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Got me a Wattmeter - what's it telling me? What can I learn using it?

Old 12-04-2011, 11:16 PM
  #1  
prof_fate
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Default Got me a Wattmeter - what's it telling me? What can I learn using it?

Bought a wattmeter yesterday and it's pretty cool - does all sorts of things (servo testing, batt check and balancing, plus watt readings).

OK, so I hook it up and hit the throttle and see that about about 40% throttle it reads 40w and at full 185w (which BTW caused the motor to tear the motor mount loose from the fuse, but that's another issue).

OK, so say I avg 50w flying it around (3 channel foamie trainer type plane, allows for takeoff, climbing, etc)). It's running on a 2 cell lipo. It's got a 1650mah battery. How long will if fly?

I know V * A = watts. I know I can switch that around to get watts/volts = amps.

So at cruise that would be 50/7.4 = 6.75 amps. I get stuck at this point.

How to get that amp figure to tell me how long a battery will last?

If I do the the math again using the max watts I saw I get 25 amps of draw - so I'd need a 25A (or bigger) esc, right?
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Old 12-04-2011, 11:35 PM
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Battery volts x battery charge in Ah = battery capacity in Watt-Hours.

In your case a two cell battery will average about 7v under load, so that's:
7v x 1650/1000 = 11.55 W.hr

You would not normally use all the battery capacity, to do so would mean flying until cut-out and landing dead stick which would shorten the life of the battery, so say you use 80% of battery capacity:

11.55 x 0.8 = 9.24 W.hr

If your plane averaged 50W power consumption the duration would be 9.24/50 = 0.18 Hours (or multiply by 60 to get minutes) = 10.8 min.

Of course this is only as good as your estimate that the plane used 50W
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Old 12-05-2011, 12:01 AM
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so then assuming the same Ah 3 cell batt (which would be heavier I know) you'd get about 50% more flight time?

11v x 1650/1000 = 18.15 W.hr
18.15 X .8 = 14.52 w/hr

14.52/50 X 60 minutes is 17.4 minutes.

So I can then use this math on any plane to get an idea of when to get down out of the sky before well, I come down out of the sky, right?
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Old 12-05-2011, 12:15 AM
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Yep, that's right.. Obviously though you could not just stick a 3 cell battery into your 2 cell model without making sure the other components could handle it. Going from 2 cell to 3 would, if nothing else changed, result in about 300% more power at WOT, which would likely burn out your motor and/or ESC.

Also adding more battery weight increases the power you need to keep the plane in the air.

Steve
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Old 12-05-2011, 12:31 AM
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OK, so can we use this math at all to come up with an idea of the ideal cell count or prop?

I tested a motor with the same prop and a 2 cell and then 3 cell batt - I got a whopping 2 watts more power with the 3 cell batt. (47 vs 45 watts).


How can I get more power out of this motor? Larger diam prop, different prop (apc vs gws, orange vs black), pitch change (more or less)? Is there a rule of thumb type thing? (more cells means more RPM so change X to keep the same power). All I"m sure of (if I even am) is that more pitch means more speed....so assuming one has enough watts to stay airborne changing from an 8x6 to an 8x4 would result in what? Less amp draw since it's easier to spin the prop, but that would probably increase motor RPM...(as would increasing cells all else being equal).

So on a nearly over powered plane (2 cell) spinning an 8x6 GWS would moving to an 8x4 mean more flite time? Or would a 10x4 be a better choice (as a larger prop would be harder to spin but perhaps take the same force to spin as a smaller, higher pitch prop).

Or is it time for me to go to bed and start again tomorrow...
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Old 12-05-2011, 12:40 AM
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Somerhing is seriously wrong with your 3s pack then. What were the volts under load?
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Old 12-05-2011, 02:08 AM
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kyleservicetech
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Originally Posted by prof_fate View Post
Bought a wattmeter yesterday and it's pretty cool - does all sorts of things (servo testing, batt check and balancing, plus watt readings).

OK, so I hook it up and hit the throttle and see that about about 40% throttle it reads 40w and at full 185w (which BTW caused the motor to tear the motor mount loose from the fuse, but that's another issue).

OK, so say I avg 50w flying it around (3 channel foamie trainer type plane, allows for takeoff, climbing, etc)). It's running on a 2 cell lipo. It's got a 1650mah battery. How long will if fly?

I know V * A = watts. I know I can switch that around to get watts/volts = amps.

So at cruise that would be 50/7.4 = 6.75 amps. I get stuck at this point.

How to get that amp figure to tell me how long a battery will last?

