Qx95S, heavy voltage sag and incorrect battery sensing

Hi guys, just bought a qx95s (frsky) and i’m having a couple of issues i can’t work out how to solve.

I’ve already flashed latest firmware and switched props around as it was completely unflyable out the box. It now fly’s pretty nice (still needs pid tuning).

First of all it’s got some pretty heavy voltage sag on the batteries, i’m using 600mah 1s lipo 35c turnigys (only 2g heavier than the stock LiHv 450mah) which should provide more than enough current for the little thing. I have Battery warning set pretty low at 3.1v at the moment just so i get “some” flight time and this is triggered after around 2-3mins flight (same with stock battery). As soon as the motors stop it jumps back to 3.6v and by the time they are on the charger they are at 3.8/3.9v so something is pulling a fair bit of current (or is this normal for this quad?)

The second more problematic issue is the quad keeps sensing a 2s lipo is fitted (which is not) completely at random. When it decides to do this and i connect to pc all voltages shown in betaflight are correct for the 1s lipo that is actually fitted, apart from it says “2s” and the low V alarm goes off continuously. After several conections/disconnections of the battery it will eventually show 1s again and the quad is usable again!

Any ideas? I’m starting to think the voltage sensing circuit has a short or is malfunctioning in some way? This is a brand new quad with less than 10 flights on it so i’m kind of puzzled.

Thanks in advance

Hey Jonathan,

To your first Problem:
It happens a lot that Batterys have a lower Voltage while they are beeing used and if you measure the battery its a tiny bit higher than before…
I dont think that this is a very dramatic Problem…
To your second Problem:
Since your Volt changes in Problem 1 were quite big, there may be an Problem with your VBAT…
Maybe get yourself a replacement and hope it will work :wink:

Luca :slight_smile:

HI luca,

I agree “some” voltage sag is to be expected and is perfectly normal during flight, however; not to this degree. generally on my aircraft i expect a sag to maybe 3.6V/cell maybe 3.4V/cell for micros, but it should then plateau to a steady rate whilst the bulk of the battery’s capacity is being used. The only time this doesn’t happen is if you are drawing more current than the battery is rated for, which in this case would have to be over 21A continuous.

To better explain the issue I have just done a timed steady hover in my living room and here are the results, I’m sure you will agree that something is not right here.

Battery starting voltage: 4.2V Battery used: Turnigy nano-tech 600mah 1s 35/70C

0m:07s - 3.5V (expected initial drop)
0m:13s - 3.4V
0m:16s - 3.3V
0.54s - 3.2V
1m:28s - 3.1V (battery alarm “flickers” once or twice beyond this point)
2m:08s - battery alarm steady

after 20s battery had recovered to 3.7V showing that very little of it’s capacity had actually been used and the quad was able to fly again and followed a similar pattern.

whilst you have already mentioned the Vbat sensor maybe on the fritz (which I hope it’s not on a brand new quad) as the test went on the quad did require more throttle input near the end which is characteristic of a dead battery. but after the 20s battery recovery it went back to flying like it was on a full battery again which sort of proves this isn’t just the board coming up with false Vbat figures.

Couple of point I can see.

  • You have swapped from a HighVolt lipo (4.35v cell) to a normal lipo (4.2v cell)
    You may be better off with a 2S. (More volts = less amps pulled and faster response)

  • Landing at 3.1v your taking at lipo really low, it’s not normally recommended to go below 3.4v - 3.6v
    Lipos will recovery a little after flight upto nearly .8v. This is normal.

  • If you have swapped your props to different ones, that may decrease your flight time, if there more aggressively pitched.

  • Why its detecting a 2S lipo when you not using one… Sorry I don’t know.