Difference between 2S and 3S - RC Groups
Feb 15, 2006, 11:51 AM
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# Difference between 2S and 3S

How big of a difference is there from a 2S and a 3S lipo? I have a 3D foamy weighing about 12 oz. I put in a 2S lipo, holding the plane vertical and full throttle, the plane doesn't have enough power to climb. I suspected that the battery might be bad (i got it used). So I installed a 3S lipo and the plane wanted to leap out of my hands, the power it had was incredible
So, is it because I went from a 2S to a 3S, or is the 2S battery faulty? Thanks.
 Feb 15, 2006, 12:04 PM Registered User In general all oher things beng equal, a given prop and motor will draw about twice the power on 3s as 2s...If the ESC motor or pack don't die in the attempt
 Feb 16, 2006, 03:32 AM Johan Beyers Eflight101 Yup, that's about right. Since the voltage is higher by 50%, the amps also should rise by about 50%, and the power is given by volts times amps. About twice the power is expected. I was very surprised when I figured this out the first time. I think this was when I computed the difference between 6 cells and 7 cells for NiCDs: about 40% more power. couldn't figure out why the plane only did a slow descent on 6 cells... Johan - Eflight101.com
 Feb 16, 2006, 05:10 AM Registered User Current increase is higher than linear, since motor is not resistance. Linear could be rotations per minute (multiplied vy volts), and power consumed by propeller demand on rotation speed quadratic. You should consider also efficiency of motor, because as You will run out of acceptable motor power, efficiency fall down and current consumption can be twice or more of all estimations. Power system should be well fitted, current have to be under limit of any of components at full throttle.
 Feb 16, 2006, 05:57 AM Johan Beyers Eflight101 Agreed. I maybe simplified too much. The only way to really be sure about any of this is to test it on the plane with the specified setup. You might want to put a smaller prop on the plane, just to bring down the power to a level where it doesn't smoke Reminds me of the instructions for tightening a bolt: Tighten until it strips, then a quarter-turn back. Johan - Eflight101.com
 Feb 16, 2006, 10:09 AM Suspended Account There is a potential advantage to 3S. If you have a target power level (in watts), more volts requires fewer amps to achieve a given amount of power. All else being equal, fewer amps means longer flights and better battery/motor life.
Feb 16, 2006, 12:03 PM
Registered User
Quote:
 Originally Posted by coro Current increase is higher than linear, since motor is not resistance. Linear could be rotations per minute (multiplied vy volts), and power consumed by propeller demand on rotation speed quadratic. You should consider also efficiency of motor, because as You will run out of acceptable motor power, efficiency fall down and current consumption can be twice or more of all estimations. Power system should be well fitted, current have to be under limit of any of components at full throttle.

Yup. All true, which is why I said 'all other things being equal' and 'about'

I just knocked up a simulation to do an actual comparison.
Its for a permax 480 long can, 3:1 gearbox and APC 10x7 prop.
a power
On 2s LIPO it draws 8.3A, (58W) at 60% efficiency.
On 3s LIPO it draws 14.9A (150W) at 58.5% efficiency.

Output power goes from 33W to 88W. 2.6 times as much output power..

So its nearly 3 times the power, and on this model the rate of climb TREBLES.

Oh, I have another rule of thumb too..when going froma good setup on 2s LIPO to a good one on 3s LIPO you need to knock 20% off the prop diameter and pitch.

So if e.g. you had a 10x7 on 3s LIPO, something like a 12x8 would be similar on 2s LIPO.
Feb 16, 2006, 12:18 PM
Registered User
Quote:
 Originally Posted by sarge There is a potential advantage to 3S. If you have a target power level (in watts), more volts requires fewer amps to achieve a given amount of power. All else being equal, fewer amps means longer flights and better battery/motor life.
There is also a very real disadvantage to 3s. The BEC has to waste a lot of energy to get the voltage down to 5V. On models with many servos and/or long flight times, a 2s setup is often the better solution (or a switching BEC).

Jürgen
Feb 16, 2006, 06:36 PM
Permanently Banned

# 2s On A Sokol Does 80+ Mph

Using Align Bl400l 2500kv 290w Brushless Motor