Power Systems Talk about motors, ESC speed controllers, gear drives, propellers, power system simulators and all power system related topics

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

Old 12-05-2011, 10:50 PM
  #26  
kyleservicetech
Super Contributor
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952
Default

Originally Posted by mclarkson View Post
That can't be right. Adding 50% more voltage surely can't triple your power.
Yes it can!

That is the main reason having access to a wattmeter is so important. What appears to be a minor change to your power system can have drastic changes in overall power pulled. (Just think of replacing the 12 Volt battery in your automobile with an 18 Volt unit. Likely instant smoke all over your car's electronics and electrical power systems)

The reason this drastic change in power exists is because the power loading caused by the prop on your motor is NOT linear. It is the rpm ratio raised to the third power. So if you have a motor that turns a prop at 4000 RPM, going to 8000 RPM requires 2 X 2 X 2 or eight times more horses at the motor shaft. And increasing your motor voltage by 50% would in theory also increase its RPM by 50%. In the real world, it likely won't increase by 50% because the motor might be severely overloaded, and its efficiency might drop like a rock. That is assuming the ESC, motor or battery will not burn up in the process.

This also applys when flying your model at full throttle, then dropping off to 1/2 throttle. The current input really drops off at 1/2 throttle, something readily observed with the use of one of those wattmeters.

The same thing happens, more or less when you go to a larger model airplane. Going from a 50 inch wingspan model to a giant scale 100 inch wingspan model will require something like 8 times more watts to fly the model. (I've got a 78 inch wingspan giant scale model that requires some 2800 watts to fly it.)
kyleservicetech is offline  
Old 12-05-2011, 10:58 PM
  #27  
kyleservicetech
Super Contributor
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952
Default

Originally Posted by prof_fate View Post
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.
FYI, the Cellpro line along with many other battery chargers actively balance the individual cells while charging.

I've got the Cellpro Powerlab 8 charger. I deliberately completely discharged one cell of a two cell A123 receiver backup battery down to 2.8 Volts DC. The other cell was at about 3.5 Volts DC. Then the pack was connected to the '8' charger. That charger immediately went into its "Safe" mode, and dropped the charge rate from the preset 4 Amps to about 1/2 amp.

It then brought up the low cell to the same exact voltage as the fully charged cell first. Then continued on to bring both cells to the proper 3.65 Volts DC value.
kyleservicetech is offline  
Old 12-05-2011, 11:16 PM
  #28  
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 6,122
Default

Originally Posted by prof_fate View Post
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.
What may have occurred is that one of the thin balance wires has become disconnected from the battery. I've had this happen a few times, the wires are quite fragile. When it happens the balancer sees the voltage of the cell from which the wire is detached as zero. A balance charger will just trip out when this occurs but a discharge balancer like yours will most likely attempt to discharge the other cells to 'match' the cell that it thinks is reading zero volts. It may keep discharging until the other cells reach their low voltage limit at which point it will stop trying.
What you now have is a battery with two almost fully discharged cells and one fully charged cell which would quite neatly explain why you read 48% capacity. It will also mean you probably will kill your battery next time you use it because the two empty cells will over discharge, or worse if you charge it the high cell may overcharge and burst!

Your balancer can confirm if it's a broken wire because one cell will always read zero volts.

Steve

I'd recommend getting a balance charger, much easier and quicker than messing with separate charger and discharge balancer.
JetPlaneFlyer is offline  
Old 12-06-2011, 12:57 AM
  #29  
kyleservicetech
Super Contributor
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952
Default

Originally Posted by JetPlaneFlyer View Post
Your balancer can confirm if it's a broken wire because one cell will always read zero volts.

Steve

I'd recommend getting a balance charger, much easier and quicker than messing with separate charger and discharge balancer.

Agreed! And, it appears that a quality balance charger really reduces the chances of a catastrophic failure of your LiPo battery when charging a really out of whack LiPo battery.

If you have access to a digital voltmeter, you can check each cell's voltage directly, one at a time through the balance connector. You can buy these meters really cheap now days, Harbor Freight has some for under $5.00. I've got two of the Harbor Freight meters, and they check out OK against my two $$$$ Fluke 87V meters.

