



Discussion
Clear this up for me
I damaged a cell in my CSRC 10C 3S1P 1650mah Lipo last weekend.
Now I have a 2S1P 1650 Lipo. It is still a 1650mah battery, right? It is still rated as 16 cont and 24 bursts according to the CSRC site, if I am reading this correctly. Other than lower voltage and less performance, what else should I expect? Based on the 1650 alone, my fly time should be the same? I would assume that I will have to give more stick to go the same or close to the same speed if flying anything other than my SS but what am I missing here? If each cell is rated at 1650 shouldn't the 3S pack be 4950mah? Thx, Rick 




Volts times amps equals watts . 746 watts equals 1 horse power. Watts are what turn props.
A given motor ,gearbox,prop will have 2/3 as much available power on 2 cells as 3cells but it is more complicated than that. If a given prop requires 100 watts to turn it at X RPMs. then with 7 volts 14.29 amps are required. With 10.5 volts only 9.52 amps are required. If you had three five gallon cans of gas for your car and one can became damaged and had to be discarded would you car travel just as far on the remaing 10 gallos as it would have on all 15 gallons? Well perhaps but not if you drove it exactly the same in both cases. Charles 


Slovakia
Joined Apr 2005
3,386 Posts

Quote:
Everything else is correct and there is no problem with removing cell from a pack. Inserting new cell would be problem. However, power will be much smaller than 2/3 with existing motor and propeller, because althrough motor rpm dependency on voltage is linear, power consumed by propeller depends on rpm quadratic ( 2/3 of voltage should result in approx 2/3 or rpm, but it means approximately half of the power). 




Each of the 3 cell lispo you have is 3.7V with 1650mAh capacity.
When you put them in series (3S) then Voltage stacked up but capacity stay. Thus your 3S is 11.1V @ 1650mAh, while 2S will be 7.4V @ 1650mAh. As Charles said, flying is all about Watt. You motor system may not enjoy a 2S voltage. For example, My Speed 400 geared system zooms @ 3S but at a petty on 2S. 



Real numbers from a real live motor test. Same watt meter,tach,prop,motor,ESC used for all testing.
Ikarus 2023011 Brush less out runner motor. GWS DD 9050 prop. 2S 6.9Vv / 3.7A / 27 watts / 5000 RPMs 3S 10.8A / 2.8 A / 30 watts /5000 RPMs Anyone who has done much motor testing will realize that there are most often slight errors in the displayed volts / amps / watts and RPMs. So some error is to be expected. My point is that it takes very close to 30 watts input to turn this prop 5000 RPMs with this motor and thereis very little differency in its' effeciency at either voltage. No every motor has different operating efficiency and this changes throughout its power range . Charles 



If last paragraph of post # 2 if read and understood then 30 watts turning prop at 5000 should be understood.
The first post in this Sticky http://www.rcgroups.com/forums/showthread.php?t=209187 covers the basics very well,IMO. There are other such Stickies at the top of most every Forum her on RC Groups. Quote:
You are correct the Throttle was not wide open during both of these test. . Charles 




I understood, it takes more throttle to get the same RPM.
It still confuses me how cells are rated at xxxxmah by themselves and not as a group. I have two of those Nippy Black brushless motors from HL on backorder and did not catch the 2S requirement for them. I guess I now have a 2S for it and my run time will be much less than I was hoping for. But, I could buy another 2S 1650 (or let my kid crash another one of my packs again ) and run them in a 2S2P and have even longer run times than I expected with my 3S packs? but slightly more weight which will shorten my run time some... Dang, I need a complex algorithm just to figure this out. Charles, did you know that, in general, a 50# weight savings on a car = 1 mpg increase. So, being I have less gas (car, not me  I have plenty ) to haul around, maybe I can drive more than 2/3 my original distance. Anyhow, in reality, I would just weld up the hole in the third gas can, fill it with gas immediately while still red hot to anneal it and continue on down the road. 



Quote:
It is #3 in the "Battery Education" category Mike www.CommonSenseRC.com 



Ijamsville, MD, U.S.A.
Joined Mar 2001
456 Posts

Li Po Handbook
[QUOTE=rampman]I understood, it takes more throttle to get the same RPM.
It still confuses me how cells are rated at xxxxmah by themselves and not as a group. Rampman: You will find the information you need and want at: http://www.fmadirect.com/tech_data/index.htm The Li Po Handbook there shows how packs are assembled and what parallel and series hooks ups are. http://www.fmadirect.com/tech_data/techdocs/ Fred@fmadirect.com 



Quote:
Ikarus 2023011 Brush less out runner motor. GWS DD 9050 prop. 2S 6.9Vv / 3.7A / 27 watts / 5000 RPMs 3S 10.8A / 2.8 A / 30 watts /5000 RPMs Charles 




Here's the simple version. Removing a cell lowers the voltage of the pack. Given the same motor/prop, the lower voltage results in lower current and therefore results in longer flight times. However, as you already know, it'll produce less power (thrust) too. If you have a copy of Motocalc, you can play with the numbers and see this.

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