Batteries Battery voltage on a new quad

[Derek]

I'll post some pics of this build in it's appropriate thread later on but I need to ask about battery voltage. I'm using four 3520 400kv T Motors. In the spec's, these motors are rated for up to 8S. I'm running them on 6S. In the specs, they state that these motors (at 6S while turning a 15x7 prop) will pull 28 max amps. Well, I'm no genius but I can do the math of 28 x 4 = 112. That being 112 amps (at full throttle). I have several 6S 3700mah 45C packs. If I do this equation correctly: 3700 x 22.2 / 1000 = 82.14 (Capacity x voltage / 1000 = Max Amp Capacity). So, one of my packs won't last long in this configuration. Since I began this project, I was thinking that using two of my packs, in parallel. That would be 7400 x 22.2 / 1000 = 164.28 Max Amp Capacity. That being said, two packs (in parallel) would handle the amp draw.

Am I thinking about this properly?

 

[vyrnguy]

hmm isn't the volts goes up and capacity remains the same. So there for 2 x 6S lipos in parallel should give you 12S. Not sure if your motor can run 12S. Correct me if I'm wrong???

 

[Tony]

actually the equation is mah*C/1000 and that will give you your discharge rating. So 3700*45/1000=166.5A. So a single battery will be able to handle those 4 motors just fine. Might get just slightly warm if you do a lot of hard loads on them. If you wanted longer flight times you could run the batteries in parallel and it will give you almost double the flight time. I say almost because you are adding quite a bit of weight to the quad.

vyrmguy, in parallel you double the mah but keep the same voltage. In series, you double the voltage but keep the same mah. Hope that makes sense.

Hope that helps Derek.

 

[vyrnguy]

thought I had mixed it up there, it's been ages. Thanks Tony

 

[Derek]

....and THAT, RIGHT THERE, is why I came to you, Tony....ha ha ha!!! I could have sworn that I had that equation right. After I finish typing this, I'm going to write down that equation and then get it laminated and then I'm gonna post it on the wall for later reference to avoid later confusion, lol.

Ok....so, even one pack will get me safely in the air. Fantastic! Thanks very much Tony!!!

Like I said above, I'll share some pics later on of this new build.

 

[Smoggie]

Only thing I'd add is that anything much above 30C is basically 'made up', almost all battery manufacturers exaggerate C ratings. You are still ok with the setup you have but if you ever did max out all the motors at once you would be working the battery VERY hard. Chances are that you wont ever see 28A from all four motors at the same time anyway but if you ever did pull 112A then expect very short flights because sucking that current out of the batteries your run time would be about 90 seconds!

 

[Tony]

It's my pleasure Derek!

 

[Derek]

Smoggie....thanks for your added thoughts too and i think youre right. I will surely keep that in mind!

 

[Derek]

It was mentioned, above, that if I reach max amps on my build that my flight times will be greatly reduced. That got me thinking....is there a way to calculate or guess-timate the flight time of a 12lbs quad with two 6S 3700mah packs?

 

[Tony]

There is a calculation, but it involves watt hours, constant current draw of the motors as well as some spikes and quite honestly, my ADD will not let me sit down and to it lmao.

 

[Derek]

Lol....i can relate to that!

 

[Smoggie]

I just had a go at working out a rule of thumb, what I came up with is 3 watt-Hours of battery energy per Kg of weight per minute of flight.

So flight time = Watt-Hours/(3 x weight)

Battery energy = 3.7Ah x 22.2V x 2 = 164.3Wh
Weight (Kg) = 12lb/2.2 = 5.5Kg

Flight time = 164.3/(3 x 5.5) = 10 minutes


This is maximum achievable flight time based on having efficient motor/prop setup and gentle/hovering flight. If you were racing around or if you have motors or props that are not working efficiently flight time could be dramatically less.

 

[Derek]

Thats interesting! So, based on these numbers.....a flight of 5-8 minutes would be very possible.

The first several flights will be gentle so that i can do some tuning and voltage monitoring. Then maybe we'll abuse the throttle more.

Thnaks for your time, Smoggie!