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Electrohub Battery/Flight Time Issue

#1
Just before Christmas I got all the thing I needed to put together an electrohub quad with a pixhawk on board. I have everything flying beautifully and am really enjoying it, but the flight times aren't great.

I purchased 4 multistar multirotor batteries hoping to get something in 20 minute flight range with everything on board: 2x4000mah 3S batteries, and 2x5200 mah 3S batteries. They are all 10C rated batteries. My voltage alarm is set to 10.6V and goes off after about 6-7 minutes of flight with the 4000mah batteries and about the same with the 5200 mah batteries. When I charge the batteries, the 5200mah batteries are taking about 1900 mah. The 4000mah batteries take about the same - far below what I expected.

I'm think what is happening is that the quad needs more current than what the low C rated batteries can provide, so it's causing the voltage to sag. I should mention that at rest, the batteries still show about 11.8V. If so, what are my options? New batteries with a higher C rating? New motors that pull less current? Maybe switch out the 1045 for 9 inch props?
 

Flat4

Senior Member
#2
Looks like you hit the nail on the head thinking it's voltage sag. Just the fact that they are triggering voltage alarm at 10.6v, then bouncing back to 11.8v resting pretty much says it all. I don't have any experience with those batteries, but it doesn't seem they are hitting their rated C-rating. Specifically with the 2x5200's. That setup alone should be good for 104 amps at a 10C rating. I find it hard to believe your quad is pulling that kind of amperage, out side of being at full throttle.

What motors are you running on the quad? How much throttle is needed to get it in a hover? Also any idea of the quad's weight without battery?
 
#3
I'm running Suppo 2212/15 930kv motors on it with 1045 carbon fiber props. In stabilize mode, it hovers at mid throttle or just a little higher. I'm not sure what it weighs, but I'll try to get a weight on it tomorrow. I'm guess somewhere in the 500-600 gram range without the battery. That's a total guess though. Thanks for the feedback! Hoping to get some more flight time soon and some decent video!
 

Topdawg

Senior Member
#4
correct me if I am wrong. I am working of memory, and if I remember right, Mah * C = amp draw. So 5.2 * 10 is only 52 amp draw. The 4000 is even less at 4 * 10 equals 40 amp draw.
 

Flat4

Senior Member
#6
correct me if I am wrong. I am working of memory, and if I remember right, Mah * C = amp draw. So 5.2 * 10 is only 52 amp draw. The 4000 is even less at 4 * 10 equals 40 amp draw.

You are correct. I read it as him carrying two at a time, but now that you brought it up, I think I may have been wrong in that assumption.
 

Flat4

Senior Member
#7
I'm running Suppo 2212/15 930kv motors on it with 1045 carbon fiber props. In stabilize mode, it hovers at mid throttle or just a little higher. I'm not sure what it weighs, but I'll try to get a weight on it tomorrow. I'm guess somewhere in the 500-600 gram range without the battery. That's a total guess though. Thanks for the feedback! Hoping to get some more flight time soon and some decent video!
Looking at thrust data for those motors, on a fully charged 3S battery, and 10 inch props, it's only pulling about 14amps static. So in the air it should be even less. Going with the extreme, at full throttle your pulling 56amps at most, which is a bit much for those batteries.

Now you said the packs would bounce back up to a resting voltage of 11.8. That's roughly 60% of that pack's capacity left. So you figure with a battery that isn't sagging you should easily double your flight times, with the recommenced 20% left in the pack.
 

Topdawg

Senior Member
#8
Flat is right, I just ran some numbers on Ecalc, and guessed the weight of the quadcopter at 1400 grams. Based off of the rough estimation as I cannot input all the exact info you should only be pulling about 18 amps at hover. Unless they are bad batteries I can't imagine those getting close to the max draw of the batteries. Do you by chance have a watt meter you can double check your amp draw?
 
#9
You are correct. I read it as him carrying two at a time, but now that you brought it up, I think I may have been wrong in that assumption.
I actually have tried both ways. I expected to fly just one at a time, but did try it with the 5200mah batteries in parallel. I had very negligible change in flight time and the batteries didn't discharge much further than when I flew a single pack. My weight without the battery as actually around 1200 g. This includes the fpv rails and a 3 axis gimbal.

I don't have a watt meter, but have a couple friends who have them. I may be able to measure the actual current draw sometime today. If I can, I'll post it up. I would certainly be more pleased with double the flight time. If that means buying some higher C rated batteries, I'm definitely up for that. I'll use these multistars to run my FPV ground station instead. They should run it for days on end. lol

Thanks for all the quick responses guys. I really appreciate all the help folks here are!
 

Flat4

Senior Member
#10
I actually have tried both ways. I expected to fly just one at a time, but did try it with the 5200mah batteries in parallel. I had very negligible change in flight time and the batteries didn't discharge much further than when I flew a single pack. My weight without the battery as actually around 1200 g. This includes the fpv rails and a 3 axis gimbal.

