I need some advice - I modified a Parallax Elev-8 quadcopter by fabricating a boom from alunimum U-channel stock to hang a Tarot 2 axis gimbal/GoPro camera. The aircraft is using an APM 2.6 FCU, with external GPS/compass, telemetry and a small video transmitter. The weight is approximately 4.4 lbs. I'm using the stock motors - KA 1050kv Outrunners with 10x4.7 propellers. My problem is after only about 2 1/2 min the voltage failsafe kicks in and it wants to land. Here is a picture of the voltage log:
You can see where the Failsafe begins - I was able to switch to RTL and the aircraft returned to the launch point.
My question is this: Is it normal for voltage to drop immediately like it did on takeoff? Should I lower the voltage failsafe threshold?
Here's a pic of my aircraft:
Replies
You need higher C rating batteries, voltage drop is normal, but the quality of your batteries is very poor...
If your ESCs are running 'Simon K' firmware (specific for multirotors), there shouldn't be any cut-offs (one of the purposes of Simon K, I believe)..
Lots of multirotor ESCs aren't. If not, check the manual for how to program them and check their parameters (using transmitter sticks plus patterns of beeps, or a more convenient programming card).
Many come with a pre-programmed low-voltage cut-off, which will either shut off your ESC like a switch, or slightly more gradually (you're still coming down fairly hard), when the voltage falls too low.
George
So I charged up a battery yesterday and set the failsafe values to 10.0V and 660 mAh, which is 20% of my 3300 mAh battery. I also switched out the props - put on 11x4.7 carbon fiber props. It took off okay and I flew it around for a couple of minutes and then it started "twitching", which got progressively worse, so I decided to bring it back and land. But it tipped over just before touchdown... .No apparent damage, but when I touched the motors they were really hot. I think the 11x4.7 props were too much and overloaded the motors.
I did get a little longer flight time however. But I've decided not to fly anymore until I get the new motors and rebuild the ship.
Adding my 2c to what others have said: those batteries cannot handle the current demanded by your system at it's current weight.
A battery's voltage drops under load depending on type, quality, load, temperature and age. For a 3 cell pack pack 10.0v is too low to really have any usable power left, and certainly not enough to slow the descent required for the actual touchdown.
After 2 minutes or so (under load) the voltage should be around 11.4v - 11.7v and often a battery that is working will actually increase it's voltage slightly as it warms up as a result of more chemical efficiency due to the warmth. (See the battery graph vault on RCGroups)
Newer and/or higher-C batteries, higher capacity (= higher effective C rate - but this also adds weight) or higher cell count (with caveats) or much lighter weight should solve the problem. Changing props will just shift the problem to other areas like overheating or not enough power.
Changing to better, correctly rated and propped motors will only add to the efficiency of the system after the battery problem is sorted.
Check the resistance of everything in the current path in the system. Most LiPo batteries have an ESR (Equivalent Series Resistance) of 4-8 milliohms per cell. For a 3 cell battery, that is around 16 milliohms. If you are drawing 60A, then the (apparent) voltage will drop 0.96V .Add the resistance of the wiring and the connectors and you can easily exceed 30 milliohms total. That results in a voltage drop of 1.2 VOLTS at 60A. You can easily see the effects of this if you put your craft in a hard climb and then land it. Check the battery voltage during the hard climb and just after you land. That will show the ESR of the battery + the wiring. If it is more than 20 milliohms or so, get a new battery (or parallel two smaller batteries) and reduce the resistance in the wiring by increasing the wire size and reducing the number of connectors.
I have actually modified APM firmware to compensate for the ESR of the batteries. It basically takes battery voltage and adds (current in Amps * ESR) to that value. The result is the TRUE cell voltage of the batteries (not the terminal voltage). Now, my (corrected) battery voltage remains relatively constant whether I'm drawing 10A or 60A. The ESR of batteries does change over time, so I measure the ESR with my TURNIGY charger and enter new values from time to time. The end result is that I have a much better picture of the state of charge of my batteries.
Where are you located?
(C-counts fall even farther with cold temperatures).
George
I'm an inmate in a lunatic asylum in Central Texas - Sun City, in Georgetown. Temperatures here have been moderate.
And if you do lower the battery failsafe threshold, make sure your ESCs don't have ideas of their own (what, if any, low-voltage cut-off actions are they programmed for?), or your RTL expectations could be severely disappointed!
George
My rationale was that it seemed that the voltage momentarily hit the failsafe threshold and if I lower it, just a tad, I might be able to fly a little longer before the actual voltage is drained to the danger point. I just plugged in one of my batteries to recharge and it's showing over 11.5 volts - I think I could safely get down to 10.0 before worrying about battery problems. But you do make a good point about the ESC's I have no idea what their low cut-off value is. How does one determine that?
A case has been made that lipo C-counts are hugely inflated, by as much as a factor of 2.
If that were (close to) true in your case, you'd have around 40A - 10A per motor, around which voltage would sag strongly.
I does seem that such a heavily loaded quad, at only 3S (lower battery voltage = higher current draw), and such weak props, might draw 10A per motor, or more.
Consider going higher on any of: C-count (too low for this set-up, imo), voltage, or max battery capacity.
You might even find a higher-capacity battery, even though heavier, would fly longer for this reason (max current = C-count x battery capacity).
Caution on dropping the failsafe threshold any lower. Down at those low voltages (below 3.3), voltage drops very, very quickly under load. Not even enough time to make a controlled landing without damaging the battery.
George.
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