Depending on the balance connector pinouts, start with putting your meters negative black wire on the balance connectors black wire's pin, and check to the other wires in the balance connector. They should measure 3.7 volts, 7.4, 11.1 volts and so on, depending on the level of charge on each cell.

And, you can zero in on each individual cell by moving the negative meters leads to a different balance connectors pin and checking the other pins of the balance connector.
kyleservicetech is offline  
Old 12-06-2011, 01:18 AM
  #30  
prof_fate
Member
Thread Starter
 
prof_fate's Avatar
 
Join Date: Oct 2011
Location: Beaver, PA
Posts: 235
Default

Originally Posted by rcers View Post
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
I have a balance charger, no probs. The wattmeter is new so I pushed a button to see what it would do. I didn't get a read on what the batt's charge was before I started. I charged the batts before leaving for the fly in, or during the day as they were used. The 93% one had some taxi time and perhaps 1 circuit on it.

Most of my batts are from my coax heli's and those all run jst connectors. Some of my planes have them too. I'm converting some over to minideans and perhps some to full size deans as are on my 450 heli batts.
prof_fate is offline  
Old 12-06-2011, 11:25 AM
  #31  
solentlife
Super Contributor
 
solentlife's Avatar
 
Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,763
Default

Originally Posted by JetPlaneFlyer View Post
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
But I have charger and wattmeters that can Balance WITHOUT charging ... so in fact I can have similar to what you say is questionable. If I have 1 cell is low - when I balance without charging - it will discharge higher charged cells down to the lowest effectively .. OK actually it's trying to balance / spread the charge but in reality ... it discharges the high cells.
Only question I have is .... if 2 cells are at 93% ... to end up at 43% means 3rd cell is basically completely discharged at start of 'balancing'......

I use a B6 charger and balance charging takes so long compared to straight charging ... that I use it only after so many normal charge cycles... or when Wattmeter reports unbalanced pack.
solentlife is offline  
Old 12-06-2011, 03:11 PM
  #32  
prof_fate
Member
Thread Starter
 
prof_fate's Avatar
 
Join Date: Oct 2011
Location: Beaver, PA
Posts: 235
Default

Originally Posted by solentlife View Post
Only question I have is .... if 2 cells are at 93% ... to end up at 43% means 3rd cell is basically completely discharged at start of 'balancing'......

I use a B6 charger and balance charging takes so long compared to straight charging ... that I use it only after so many normal charge cycles... or when Wattmeter reports unbalanced pack.
they were DIFFERENT BATTERIES. Sorry to yell, but i've said this a couple of times now and it's being read.

Balance charging from what I understand should be done consistently, but at no specific interval - so every 5th charge or 10th may be fine.
prof_fate is offline  
Old 12-06-2011, 04:25 PM
  #33  
solentlife
Super Contributor
 
solentlife's Avatar
 
Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,763
Default

Originally Posted by prof_fate View Post
they were DIFFERENT BATTERIES. Sorry to yell, but i've said this a couple of times now and it's being read.

Balance charging from what I understand should be done consistently, but at no specific interval - so every 5th charge or 10th may be fine.
Okee Dokee .... your wording was unclear ... it;s always the same with posts - author knows exactly what it means, but readers don't have person in front of them to get the perception of whats meant.

Based on it being a different pack - then as another says - it should not drop to 43% just by balancing unless the pack is seriously low on overall charge before you start. I would suggest based on what I've seen with my own packs .... average charged unbalanced pack - 3S has dropped after balancing to sub 90% or maybe high 80's in bad sits ...... but never approached such low as 43% ...
solentlife is offline  
Old 12-06-2011, 05:56 PM
  #34  
FlyingBrick50
FlyingBrick50
 
FlyingBrick50's Avatar
 
Join Date: Sep 2010
Location: Florence, SC
Posts: 418
Smile Balancing batt

Originally Posted by solentlife View Post
But I have charger and wattmeters that can Balance WITHOUT charging ... so in fact I can have similar to what you say is questionable. If I have 1 cell is low - when I balance without charging - it will discharge higher charged cells down to the lowest effectively .. OK actually it's trying to balance / spread the charge but in reality ... it discharges the high cells.
Only question I have is .... if 2 cells are at 93% ... to end up at 43% means 3rd cell is basically completely discharged at start of 'balancing'......

