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Battery selection on quad with DT700 motors.

#1
So some of you might have been following my other thread reference my tri to quad conversion. I keep running into snags and finally have everything almost figured out. However, I am still have slight issues with a couple of things and would like to know if I am on the right track.

I am running a 3 cell 2800 mha battery that is rated for 30c. Since I added the forth motor the throttle feels really sluggish and when I push the thing to max it sounds like low RPMs. I have calibrated the RX so it is definitely getting full throttle to the unit. This was not an issue when running 3 motors.

So here is the config.

4 X DT700 motors
4 X 20a hobbyking ESCs
4 X 10x6 props

I have a higher C 4s battery available but these DT700s are not rated for it.

Am I on the right track of narrowing my issues down to the battery???
 

xuzme720

Dedicated foam bender
Mentor
#2
I would first check your amp draw at full throttle. That will tell you (and us) a lot more. Do you have a wattmeter or anything that can show you total amperage?
 
#4
If my math is right,

30c X 2.8 = 84 amp rating on my battery

18a x 4 = 72 amp draw from the motor and max of 80 the speed controller can handle.

Is it realistic for a battery that could theoretically push 84 amps to actually roll with 72 reliably?

Or is my problem elsewhere????
 

xuzme720

Dedicated foam bender
Mentor
#5
Without some sort of wattmeter you could be chasing your tail, or throwing money at the wrong thing. All other factors considered, it could still be the battery if you are close to hitting full amperage on the esc's. But without knowing actual draw, I can't say for sure. Plus if the battery isn't putting out it's full C-rated amperage....
This one is pretty cheap but I don't know if it keeps peak readings like this one does. And you'll need connectors for it as well but you'll be able to properly prop your motors and match your esc's in a more practical way.
 
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xuzme720

Dedicated foam bender
Mentor
#6
I'm kinda leaning towards elsewhere though since you are at almost full throttle before lift-off. In adding the extra motor and boom you may have pushed the weight above where you should be for the combo, I doubt it since a lot of people are using the same gear. I don't know but maybe larger props could help like an 11x4.7. I'm wondering if the 6 pitch props are drawing too much amperage, but since these motors are rated for 11x4.7, that's a little far fetched.
 
#7
Yeh I should really get one of those.

Just doing the online calc tool it says I need at least a 30c battery with no extra amp draw from anything else like the board or GPS. Each one of my ESCs also function as 5v 2.5a ubec so I am getting losses there. I will just slap my 4 cell battery on for a quick test. If that fixes it I guess I am good.
 
#9
Worst case scenario I end up with more batteries.

Might not get back to this until tomorrow. I have to do work to my house and did surgury on my leg last night. So I have to stop the bleeding before doing work. I then have to do the work before I am allowed to play with my "drone." LOL
 

Tritium

Amateur Extra Class K5TWM
#10
If you are not lifting off at half throttle or less then something is absolutely wrong. I bet that the prop pitch of 6 is drawing closer to the 90 to 100A range which the battery cannot sustain (and neither can the electronics, ESC, Motors for long). Are they hot to the touch afterwards? That is usually a sure indicator of overload. A hot battery after flight is also a sure sign of too much amp draw.

Thurmond
 
#11
Nope not really hot but not taking off either. That is a good point though about the pitch. I have some larger props at a smaller pitch on the way. So far it has been a pain to find props state side.
 
#12
Found a pack of 8 on amazon also, now some larger ones with less pitch are on the way from somewhere stateside.

Bigger higher c battery on the way too!
 
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#13
Tried the new battery and new props today. I think the improvement was probably the props since the motors dont sound like they turn any faster. Anyway, I zipped the props to the motor though the holes in the top to prevent prop loss. The wires on the new battery were so big I couldnt solder them with my gas iron because I couldnt get them hot enough to get the solder to flow. This was a simple fix and I removed the tip soldering them HVAC stile to the leads with the flame. Discolored the connector a bit but who cares.


The whole damn thing flips over forward still. I really think that the whole V frame setup in megapirate requires precise angles or the thrust is all wrong. Right now I have two motors that are lifting on almost the same axis with very little on the others. I then have two motors that are split evenly between two. I dont think the firmware sees a difference between X and V. So, I am going to put these on the ATG frame I got that was going to be my second build. I really wanted to make something with motors stronger than DT700s.

Oh well, these are very easy to reconfigure and I can always swap the 700s and ESCs for something bigger and put the 700s on something smaller. I do have a spare KK2 board now for a future project. I really want to get this project rolling this month in time for my beach trip.
 
#14
Success, I moved all my electronics to the ATG 650 frame and it ran great. The bigger battery helped too.

I did notice that after flying for a bit the thing got sluggish and would not climb at max throttle. I assumed the battery was dead but when I went to charge it the thing was 70%. So either the thing requires much more juice than I thought or something else is going on. Oh well I got a pretty decent amount of time to mess with it. The battery is a 3s 3000mah. Maybe 7 minutes flight time I think.