I don't have a watt meter, but have a couple friends who have them. I may be able to measure the actual current draw sometime today. If I can, I'll post it up. I would certainly be more pleased with double the flight time. If that means buying some higher C rated batteries, I'm definitely up for that. I'll use these multistars to run my FPV ground station instead. They should run it for days on end. lol

Thanks for all the quick responses guys. I really appreciate all the help folks here are!
More data never hurts, can at least get a better idea of the true C rating of those batteries. Do any of your friends have a higher C rating battery you could try for a flight?
 

pressalltheknobs

Posted a thousand or more times
#11
As I understand it theoretical max flight time can be calculated from the average amp draw and the battery capacity. Here's some rough calculations...

If you have 4 propped motors and their max static draw is apparently 12A each so lets say their cruising draw is 6 amps each so that 6*4 = 24amps or 24000 mA

You have a 4000mAH battery and we can use about 80% of that so 4000 *.8 = 3200 mAH
and a 5200mAH so that's 5200 *.8 = 4200 mAH

mAH = current * time

so

mAH/current = time

3200/24000 = 0.13 hrs = 7.8 mins theoretical max flight time.
4200/24000 = 0.18 = 10.8 mins theoretical max flight time

Your mileage will vary and will likely be less. 6 or 7 minutes doesn't seem that far off what you might expect from the numbers.

As mentioned in other replies, a 10C 4000 can supply 4*10 = 40 Amps and an 10C 5200 can supply 5.2*10 = 52 Amps. Assuming 12A a motor, your max is 48 Amps so these batteries are a bit low C for aggressive flying but should be fine for cruising around assuming they perform reasonably to spec.

If the batteries are sagging it could be the way you are flying. More aggressive flying will give you shorter run times. If you are NOT flying aggressively you might try setting your battery alarm at 3.2v per cell instead of 3.5v which is fairly conservative. You have to take it under 2.7v per cell for a while to do damage so as long as you have a good idea as to actual flight time and can land reasonably quickly you can be a bit more aggressive on the voltage alarm.

Another question is how are you charging these batteries? Are they well balanced with every cell within 0.02 v or better?

I'm surprised you say doubling up makes no difference. I would expect some increase. Won't be twice the run time but it shouldn't be nothing if it's still flying reasonably well. It depends how much the Amp draw goes up for the increase in weight. Measuring the actual amp draw will certainly help figure out what you can expect.
 
#12
More data never hurts, can at least get a better idea of the true C rating of those batteries. Do any of your friends have a higher C rating battery you could try for a flight?
I have some 2200mah 25C rated batteries I may try just to see what they do both as a single battery and in parallel. As far as having a battery with a comparable capacity but higher current rating, I don't really have any or know of anyone who does other than the ones a friend has for a DJI inspire, but I can't really use those.

As far as the calculations go, that makes sense, except the batteries don't seem to be putting out anywhere close to 3200 or 4200 mah. When I charge them, they consistently only take 1800-1900 mah. The batteries are balanced to within .2v per cell as well. When I ran them in parallel, I did see some improvement, just not very much at all. My thought is that the extra weight of a 5200 mah battery must have nearly offset the additional current it provided since they still only took about 1900 mah eachto charge.

I'm flying pretty gently. I built it to do aerial videography/photography, so I'm pretty easy on the sticks - a lot of slow ascension and light cruising. I definitely fly my 250 tricopter a lot harder, but it serves a different purpose. If get time today, I'm going to measure the current draw and see how it goes. I could always lower the voltage alarm level on the pixhawk, but I'll be keeping it close and taking it easy toward the end of the battery for sure.

The other thing I thought about doing is double checking the actual voltage during flight. I have a small voltage alarm that plugs into the balance plug and displays the voltage. I calibrated the pixhawk, but it probably isn't a bad idea to double check that during flight. I'll just aim the camera at the little voltage alarm and see how it compares to the pixhawk readout in mission planner.
 
#13
The power went out at my office which took our computer systems and machinery down, so we had the opportunity to do some testing with a nice Fluke clamp meter that belongs to an electric engineering friend. The quad is pulling 60A at full throttle and I believe it would pull more given the opportunity since there was not much change in prop speed or amp draw above 3/4 throttle. This was done with a 5200 mah battery. These batteries have a burst of 20C for 10 seconds, but only 10C sustained. I believe we saw the battery reach it's peak and taper off a bit. It also was getting fairly warm during the test. The 4000mah battery didn't really stand a chance with higher throttle input.

Long story short - I believe the batteries can't hack the current draw of the quad. I'm going to order a battery with a higher current rating to see if that makes a difference. If this is correct, I should see some longer flight times and a little more punch throughout the throttle range.

I'll come back to this post and update you guys on what I find out as I attempt to get longer flights. Thanks again for all your help. As I've worked through this problem over the last few days, I have learned a lot about quad power systems and current ratings.
 
#14
Hi ,
You need some hight C-rate (75c or 95c ) battery for your FPV, 10C is not enough power. Don't strive merely for capacity and do not ignore the weight of battery. Bigger lipo battery can be powered more energy. In fact As the battery gets larger, the increase in flight time becomes ineffective, because of the weight of battery. you can calculate quadcopter flight times by this formula : Quadcopter flight times =(Battery Capacity * Battery Discharge /Average Amp Draw)*60 , which I found it from here http://www.rcdronegood.com/quadcopter-battery-calculator-flight-times/, Maybe help you.