52:33 PM »
John,
I did a quick search on the Hyperion 3S 1600 at 5.1oz, on the same site the 3S 1800 pack was only 4.6 oz, for $2 more you save 1/2oz on a Ringmaster that's pretty good deal. One of the things I am considering is moving the battery pack from the wing pocket to the front fuse side. This gets the CG at around 1 1/2" back from the leading edge without adding extra weight. The 5S pack is shorter then the 4S packs and fits well, problem is once I make this change I need to cut some slots in the fuse for the hold-down straps so it is a commitment and that's why I'm trying to get a feel for if the 5S pack will work before cutting into the fuse.

In the setup thread you indicated you used a 9x6 EP prop (Tom's recommendation) in the test you listed a 10x6 EP, which prop draws the 20 amps? With your setup can you get through the OTS pattern (particularly the vertical 8's)?

Best,    DennisT
OOOPS again.  Another old age brain fart.  It is indeed a APC 9 x 6 EP.  Thanks for noting the error.   mw~.   Don't know about OTS pattern. All I ever do is a little sport flying.
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Offline Teodorico Terry

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #25 on: November 07, 2021, 03:07:03 PM »
Tim,

I would like to ask you a question regarding heat.  I was of the understanding that the ESC is in principle a switch which sends full power pulses which are modulated to deliver the power desired.  Using the same prop, going from 4S to 6S will result in a significant increase in current for each pulse.  At the higher voltage, given the higher current as well, fewer pulses are need to accomplish the task.  I believe this is why the measured current goes down; the Whatt meter measure the average current draw.  Is this roughly right or am I off?

Now to the question about heat a portion of which is generated by the resistance of the coils.  This varies with the square of the current. I was wondering is if the heat generated by the motor is more of a reflection of the peak current averaged for the duty cycle rather than the average current. 

For example; tested a motor on 4S and got a current draw of 24 amps at full throttle.  Same set-up at 6S and the current went up to 48 amps. Obviously different RPM in both tests.  To achieve the RPM of the 4S set-up using the 6S battery the current dropped to about 16.5 amps but the motor did appear considerably warmer after a 20sec run.  I do not have an IR thermometer so this was just a finger test.  Does this make sense or is my interpretation of how things work incorrect?

Thank you,

Teo

Offline Tim Wescott

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #26 on: November 07, 2021, 07:10:36 PM »
I would like to ask you a question regarding heat.  I was of the understanding that the ESC is in principle a switch which sends full power pulses which are modulated to deliver the power desired.  Using the same prop, going from 4S to 6S will result in a significant increase in current for each pulse.  At the higher voltage, given the higher current as well, fewer pulses are need to accomplish the task.  I believe this is why the measured current goes down; the Whatt meter measure the average current draw.  Is this roughly right or am I off?

Roughly right.  It's more accurate to say that the ESC switches full voltage pulses to the motor.  What you're leaving out is that the motor has inductance, so when the ESC is switching rapidly enough, the current generated in the motor coils is somewhat smoothed out.  There's a lot of variation in motor characteristics and ESCs, so exactly how much smoothing is taking place varies a lot from setup to setup.

At any rate, the average current from the battery should reflect the actual amount of power going into the ESC.

Now to the question about heat a portion of which is generated by the resistance of the coils.  This varies with the square of the current. I was wondering is if the heat generated by the motor is more of a reflection of the peak current averaged for the duty cycle rather than the average current. 

Now this is getting complicated.  When you design a motor driver for a specific motor, you design things so that the I2R losses are no more than 10% for current variation, with the rest of the I2R losses being for the actual average motor current.  But if you grab some random combination of motor, ESC and battery off the shelf, this may not happen.

For example; tested a motor on 4S and got a current draw of 24 amps at full throttle.  Same set-up at 6S and the current went up to 48 amps. Obviously different RPM in both tests.  To achieve the RPM of the 4S set-up using the 6S battery the current dropped to about 16.5 amps but the motor did appear considerably warmer after a 20sec run.  I do not have an IR thermometer so this was just a finger test.  Does this make sense or is my interpretation of how things work incorrect?

It's hard to say.  I'd want to do the test with an ESC that does speed regulation, and have an IR thermometer at hand.  I'd want to do controlled tests, running the motor for about the same amount of time each time because it can take minutes for a motor to heat up fully, and letting the motor cool for a good long time between tests because it takes ages to cool down without help.

For quick measurements of system efficiency it's probably much better to use a Wattmeter or two meters to independently measure the motor voltage and current -- without very careful thermal management, trying to determine motor power usage by temperature is going to be very inaccurate.
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Offline Teodorico Terry

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Re: Battery/amps conversion 4cell to 5cell?
« Reply #27 on: November 08, 2021, 08:24:06 AM »
Tim,

Thank you for taking the time to answer my question.  I am aware that the ESC allows the full voltage to be applied and that the current develops in response to the load.  Maybe I should get an IR thermometer so that I can actually measure my temperatures.

My concern was not so much the efficiency of the set-up per se (pick a large enough battery and you will be able to fly for as long as you want provided nothing melts) but rather how much heat can be generated if the wrong prop is used given a motor and battery.  I have noticed that in my full bodied stunter it is difficult to get good airflow over the motor.  With limited cooling available, the motor will get warm.  This got me thinking that maybe the heat generated by the motor is a function of the square of full throttle current scaled to represent the average current when the motor is governed.  In essence:

Heat is proportional to (Imax*Imax)*(Igov/Imax)   

Imax = open throttle current
Igov  = governed current

This does not give me an actual temperature number but simply a way of comparing set-ups if I have a reference that I know works.  For example I know that the motor can operate at 24amps on 4S at full throttle 100% of the time and not get cooked.  So the reference number would be:

Heat proportional to (24*24)*(24/24) = 576 This is a "safe" value as the motor can operate in that range all day long.

Now if I run the system on 6S the peak current is of 48 amps while the governed current is of 16.5 amps:

Heat proportional to (48*48)*(16.5/48) = 792

Relative to the safe value, the new set-up generates 1.375 times more heat, maybe high enough that I would be better off trying a smaller prop.  Is this a sensible way of comparing things?

Thank you,

Teo





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