Amps, volts & watts comparison on quad with 4S & 3S batteries

I have an old BNB Digital Power recorder so I wanted to have a look at the hovering power requirements of my quad. The weight of the quad with either battery is very similar.

Quad specs:

APM2

KDA20-22L motors from Hobbyking

2 x Super Simple 25-30A ESC's from Hobbyking

2 x Hexfet 25A ESC's from Hobbyking (2 of 4 were DOA)

10 x 4.5 props from Goodluckbuy.com

Batteries: 4S 2200mAh Nanotech 25C

3S 3000mAh Turnigy 25C

X525 frame

 

 3S TEST:

*taken from level portion of graph excluding full power climb

 4S TEST:

*taken from level portion of graph excluding full power climb

Current Max: 17.58 A
            Avg: 15.14 A
Voltage Max: 12.12 V
            Avg: 11.80 V
            Min: 11.60 V
Power Max: 206.14 W
            Avg: 178.63 W

Full power climb: 41.99A, 464W

Weight: 1263g

Current Max: 14.89 A
            Avg: 13.33 A
Voltage Max: 15.02 V
            Avg: 14.70 V
            Min: 14.49 V
Power Max: 220.04 W
            Avg: 195.73 W

Full power climb: 41.50A, 552W!

Weight: 1268g


Full Graph

Full Graph

Views: 1372

Comment by Jonathan M on March 1, 2012 at 2:02pm

Very cool!  So it looks like you are getting longer flights with the 3s.  I wonder how a lower kV motor would perform.  I have an eagle tree and have been doing similar analysis.  Haven't tried a 4s though

Comment by PACEFE on March 1, 2012 at 5:55pm

Hi,

   it looks to me like 4s will deliver longer flight times. Averange amps is 1.81 amps less.

At 3000mAh:

3s flight time with avg 15.14 is about 12 minutes.

4s flight time with avg 13.33 is about 13 minutes 50 seconds.

Comment by Jonathan M on March 1, 2012 at 6:31pm

The 4s was 2200mah.  If you upped it to 3000mah, the weight would increase and thus the hover amps.  If you could find a 4s 3000 that weight the same as the 3s 3000 post a link! :)

Comment by Dusty Starbucks on March 1, 2012 at 7:54pm

Not sure how that was calculated.  Back of the envelope I get 3s = 11.88 min, 4S = 9.90 min.  4S has more lift power, 3S has longer flight time at that weight, but as weight increases this will have a crossover and the 4S will get longer flight times at some point on the curve.


Moderator
Comment by Graham Dyer on March 1, 2012 at 10:33pm

Yes, although the average current with the 4S pack is lower, the 3S 3000mAh pack will fly longer. It would be interesting to know how a 4S 3000mAh pack with it's increase in weight would affect flight time.


Developer
Comment by John Arne Birkeland on March 2, 2012 at 1:39am

At 4S the motor/prop size is probably mismatched. As a general rule of thumb it is more efficient to raise the voltage then to raise the current in electrical application. A lower kv motor would give better 4S results.

Comment by David M Eno on March 2, 2012 at 3:49am

I don't believe that the starting voltage of these batteries is indicative that they were charged to capacity.  Maybe  I'm reading your stuff wrong.  You also need to adjust for mass of the battery to get a good comparison between the two setups.   You should really buy one of those Eagletree loggers, calibrate it, and then you could provide the graphs in a much easier fashion and show one of those mAh vs. time plots.  There are also quite significant differences between battery vendors ....   I also totally agree with @John that prop matching would help.       

3S should start at ~12.60 V if properly charged

4S should start at ~16.80 V if properly charged

Don't think of this is you did something wrong.  Just think of it as the start of a long an interesting project.  


Moderator
Comment by Graham Dyer on March 2, 2012 at 4:09am

Yup, the 4S pack's max voltage seems a little low but once hovering/discharging the voltage of a LiPo doesn't vary considerably except the very beginning and the very end of a discharge, so this is fairly indicative of what's happens. It's also not intended to be scientifically accurate at all, this was just for interests sake. I may do another with the 4S pack starting at 16.8V but I doubt the numbers will vary that much. As for other loggers or prop matching, sorry, I'm not really worried :)


Developer
Comment by John Arne Birkeland on March 2, 2012 at 5:13am

The voltage of a fully charged cell (4.2) will drop like a rock and stabilize and stay more or less linear around the nominal voltage (3.7v) under load. After you remove the load the cell voltage may rise again to a higher if there is enough energy left in the cell.


Moderator
Comment by Graham Dyer on March 2, 2012 at 5:36am

Voltage of the 4S graph at the end of the flight (but still flying) was 3.6V per cell, so yes, very close.

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