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Old Jan 06, 2013, 09:15 PM
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Joined Apr 2008
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I have question for you guys about regulating voltage. The plane im putting together has 8 servos. Im planning on using a castle BEC to regulate the voltage. my question is:

If i use a 3000mah 3s lipo, will that last longer than running a 3000mah 2s?

harv
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Old Jan 06, 2013, 09:49 PM
ex-noob
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Sammamish, WA, USA
Joined Jan 2010
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Considering some batteries (not hobby lipos) have their capacity indicated at "Watt hours" then it would stand to reason the a higher voltage 3000mAh pack should last longer at a given current rate.
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Old Jan 07, 2013, 06:26 AM
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United States, WA, Seattle
Joined Mar 2012
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If your BEC is a linear mode type, it will draw the same current from both batteries and they will last the same time, but it will dissipate more heat with the higher voltage battery as "waste" because it has to drop more voltage in the regulation process. Linear mode types are not very efficient.

If your BEC is a switching mode type, then it will be more efficient and will make use of the extra voltage and draw less current from the higher voltage battery, and it will last longer. But most inexpensive BECs are linear mode types.

However, if we consider the BEC as a constant load with resistance R, then the current I(2s) = V(2s) / R and I(3s) = V(3s) / R. In this case, the current is greater for the 3s battery, and therefore the 3s battery will not last as long as the 2s battery.
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Old Jan 07, 2013, 10:56 AM
aka JetMan Joe
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Joined Feb 2010
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Quote:
Originally Posted by harv_d View Post
I have question for you guys about regulating voltage. The plane im putting together has 8 servos. Im planning on using a castle BEC to regulate the voltage. my question is:

If i use a 3000mah 3s lipo, will that last longer than running a 3000mah 2s?

harv
Since the CC Bec is a switcher as opposed to a linear and If I were doing the calculations I wouldn't model it as a constant load, I'm with Lardog. I think the 3S would last longer. You could do a test on both types for the same duration and load and see which one takes more mah to recharge. Maybe lock the same robust servo at full load for 60 seconds, or something similar.
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Old Jan 07, 2013, 12:22 PM
Seattle, WA - USA
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Joined Sep 2003
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3S will last longer. It's a power problem, plain and simple.
If you disregard losses and inefficiencies...

At 1C:
A 3S 3000mah battery will provide 3 amps x 11.1 volts for one hour or 33.3 watt/hours.
A 2S 3000mah battery will provide 3 amps x 7.4 volts for one hour or 22.2 watt/hours.

If your electrical system, on average consumes 25 watts, the 3S battery will give you 79.9 minutes of run-time.
On 2S you will get about 53.3 minutes of run-time.
So, the 3S will last 1-1/2 times longer.
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Old Jan 07, 2013, 03:19 PM
I think I forgot how to fly RC
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United States, WA, Woodinville
Joined Mar 2007
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Quote:
Originally Posted by harv_d View Post
I have question for you guys about regulating voltage. The plane im putting together has 8 servos. Im planning on using a castle BEC to regulate the voltage. my question is:

If i use a 3000mah 3s lipo, will that last longer than running a 3000mah 2s?

harv
One thing that the others might be forgetting is that the CC BEC Pro reduces max amp output as the input voltage climbs.
it would be advisable to use no more than a 3s pack for input voltage with those 8 servo's.


Max continuous current varies with the input voltage. The ratings are:

Input Voltage: Max Continuous Output Current
16V 15A
24V 13A
32V 11A
40V 9A
48V 8A
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Old Jan 07, 2013, 10:39 PM
Quadrophenia
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Seattle
Joined Mar 2009
915 Posts
I'm curious about your 2s/3s plane with eight servos.

e
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Old Jan 07, 2013, 11:07 PM
This ain't my first rodeo...
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United States, WA, Orting
Joined Jan 2010
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Don't get too excited, it's still pumpin gas through the business end...
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Old Jan 08, 2013, 12:23 AM
I think I forgot how to fly RC
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United States, WA, Woodinville
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Quote:
Originally Posted by egaus View Post
I'm curious about your 2s/3s plane with eight servos.

e
I'm fairly certain he is planning out the electronics power for a gasser
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Old Jan 08, 2013, 08:15 AM
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Chip Hyde double vision
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Old Jan 08, 2013, 09:27 AM
I think I forgot how to fly RC
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United States, WA, Woodinville
Joined Mar 2007
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Harv, assuming the roads are clear..what day were you thinking of coming down?
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Old Jan 08, 2013, 10:27 AM
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Yes. I plan on coming down Friday evening. Figured I'll stop by your place first on the way down. Pm me your address.
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Old Jan 08, 2013, 11:33 AM
I think I forgot how to fly RC
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United States, WA, Woodinville
Joined Mar 2007
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Quote:
Originally Posted by harv_d View Post
Yes. I plan on coming down Friday evening. Figured I'll stop by your place first on the way down. Pm me your address.

done
thanks
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Old Jan 08, 2013, 12:15 PM
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Renton, Wa.
Joined Feb 2006
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Hey guys. I've got a couple large outrunners for sale if anyone is interested, priced cheap to sell
http://www.rcgroups.com/forums/showthread.php?t=1804350
http://www.rcgroups.com/forums/showthread.php?t=1804344
Each one would work great in a 10s set up for a 12-13 Ilb plane. Also have some 18x8 Xoar props
http://www.rcgroups.com/forums/showthread.php?t=1804363
And I have 2 TFR6 FR Sky Futaba Fasst compatable receivers, I'd let both go for 30.00. Thanks everyone.
Steve
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Old Jan 08, 2013, 02:40 PM
Gas Monkey Vic
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Snohomish, WA
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Quote:
Originally Posted by harv_d View Post
Chip Hyde double vision
NICE! I love biplanes
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