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Li-Po power

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Philip Harrison23/12/2008 21:13:00
14 forum posts

can some one help me on this,

if i have 2x 11.1v lipos 20c i think they are 2000mah will have to check on that at work,

which would give me the best flight time in series or par,

which would give me the best power,

Brian Lambert23/12/2008 21:35:00
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Well the experts will be along soon I am sure, but series should give greater efficiency as the IR losses will be less, but I bet it will not be that simple
Tim Mackey23/12/2008 21:45:00
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Paralleling two identical packs will double the capacity and therefore increase flight time ( over that of a single pack in the same rig ) however volts remain the same and therefore power will remain the same too. Flight time will not simply double however, as weight will have increased etc.

Series will double the voltage and therefore current drawn and power produced will also increase, and duration reduce. 

Of course simply connecting 2 of them in series, on a rig that was setup to use the one only, will almost certainly let out the magic smoke. 

Simon Chaddock23/12/2008 22:07:00
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Philip

As far as the lipos are concerned series or parallel doesn't make any difference to the power available. In series the voltage is doubled, in parallel the current is doubled but the total power output is the same. So it really comes down to your ESC, motor and prop combination as to which configuration best keeps within the 20c (40 amp) battery limit.

If you do have a suitable ESC, motor & prop, a series configuration (higher voltage, lower current) is slightly more efficient as the electrical losses are lower but remember it is your airframe that is absorbing all the power so even a slight improvement in that will result in an improvement in overall performance. 

Tim Mackey23/12/2008 22:54:00
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Not wishing to sound pedantic here Simon, but in parallel the current is NOT doubled - the capacity is.

Philip Harrison24/12/2008 11:45:00
14 forum posts

thanks for the reply lads,

had  a few different answers,

Tim Mackey24/12/2008 12:07:00
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Thats normal really...but I think you should be able to decipher the gist of it - all answers are basically saying the same thing really.
Frank Skilbeck24/12/2008 14:47:00
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Timbo - forum moderator wrote (see

Not wishing to sound pedantic here Simon, but in parallel the current is NOT doubled - the capacity is.


But if you have 2 x 20C 2000mah lipo's then in series the max current you can draw is 40A but if you put them in parallel then they become a 20c 4000 mah pack so 80A max.
Eric Bray24/12/2008 15:36:00
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Yes, Frank, the AVAILABLE current is twice, but the DRAWN current, as nothing else has changed, remains the same, so you can draw the same current for twice as long.
Tim Mackey24/12/2008 17:20:00
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As Eric says.

You said ....Quote...

If you have 2 x 20C 2000mah lipo's then in series the max current you can draw is 40A but if you put them in parallel then they become a 20c 4000 mah pack so 80A max"

Yes of course they do, but that is my point...the current CAPACITY increases not the current.

Simon Chaddock24/12/2008 20:27:00
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Timbo

Yes, it would have been better if I had said "the current capacity is doubled" but I was trying to show that as far as the batteries are concerned the power (i.e. the rate of doing work) available is the same either way and is actually limited by the cells c rating and the ability of the motor/prop combination to use it.

Tim Mackey24/12/2008 22:13:00
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Its fine Simon,,,as I said I am sure the OP gets the idea - lets not drag the thread out over something minor .
Philip Harrison25/12/2008 21:08:00
14 forum posts

cheers lads you are a pool of information,

so if i put them in series it will give double the 20c to 40 c.

Always broke25/12/2008 23:34:00
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No If you put them in series it will sill only be 20C but twice the voltage.

So

2x11.1  in series  = 22.2v at 2200ma capacity.  20c is 44A  .In series you have done nothing to increase the capacity only the voltage but the amount of power available is twice that of a single pack.

volts x amps = watts so  22.2 x 44 = 976  watts . High voltage low current

2x 11.1 in parallel = 11.1v at 4400ma capacity.  20c is 88A .

volts x amps = watts so  11.1 x 88 =  976 watts Low voltage  High current

If the set up you were using was an 11.1 v set up and you were using a 2200 ma pack and you added an identical pack in parallel to it you would get nearly 2 x the flight time ( remembering it is heaver)

If the set up you were using was an 11.1 v set up and you were using a 2200 ma pack and you added an identical pack in series you would  smoke the system unless you changed the motor or prop or both as you are trying to get the prop to spin faster as the voltage has increased . Remember one of the specs of a motor is RPM per volt.

Its Late on Xmas Day

Good night.

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