I use a B6 charger and balance charging takes so long compared to straight charging ... that I use it only after so many normal charge cycles... or when Wattmeter reports unbalanced pack.
My charger does not take the other cells down, it brings up the low one/s up.
FlyingBrick50 is offline  
Old 12-06-2011, 06:06 PM
  #35  
rcers
Super Contributor
 
rcers's Avatar
 
Join Date: Jul 2005
Location: Trophy Club TX
Posts: 6,314
Default

Originally Posted by FlyingBrick50 View Post
My charger does not take the other cells down, it brings up the low one/s up.
But standalone balancers can't do that, since they have no provision to charge. They simply discharge until all three are the same.
rcers is offline  
Old 12-07-2011, 02:29 AM
  #36  
prof_fate
Member
Thread Starter
 
prof_fate's Avatar
 
Join Date: Oct 2011
Location: Beaver, PA
Posts: 235
Default

OK, let me see if I have this right...

a jst is only good for 5 or 6 amps - so if i've got a 10 or 12 amp ESC I'm never gonna get that kind of throughput with the included jst...

so a mini deans may be the way to go.

OK, i've got 10 amp capacity and approximately 8volts (2 cell, charged can be 4.2/cell).

If watts = volts X amps the most power I can get is 80w without overloading the esc, right?

If I go to 3 cells that's ~12v, so 12 X 10 = 120 watts without overloading things.

So if I'm looking to get 100w per pound of plane this is how I start, right?

Then I need to go find a motor/prop that stays within that limit, right?
While providing sufficient thrust of course.
prof_fate is offline  
Old 12-07-2011, 07:48 AM
  #37  
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 6,122
Default

Prof,

Yes, your thinking is correct, but there are other options such as staying with two cells and getting an ESC (and possibly motor) that will take more Amps.
Also you don't necessarily need 100W per lb to fly. Most planes will fly on half of that, it all depends what sort of performance you are looking for from the model.

Steve
JetPlaneFlyer is offline  
Old 12-07-2011, 03:17 PM
  #38  
prof_fate
Member
Thread Starter
 
prof_fate's Avatar
 
Join Date: Oct 2011
Location: Beaver, PA
Posts: 235
Default

I bought an epp foamie glider conversion RTF and and am building a smaller one for my son so these will sorta be test beds for learning about power/props.

I hooked up the esc/motor from my first test and tried 2 and 3 cells and 2 props. With the least powerful setup I get 50w and with the most I get 100w, at 'normal' volated (very little drop) and the total amps stay under 10 that the esc is rated for.

The plane should weigh in at about 8 ounces, depending on battery of course (7.5 to 8.5 range w/ 2s 1000mah or 3s900mah)

I suppose it's possible to have the motor off and cycle the servos and watch the draw to determine, roughly, how much power the rx and servos consume, right?

I was reading recently some threads on 'brown outs' and folks using BEC's (on bigger planes mostly) to eliminate this - their thinking is that you get the esc hot/overloaded and it shuts down, resets/restarts and you lose rx duriing that time and of course, crash. I guess the real way to verify this is to get an onboard real time datalogger device (gees, "black boxes" for rc!) or telemetry.
prof_fate is offline  
Old 12-07-2011, 03:26 PM
  #39  
rcers
Super Contributor
 
rcers's Avatar
 
Join Date: Jul 2005
Location: Trophy Club TX
Posts: 6,314
Default

Originally Posted by prof_fate View Post

I suppose it's possible to have the motor off and cycle the servos and watch the draw to determine, roughly, how much power the rx and servos consume, right?
No it is not sensitive enough for that. They do make in-flight servo power monitors. I use the Servo Sense Plus (no longer produced) by Dimension Engineering. It rocks but sadly is not available anymore.

It was shocking how little some servos draw and how much others do. The cool thing about the device is it works in flight with real flight loads.

Also here is a good spot with ultra accuracy on some common servo types. Note the large differences in power draw:
http://homepages.paradise.net.nz/bhabbott/Servo.html

This clearly shows those well loved cheap HXT 900's draw very near twice the current of a Hitec HS55. So you can use almost twice the number of 55's as 900's on the same BEC. That is very meaningful. You can also see some Hitec digitals that draw nearly 2 amps EACH!