If I do the the math again using the max watts I saw I get 25 amps of draw - so I'd need a 25A (or bigger) esc, right?
Another way to figure out flying time is by using Ampere Hours. With your 1650 Milliampere Hour battery, that's 1.65 AMPERE HOURS. Which translates to 1.65 times 60 minutes/hour or 99 Ampere Minutes. (Note that you have Ampere Hours times 60 Minutes/hour, so the Hours drop out, leaving your result in "Minutes")

Since you are pulling 6.75 Amperes out of your battery, take your 99 Ampere Minutes and divide by the 6.75 Amperes, that is 99/6.75 or 14.66 minutes. (Again note that the units are ampere minutes, and amperes. Since you are "dividing" Ampere Minutes/Amperes, the unit "Ampere's" drop out, leaving your answer in minutes.)

Now, depending on your mode of flying, I never fly more than about 60% of the battery on any given flight. That way, if someone is on the flying field, if you do more acrobatics, or if you've got to go around, there is power left in the battery to do so. And, IMHO, running your LiPo battery down to where the ESC cuts power can be a little hard on your $$$ batteries. Plus, the larger higher powered airplanes usually don't do well with an off field landing due to the ESC shutting down power due to a low battery.

If you update your current to 25 Amps, (assuming you don't burn up anything), the flying time would be that 99 ampere minutes/25 amps, or just under 4 minutes. That'd be a little under three minutes in the real world.

You can also work with that terminology "C" that defines the discharge current on a percentage of the battery ampere hour capacity. So, if you are discharging 6.75 Amperes on a 1.65 Ampere Hour battery, that gives "C" as 6.75/1.65 or "C" equals 4.09. Since "C" is based on 60 minutes, dividing 60/4.09 results in that same 14.66 minutes.

Either method works on calculating flight time, use what ever works for you. Note that "C" is also used in charging currents on your LiPo's. Most times a charge rate of 1 C is used. So that would be 1.65 Ampere Hours with C=1 or a charge rate of 1.65 Amperes. Using a charge rate of 3C would be 1.65 times 3 or 4.95 Amps. That high of a charge rate can cause damage to a LiPo battery not designed for it. Oh, and by the way, if C equals 3, your charging time is 60 minutes / 3C or 20 minutes.

Hope that helps.
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Old 12-05-2011, 07:15 AM
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Originally Posted by prof_fate View Post
I tested a motor with the same prop and a 2 cell and then 3 cell batt - I got a whopping 2 watts more power with the 3 cell batt. (47 vs 45 watts).
As already noted, there is something very seriously wrong with your 3cell battery if you only get a 2 Watt increase.

Motor Watts increases by roughly the voltage increase to the power of 2.6 . Going from 2 cells to 3 cells is a 1.5 x increase; 1.5^2.6 = 2.86. So that's 286% more power by going to three cells. All this extra power comes by pulling a lot more amps (2.86/1.5 = 1.9 times the Amps). So you need to be VERY careful if you go to a higher cell battery.

I can only assume your 3 cell battery was either too small (and/or too low c rating) to deliver the required Amps, or the battery was just old and very tired.

Steve
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Old 12-05-2011, 08:15 AM
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Originally Posted by JetPlaneFlyer View Post
Motor Watts increases by roughly the voltage increase to the power of 2.6 . Going from 2 cells to 3 cells is a 1.5 x increase; 1.5^2.6 = 2.86. So that's 286% more power by going to three cells.
That can't be right. Adding 50% more voltage surely can't triple your power.
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Old 12-05-2011, 10:00 AM
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Well 50% more voltage will generally give you at least 50% more current too if you don't change anything else which would be 1.5 x 1.5 = 2.25 times the power.

But I'm not sure where Steve JPF's power of 2.6 comes from, that's new to me. But I'm ready to learn .

Steve
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Old 12-05-2011, 10:16 AM
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Yeah, it's right, makes you think doesnt it!

Power to drive a prop is proportional to the cube of the speed of the prop. A 50% increase in voltage will attempt to drive the prop 50% faster; so that's 1.5^3.

In reality it wont quite go 50% faster because of the extra load hence my rule of thumb of ^2.6.

You can see this relationship if you look at motor test data such as: http://www.flybrushless.com/motor/view/610

Taking an example at random from those tests, a GWS 9x5 at 6.7V draws 85 Watts. At 9.5V on the same prop it draws 213 watts.


Now lets check against my 'voltage ratio^2.6' rule of thumb:

9.5/6.7 = 1.42

1.42^2.6 = 2.48

2.486 x 85W = 211W

So less than 1% varience from actual test results. Feel free to try it against other data for other motors, it usually comes out very close.

Steve
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Old 12-05-2011, 11:18 AM
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just a comment .... based on what an electronics guy told me and my own limited qualifications .....

Part load readings are subject to large error, particularly those based on amps in the equation. Only WOT readings would be near accurate.

