Maiden flight test and voltage sag

[FJJ]

Hi,

I had my first flight test of my new DJI F550 build yesterday. It's an F550 kit with Naza V2, E300 propulsion with stock 9.43inch props, iosd mini, ImmersionRC VTx, FrSky X8R Rx, AeroXCraft landing gear, Zenmuse H3-3D gimbal and GoPro HD Hero 3+ Black. Power is 4S from a Zippy Compact 5800mAh 25C battery.

Weight is about 2kg excluding gimbal and gopro.

During testing in gig left the gimbal and gopro off so the flying weight was 2kg, it took off perfectly and was very responsive, I was monitoring voltages using the feed from the iOSD Mini.

There re seems to be some big voltage sag going on, after a bit of use with the pack sitting at 15.3v when I start the motors the voltage dropped to 15.1, then on take off with full throttle and forward pitch, it drops to 13.9v trigger the voltage protection (set at 14.8/0loss and 14.2/0loss respectively). I charged the pack after that and it took 2700mA according to my balance charger, so it wasn't even 50% depleted and slightly warm to the touch after flight. I'm thinking that this level of sag must be an indicator that something is wrong? I'm using XT60 connectors which I guess could be a bottle neck - I'd imagine maximum current would be 15A (rating of E300 ESCs) x 6 so 90A way above the rating of XT60. I also found them pretty difficult to solder onto the big gauge wires on the LiPo so I guess that the soldering could be an issue. The battery at 25C 5.8Ah should be able to deliver a constant 145A so don't think that's the issue.

Any ideas?

thanks,
FJJ

 

[Tahoe Ed]

Don't always believe the manufacturer's spec's. They are often over stated. I am using XT60 on an S1000 parallel cable. I do notice some drop with that connection. But on single lipo connections even DJI has used them on the S800 and S800 EVO.

 

[Av8Chuck]

Curious why you don't use the XT90? Also do you know what Deans connectors are rated at? I find it challenging to solder the larger 10 gauge wire to the Deans connectors so I often solder 12 or 14 gauge "extension" between the two. I've never had a problem that I'm aware of but I know that's not the right thing to do.

Is that a new battery?

 

[FJJ]

I went with XT60 as I had a bunch in my drawer! I'll maybe try XT90 or a resolder of the connections, incase any are poorly soldered.

It it is a new battery, I've got another on order so should have something to compare it to soon - I've also ordered a LiPo alarm so that I can see if the actual cells are dropping to that voltage or if it is just the voltage available at the PMU.

 

[FJJ]

Did a little more testing, resoldered the XT60 on the pack and checked the soldering on the F550 - all looks good, although the AWG of wire supplied with the F550 looks a little on the small side?

With throttle on and a full pack, it drops from no load of 16.7V to around 14.9V with me restraining the F550 so that it can't fly - I guess a word case scenario - but that is a voltage drop of around 2V!

Should I get some heavier gauge wire, some XT90 connectors and redo the main power connector?

 

[Av8Chuck]

So I'm guessing that a 2V drop is a lot?

I built a Hex with inexpensive RCTimer motors, don't remember which ones, it was the first Hex I'd built so when I experienced a similar thing I thought maybe it was the 25C rating of the battery, the wiring etc., but it seemed to fly well just for not as long which I also chalked up to having six motors. I switched out the motors with Avroto 2814's and the performance was much better, they were bigger motors so I expected that but the efficiency was a lot better as well.

 

[FJJ]

That's pretty much what I want to know - is a 2V drop a lot? Should I dial that into my Naza settings - I can only fly for a couple if mins before low voltage cut off kicks in and only 40% of the pack has been used!

 

[Av8Chuck]

That's pretty much what I want to know - is a 2V drop a lot? Should I dial that into my Naza settings - I can only fly for a couple if mins before low voltage cut off kicks in and only 40% of the pack has been used!

I don't know either and I don't fly DJI anymore but when I did, I turned that feature off, flew with fully charged batteries for s specific amount of time, a minute, then measured how much I had to put back. Did that a couple of time to get a good average then extended the time to three minutes, step and repeat until I got to where I knew how long I could fly and still have 40% remaining. Once I knew that I set the timer on my transmitter and then I knew I had plenty of reserver if I flew in the same conditions I did the testing or if I flew more weight or more aggressively I would still land with more than 20%. After a while I got a really good feel for how long I could fly and how accurately I knew how much battery I had left when I landed.

Even after figuring out how to setup the power/battery feature on the NAZA I decided not to use it because once it starts to land you can't stop it and it might be landing in a place that could do some damage so I just felt more comfortable figuring out how long I could fly on the batteries I had and just timed the flights.

 

[Tahoe Ed]

My guess is that you are over the max amp draw of your lipos. I experienced similar behavior of some inexpensive packs, not cheap but less than top of the line, on my S1000 with a 5D.

 

[FJJ]

I'll try thick wire, XT90 connectors and a new LiPo then.