EDIT:
Here is one for individual servos:
http://www.horizonhobby.com/products...t-meter-HAN172


Mike
rcers is offline  
Old 12-08-2011, 01:15 AM
  #40  
kyleservicetech
Super Contributor
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952
Default

Originally Posted by rcers View Post
No it is not sensitive enough for that. They do make in-flight servo power monitors. I use the Servo Sense Plus (no longer produced) by Dimension Engineering. It rocks but sadly is not available anymore.

It was shocking how little some servos draw and how much others do. The cool thing about the device is it works in flight with real flight loads.

Also here is a good spot with ultra accuracy on some common servo types. Note the large differences in power draw:
http://homepages.paradise.net.nz/bhabbott/Servo.html

This clearly shows those well loved cheap HXT 900's draw very near twice the current of a Hitec HS55. So you can use almost twice the number of 55's as 900's on the same BEC. That is very meaningful. You can also see some Hitec digitals that draw nearly 2 amps EACH!

EDIT:
Here is one for individual servos:
http://www.horizonhobby.com/products...t-meter-HAN172


Mike

Yeah
I did some testing on the use of those linear voltage regulators used in many ESC's for receiver/servo power. The results found were quite interesting. Bottom line, IMHO, don't use those linear voltage regulator BEC's on any model with more than two LiPo cells for power.

Take a look:
http://www.wattflyer.com/forums/showthread.php?t=63779

I've got a model that uses seven Hitec 645MG servos. The peak current of all seven servos measured at 14 Amperes (not a misprint, 14 Amps) when moving the transmitter joy sticks back and forth. This 14 Amps was measured by the peak reading feature of a $350 Fluke 87V digital multimeter.
kyleservicetech is offline  
Old 12-09-2011, 02:45 AM
  #41  
AlexK
New Member
 
Join Date: Sep 2011
Posts: 2
Default Wattmeters and Batteries

Folks this has been a very interesting and educational read for me. I have but two comments sort of related to the topics being discussed.
1. Had a friend who kept complaining to me that he was destroying his Lipo battery packs at a consistant rate. I asked him if he had a Wattmeter of any type, no was his answer. About one year later, he called me and said he had just purchased a Wattmeter, and low and behold he was drawing more current than the battery could supply. He also went to 35C batteries and reproped his motors.

2. On the order of battery capacity and minutes to fly. I have not been using any mathmatical means to determine my flight times. I guess I have been doing it incorrectly but have never destroyed a pack by Overflying it.
I put my wattmeter on the electrical system and prop the motor till I get the correct number of watts to fly the airplane. If the current draw is say 12amps and I have a 1500mah 35C pack, in my head I say OK thats 1.5A in one hour, in 30minutes its 3amps, in 15 minutes its 6amps and in 7.5minutes its 12amps. So I say to myself I can fly WOT for 7.5 minutes and completely totally drain the battery, not good. So using my own goofy power management , I guess at flying 1/2 throttle except when taking off or going for more altitude, so I allow myself 10 minutes of flying joy.

TOTALLY all wrong aren't I ??? Oh well, I did say this thread was educational. THANK you all.

Alex
AlexK is offline  
Old 12-09-2011, 07:02 AM
  #42  
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 6,122
Default

Alex,

What you said sounds perfectly reasonable. I don't think many would rely only on the math to work out their flight time but math is a useful estimating tool when you are deciding what battery to select for a certain model (if there is no other guidance in the manual, or if it's a scratch build etc)

Of course once you start flying the model you can check actual duration by measuring remaining capacity in the battery after flying and adjusting you flight time so that you are landing with what you consider to be a safe margin left.. That's fine once you are flying but it doesn't help in selecting what battery to buy initially.

Steve
JetPlaneFlyer is offline  
Old 12-09-2011, 01:59 PM
  #43  
prof_fate
Member
Thread Starter
 
prof_fate's Avatar
 
Join Date: Oct 2011
Location: Beaver, PA
Posts: 235
Default

OK, Alex brings up a point I'd not thought of...pulling power in excess of the C rating of the battery.

I know C rating is simple to understand, but I have a mental block on it for some reason.

If I have a 900mah pack and I want to charger it at 1C that's .9 amps, right? Or is it .9 mah?

So if that batt is a 15c then I can draw 13.5amps and be Ok?

