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Old Feb 20, 2012, 06:42 PM
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United States, CA, Los Angeles
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Question
Two prop system with one battery pack

Hi all,

Right now we are running two completely independent propulsion systems (battery pack, esc, motor), but we are thinking about running the two esc's in parallel off of ONE battery pack.

The voltage should then be the same for the two systems in parallel, but the current will be split, right? Will this mean a 50% drop in motor rpm (and hence thrust)?

Our system is listed below, but this is really more of a conceptual question.

Thanks.

Setup right now:
13 cell Elite 1500 in series -> 20A fuse -> Hacker X-40 SB Pro -> ARC 28-47-3 -> 4.75 gearbox -> 16x10 prop
(x2 because two independent systems)

What we are thinking about:
~13 cell Elite 1500 in series -> 20A fuse => 2 Hacker X-40 SB Pro => 2 ARC 28-47-3 => 2 4.75 gearbox => 2 16x10 prop

We are limited in our choices, but we are able to change the number of cells, gear ratio, and propeller. I just want to know if we will experience a large drop in thrust by using the second setup.
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Last edited by nekoji; Feb 20, 2012 at 07:46 PM. Reason: Added detailed configuration
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Old Feb 20, 2012, 07:05 PM
hass-alfed and bass-ackwards
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United States, AZ, Chandler
Joined Jun 2008
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It is certainly better to post your system, concept or not. It's easier to explain what will happen if we have an idea of the gear you are using, as we can give practical examples.
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Old Feb 20, 2012, 07:26 PM
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State College PA
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Your RPMs will not reduce by 50%, your flight time will.
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Old Feb 20, 2012, 07:45 PM
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United States, CA, Los Angeles
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Originally Posted by TomM View Post
Your RPMs will not reduce by 50%, your flight time will.
So if we upgrade the pack to a higher capacity, but the same number of cells, we can maintain our flight time AND our thrust?

Thanks!
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Old Feb 20, 2012, 07:59 PM
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State College PA
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Quote:
Originally Posted by nekoji View Post
So if we upgrade the pack to a higher capacity, but the same number of cells, we can maintain our flight time AND our thrust?

Thanks!
Yes that's true. I'm running a smaller 2 esc / 2 motor setup in a P-38.
The half run time is only for full throttle. In reality at partial throttle the run times are harder to calculate, but much better than 1/2. Two motors draw more current, but you have twice as much thrust, so the motors are working less and draw less current than an equal model at equal speed running one motor.
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Old Feb 21, 2012, 02:51 AM
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Letchworth, Great Britain (UK)
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Quote:
Originally Posted by nekoji View Post
So if we upgrade the pack to a higher capacity, but the same number of cells, we can maintain our flight time AND our thrust?

Thanks!
If your present packs are still in good condition, you can simply parallel-connect them to make your single supply source. Otherwise, with a single battery you need twice the capacity to achieve the same run-times and performance as before. C rating can stay the same because doubling the capacity also gives you double amps capability.

Two motors will take twice as many amps from one battery than one does, so if you increase your pack capacity, but not as much as double size, you need to check if its amps capability (= Ah x C) is sufficient.
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Old Feb 21, 2012, 03:23 AM
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Staffs, UK
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Quote:
Originally Posted by nekoji View Post
Right now we are running two completely independent propulsion systems (battery pack, esc, motor), but we are thinking about running the two esc's in parallel off of ONE battery pack.

The voltage should then be the same for the two systems in parallel, but the current will be split, right? Will this mean a 50% drop in motor rpm (and hence thrust)?
It might be better to think of the current fom the battery being DOUBLED. I.e. if each motor is currently pulling 20A from it's own pack then the two motors together will pull 40A from the now single pack.

Because the voltage is still the same the performance of the motors will be the same (provided the battery can handle the increased current) but obviously the pack will need twice the capacity to maintain flight time.

Steve
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Old Feb 21, 2012, 07:31 AM
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Chattanooga, Tennessee, United States
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If the capacity of the single pack is exactly the same as the combined capacity of the two packs... there will be no difference in current draw, performance or run time.

The advantage of using a single pack is that each motor will receive exactly the same voltage, and will draw exactly the same current.... so exactly the same performance... and both motors will run out of juice simultaneously. If you use two packs, one may be weaker than the other and give one motor lower voltage, lower current draw, less performance.....and perhaps run out of juice at a different time than the other.

Cheers, Phil
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Old Feb 21, 2012, 01:27 PM
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Originally Posted by Dr Kiwi View Post
... advantage of using a single pack is that ... both motors will run out of juice simultaneously. ...
If you're very lucky

In most cases one lvc will kick in slightly before the other, due to manufacturing variations and, once it does, the voltage will rise due to less amps load, so the second ESC may take a while to reach its lvc.

The simplest way round the problem is to time your flights so that neither motor reaches lvc.
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