His reasoning being that the Brushless motor is fed "pulsed" power frrom the ESC ... which can lead to serious error in ampage readings. It's ok as long as the meter averages out though.

It's a bit like the old days when you could charge a Tx and Rx battery of the glow plug socket on the field power panel .... it pulses 12v to give an effective 1.5v for the plug ... but being 12v pulses will actually feed a larger battery system ... Oh for the Good Old Days !!
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Old 12-05-2011, 03:57 PM
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Regardng the 3 cell output I used a battery I used in a small heli - it and the 2 cell also, had JST connectors.
These are of course connected to small wires going to the battery.

What affect does the size of battery wire (since it can't be changed) have on all this? The new plane that read 185w on 2 cells (the 1650mah batt) has a deans and a big fat wire (14awg or so) compared to the other batts (18 to 22awg)
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Old 12-05-2011, 05:14 PM
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Wire size and connectors are a huge impact. Those JST connectors are rated (depends on source) anywhere from about 2-5amps. I have used them to about 10 amps, but they get fairly warm (indicating loss).

Again what were the volts under load of the 3s pack? I think it is unable to do what you need it too and the JST's are really not happy at much over 5-6 amps.

Mike
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Old 12-05-2011, 05:43 PM
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How about mini Deans - i've but them on some planes (my F3p 3d foamie)

the 3cell pack was 11.1 v or there abouts - I checked all my packs on the meter for balance and charge, balanced one just to see what it did and it takes all cells to the lowest cell -all batts were 93% or better capacity except for the balanced one, it dropped to 43%.
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Old 12-05-2011, 06:06 PM
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I have run Dean's mini connectors to 15 amps with zero issues. A much better choice. If you are getting 11.1v under load that is 3.7v/cell and is fine.

I don't understand your last statement of balancing...sorry. You are starting with the battery fully charged - 12.6v right?
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Old 12-05-2011, 06:32 PM
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Originally Posted by rcers View Post
I have run Dean's mini connectors to 15 amps with zero issues. A much better choice. If you are getting 11.1v under load that is 3.7v/cell and is fine.

I don't understand your last statement of balancing...sorry. You are starting with the battery fully charged - 12.6v right?
THere is charged and there is balanced - is each cell putting out the same power? Taking the same charge? If you balance charge your battery what you are doing charging each cell individually to it's max potential. Having a cell .05v less than another is considered out of balance.

So 12.6 is fully charged - or 4.2 volts per cell. When does your charger stop charging? When it hits 12.6? What if it never does since as batts age they may not hold a 'full' charge - will it keep pumping in volts, overcharging a cell or two to make up for one that's not taking a full charge? How do you know a batt is reaching the end of it's life, or not performing up to full expectations?

This is why multi cell lipos have 'balance' plugs - you could just shove voltage back in the plug you use to run your plane with, but you'd not be able to balance the cells that way.

My charger can charge or balance charge and I noticed the other day that I'd had it on charge, not balance, and since the wattmeter can show me each cell's specific charge I thought I"d have a look and see. It will also tell me what percentage charge is on the batt. My charger might do this I'm not sure - the chinglish manual and nested menus are difficult to decipher and navigate.

The RC car guys tell me it can make a big performance difference and your batts will live longer if you balance charge them. I remember back in the day of nicad cells the car guys would go to great lengths to find and maintain matched cells.

I have a batt checker for 'regular' type batts and have noticed over time, even though I use a set of 4 nimh AAs as a set from new, use all 4 in my flash (i'm a photogrpaher) and charge them in sets of 4 they are rarely equal in charge after a few dozen uses, and the weakest one determines how long my flash runs.

Last edited by prof_fate; 12-05-2011 at 07:38 PM.
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Old 12-05-2011, 06:56 PM
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Not quite sure why you're telling us what balancing means . But I do think you keep answering the wrong question....

Let's try again. What was the voltage under load of the 3 cell battery i.e. when the watt meter was reading 47 watts what were the corresponding voltage and current measurements ?

It's likely that the battery could not handle the current being asked of it and the voltage dropped well under 11V.

Steve
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Old 12-05-2011, 06:58 PM
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prof,

I'm guessing Rcers is well aware of what balance charging is. I think it's the "all batts were 93% or better capacity except for the balanced one, it dropped to 43%. " that is leading to the confusion, certainly i dont know what you mean.