And if that math works, some batts like that have very tiny wires and JST connectors that aren't rated/can't flow 13 amps...
prof_fate is offline  
Old 12-09-2011, 03:20 PM
  #44  
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 6,122
Default

I did try to say that i thought you were exceeding your battery discharge limit

Your math is correct;
A 900mAh (0.9Ah) battery will charge at 0.9A (1c charge rate) and if it has a 15c discharge rating would discharge at 0.9 x 15 = 13.5A

I can only guess that they used the JST in this application because for the particular system that it was installed in 5A was enough, even though the battery was capable of more. You should find that you can pull a few more than 5A through a JST without any big issues, especially if only for short bursts.
JetPlaneFlyer is offline  
Old 12-09-2011, 04:11 PM
  #45  
kyleservicetech
Super Contributor
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952
Default

Originally Posted by prof_fate View Post
OK, Alex brings up a point I'd not thought of...pulling power in excess of the C rating of the battery.

I know C rating is simple to understand, but I have a mental block on it for some reason.

If I have a 900mah pack and I want to charger it at 1C that's .9 amps, right? Or is it .9 mah?

So if that batt is a 15c then I can draw 13.5amps and be Ok?

And if that math works, some batts like that have very tiny wires and JST connectors that aren't rated/can't flow 13 amps...
That term "C" is simply a ratio between your batteries milliampere hour rating, and how much current is being put into, or out of, that battery.

A battery with a rating of 900 mah is rated to put out 900 milliamperes (0.9 Amps) for one hour.

So, if you have a battery that is rated at 15C, that would indicate that this battery is capable of putting out 15 times as much current as would be taken out of that battery to discharge it in that one hour. In doing so, it will last 1/15 as long as that hour, or 60 minutes/15. That's four minutes. The discharge rate would be 0.9 times 15 or 13.5 Amps as you indicate.

Problem is, discharging some of those very low priced LiPos at their claimed discharge rating can result in very short life of that Lipo. One Electric flyer in SE Wisconsin was only getting 10 flights out of a 25C LiPo discharged at 25C before the battery gave up.

That milliampere hour rating needs to be clarified though. A lot of batteries are rated at something like a 20 hour discharge cycle. Take a common "AA" Alkaline cell, discharge it in 20 hours, and you'll get around 1500 milliampere hours, or 1.5 Amp Hours. Discharge that same AA cell in one hour, and your amp hours can drop in half.

Same with a common lead acid deep cycle battery. They also are rated on a 20 hour discharge time. I'd been discharging my 120 Ampere hour deep cycle batteries in two hours or so, and have had those batteries quit in one year, and only 75 cycles or so.
kyleservicetech is offline  
Old 12-09-2011, 04:20 PM
  #46  
prof_fate
Member
Thread Starter
 
prof_fate's Avatar
 
Join Date: Oct 2011
Location: Beaver, PA
Posts: 235
Default

Such good info!

I think I understand the C rating now..and mah. Today anyway.
prof_fate is offline  
Old 12-09-2011, 04:35 PM
  #47  
kyleservicetech
Super Contributor
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952
Default

Originally Posted by prof_fate View Post
Such good info!

I think I understand the C rating now..and mah. Today anyway.

Yup
Milliamperes is a rate of battery discharge current. And milliampere hours is that rate of discharge current for a length of time in hours.

"C" is the rate of discharge of a battery, defined as a ratio of the battery actual discharge current divided by the ampere hour capacity.

So if you have a battery with a rating of 30C, you can pull 30 times the one hour rate, your battery will last two minutes.

And following up on that, a 2.5 Amp hour (2500 Milliampere) Hour battery discharged at 20 Amps would be 20/2.5 or C=8.

As for the terms, that is Amps/ampere hours, so C represents 1/hours. If you discharge your battery in one hour, C=1/hours, or 1/1 or C=1. If you discharge your battery in 3.5 minutes, that is 3.5/60 or 0.058 hours. So, in discharging your battery in 3.5 minutes, C=1/0.058 or C=17.14.

Whew!!! Stuff to ask your 5th grader about.
kyleservicetech is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
KiyHatarimaru
Beginners
12
09-13-2011 11:43 PM
cbatters
Batteries & Chargers
6
05-22-2011 07:25 AM
Big Johnny
Power Systems
20
03-22-2011 01:29 AM

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


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


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.

Page generated in 0.12044 seconds with 15 queries