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Sep 03, 2019, 07:06 PM
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Robert Pulse's Avatar
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Astroflight watt meter


Anyone ever used a watt meter to check amp draw of a motor /prop under load?
Bob
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Sep 03, 2019, 09:16 PM
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scirocco's Avatar
Quote:
Originally Posted by Robert Pulse
Anyone ever used a watt meter to check amp draw of a motor /prop under load?
Bob
That is its primary purpose.

If you meant in flight under load, there are dedicated data loggers for that purpose.
Sep 03, 2019, 09:27 PM
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vollrathd's Avatar
Quote:
Originally Posted by Robert Pulse
Anyone ever used a watt meter to check amp draw of a motor /prop under load?
Bob
I've had my Astroflight Wattmeter for many years. It works well up to around 80 Amps or so. I designed my own Wattmeter that reads up to 200 Amps for the really big models.

DIY 200 Amp Wattmeter project
https://www.rcgroups.com/forums/show...r#post42024531
Sep 03, 2019, 10:54 PM
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Robert Pulse's Avatar
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Watt meter


Quote:
Originally Posted by vollrathd
I've had my Astroflight Wattmeter for many years. It works well up to around 80 Amps or so. IWattmeter#post42024531[/url]
What I want to know is I have 3 outboard motors 540 35 turn and I need to know the total amp draw under load at 12 volts for the three to determine what size lipo to buy. Will the watt meter provide that info running in water with a 12 volt gel cell through the watt meter with no esc? I just need a close estimate
Bob
Sep 03, 2019, 11:01 PM
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vollrathd's Avatar
Quote:
Originally Posted by Robert Pulse
What I want to know is I have 3 outboard motors 540 35 turn and I need to know the total amp draw under load at 12 volts for the three to determine what size lipo to buy. Will the watt meter provide that info running in water with a 12 volt gel cell through the watt meter with no esc? I just need a close estimate
Bob
That is what a Wattmeter does. But, the boat needs to be stationary in the water in order to read the meter.

Another option is an AC and DC clamp on Ammeter. Most clamp on meters are AC only.
Sep 03, 2019, 11:07 PM
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Robert Pulse's Avatar
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Watt meter


The outboards are presently mounted on a frame that I can put in the water at the proper depth and run them in a fixed position.
Bob
Sep 03, 2019, 11:52 PM
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vollrathd's Avatar
Quote:
Originally Posted by Robert Pulse
The outboards are presently mounted on a frame that I can put in the water at the proper depth and run them in a fixed position.
Bob
I have this unit along with a Craftsman meter that is hard to locate.

https://www.amazon.com/Uni-T-B4Q094-.../dp/B00O1Q2HOQ
Sep 04, 2019, 06:03 AM
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scirocco's Avatar
Quote:
Originally Posted by Robert Pulse
What I want to know is I have 3 outboard motors 540 35 turn and I need to know the total amp draw under load at 12 volts for the three to determine what size lipo to buy. Will the watt meter provide that info running in water with a 12 volt gel cell through the watt meter with no esc? I just need a close estimate
Bob
Yes, assuming "540 35 turn" = brushed motor. But I'd just run 1 motor and triple the current. That way you'll get less voltage sag on your gel cell.
Sep 04, 2019, 09:14 AM
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Robert Pulse's Avatar
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Ok thanks
Bob
Sep 11, 2019, 12:59 PM
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Robert Pulse's Avatar
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Amp draw, battery c rating and mah


Ok did the single test. One motor full throttle 12 volt, just under 6 amps in the water. All three in water 12 volt, just under 17.5 amps The battery I would like to use is a 6000 mah 70-120 c 3s battery. I wont be going full throttle all the time.
Will it work? Run time?
Bob
Last edited by Robert Pulse; Sep 11, 2019 at 02:13 PM.
Sep 11, 2019, 06:34 PM
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scirocco's Avatar
Even at only 18A, the battery voltage will probably drop to just below 12V and then continue to drop during discharge. This means your current (and rpm) will gradually fall off as well.
You should also aim not to discharge more than 80% of the pack capacity.
So if you have 0.8 x 6000 = 4800mAh available and average current is 16A, you should get 4.8/16= 0.3 hours or 18 minutes running full throttle the whole time
Sep 12, 2019, 07:32 AM
Registered User
The problem I see here is that you are running the motors on a fixed stationary platform.
This does not take into the effect of "unloading" during actual operation.
I use the same watt meter but also aware the current unloads a bit during actual operation while flying my planes.
To get a better knowledge of just how much current the motor/prop combination is drawing I would use a dedicated data logger such as Eagle tree that will log the current actual be used while running the boat. But then again it may be a bit more money to spend than you may wish to.
Sep 12, 2019, 08:17 AM
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pilotpete2's Avatar
Bob,
You're setup is going to be very easy on a 6000mAh battery. I personally would not waste money on batteries with extremely high C ratings, which in too many cases are unrealistic and inflated anyway. Decent quality packs rated at 30 or 35C will perform very well under the very light load your setup is drawing.
If your radio has the capability to have a timer controlled by the throttle, you can do trial runs of increasing duration, this will give you a good real world of how long you can run at your normal throttle settings. Low tech, but it works!
Pete
Sep 14, 2019, 04:36 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by pilotpete2
... batteries with extremely high C ratings, which in too many cases are unrealistic and inflated anyway. ...
See
Battery Load Test Comparisons - RCG

&
www.liporatings.com


... a lot of the time, C stands for crap ...
Have you ever wondered how a 100C battery gets away with an XT60 connector that is only rated for 60A? If you use lipo batteries in your RC planes, helis or drones then you need to watch this video and learn why the C rating of your batteries might not be what you think it is. There's also some other lipo info in this video that hopefully everyone will find interesting and of some value.



Lipo C ratings - fact, fakery and fire (23 min 37 sec)
Sep 16, 2019, 06:44 AM
Registered User
Quote:
Originally Posted by pilotpete2
Bob,
You're setup is going to be very easy on a 6000mAh battery. I personally would not waste money on batteries with extremely high C ratings, which in too many cases are unrealistic and inflated anyway. Decent quality packs rated at 30 or 35C will perform very well under the very light load your setup is drawing.
If your radio has the capability to have a timer controlled by the throttle, you can do trial runs of increasing duration, this will give you a good real world of how long you can run at your normal throttle settings. Low tech, but it works!
Pete
Well put. "C" labels are much like anything else you buy, mostly hype so don't get involved with spending money on batteries with overhyped "C" ratings. There are some decent brands available, at the same time, price is not necessarily an indicator of quality, motors being the exception.


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