A fully charged and balanced LiPo will measure 4.2V per cell, or 12.6v for a 3 cell pack, give or take a few hundredth of a volt. Even old batteries will charge to 4.2v per cell, where batteries show their age is what voltage they maintain when you put them under load. If your 3 cell pack was measuring 11.1V 'resting' (i.e. not in use) then it was basically fully discharged.
What size in mAh and 'c' rating was the 3 cell pack? If the motor pulled 25A on a two cell pack then 3 cells would want to pull nearly 50A, so unless the 3 cell was something like 2200mAh and 20-25c then the battery discharge limit would be drastically restricting performance. The fact that it battery has a JST fitted screams to me that it was not rated for even close to 50A, more like 5A! The surprise is the the low voltage trip didn't activate but perhaps that was set for two cells?
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Old 12-05-2011, 07:09 PM
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Lipos have balance plugs not to charge through but for feedback purposes. A decent quality Lipo charger will have a receptacle for this plug and will use the info to balance the charge rate. Some chargers will only balance when discharging or 'storage' charging. You'll have to figure that out if the documentation isn't useable.

I find I most use my wattmeter to test props. The propeller functions like a transmission and changing diameter or pitch will alter the power draw from your battery at the same time it changes the thrust. In this way you set the amount of thrust produced; the airspeed of the aircraft; the rate of power consumption and thus flight duration; and last but most important: keep from overloading your motor, battery and ESC so as to keep the magic smoke where it belongs.
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Old 12-05-2011, 07:52 PM
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Originally Posted by JetPlaneFlyer View Post
prof,

I'm guessing Rcers is well aware of what balance charging is. I think it's the "all batts were 93% or better capacity except for the balanced one, it dropped to 43%. " that is leading to the confusion, certainly i dont know what you mean.

I used the meter to read the charge of each batt. All read 93% or better. I balanced one and after balancing it read 43% state of charge. Point being, if you don't balance charge your batteries then the real capacity is determined by the lowest cell. Which can be much lower than the total capacity might show.

I did not use that battery for the testing of the motor.


What size in mAh and 'c' rating was the 3 cell pack
?

factory 3 cell batt - no C rating marked on it.

If the motor pulled 25A on a two cell pack then 3 cells would want to pull nearly 50A, so unless the 3 cell was something like 2200mAh and 20-25c then the battery discharge limit would be drastically restricting performance.

This makes sense. I"ll have to retest and see what I get.
The plane I was having trouble with has a 10a esc and jst connectors - it came that way. I'd flown the plane on 2s and 3s but after making changes to it it wouldn't get off the ground - hence the interest in getting a wattmeter and seeing what's going on. a 15oz plane and 50w of power tells me it won't fly. Before the mods it was a 10oz plane and flew on the 50w (assuming that didn't change for some reason - I changed nothing on the plane)
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Old 12-05-2011, 08:03 PM
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It would be most unusual for a balancer, in the process of balancing, to discharge the battery to 43%.
In fact if it does then it's pointless because you wont ever be able to get a battery to be both fully charged AND balanced, which is after all the object of the entire exercise.

Most chargers these days do charging and balancing in one process so what you get at the end is a fully charged and fully balanced battery.

It seems certain to me that the 3 cell battery you were trying on your plane was hopelessly undersized for the duty (50A). Probably a good job too because a properly sized battery would have quite likely burned out the ESC and/or motor.

Steve
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Old 12-05-2011, 08:16 PM
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Balancing while charging will bring up the low cells to the high cell.

A balancer that cannot charge has only one way to do it - bring the high cells to the low one. How it does it I don't know, but that's the explanation it gave me.

http://www.ronlund.com/rcheli-prod/B...NTWOMETER.html is the device
After balancing it showed me 43% full. Just telling you what I saw.
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Old 12-05-2011, 09:20 PM
  #24  
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I just picked up the AstroFlight Whattmeter. They are invaluable for telling you the numbers to help prevent damage to your gear.
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Old 12-05-2011, 10:18 PM
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Prof - yes I understand what balance charging is.

So to get to some of your questions. Yes all three cells should discharge at the same rate (or very close).

Yes fully charged is 12.6v total (for a 3s pack). Your charger should stop there or very close to that...

The way you know your battery is working well is with your wattmeter. So you hook it up to a load and see what voltage under load (while the motor is running with a prop) it is holding. Good batteries should hold at least 3.5v/cell many new high C packs do MUCH better than that. When you remove the load they will quickly rebound to the resting voltage of the pack (remaining voltage).

If it is holding that voltage (say 10.5v for a 3s pack) while running it is happy with the load it is asked to do. If it is dropping below that - you have a battery that is not up to that task.

You MUST change the connectors out JST's simply won't cut it at your amp loads.

If you are using a stand alone balancer (like the one you list) is starts at 93% for all three cells and cuts with all cells are 43% charged you have a issue with a cell in the pack or the balancer is calibrated improperly.

As pointed out above - I don't generally use stand alone dischargers. I use balance chargers and those have the ability to lift the voltage and balance all the cells WHILE charging.

Mike
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