Hello, at what percentage of critical battery the RTH is triggered?

It can vary, but the simple answer is when the aircraft figures out it cannot get back home on the remaining battery capacity.

You're referring to smart battery RTH.  That's a little different than critical.  

I believe the range is 66% max to 15% min.  That's what it says in my log.  So if you flew out with a fully charged battery and just kept going, you'd go into smart battery RTH at 66%.  But if you are close to home, it won't be triggered until much later - as low as 15%.  As Geebax says, it varies depending on your distance.  

Ten percent battery is critical auto land.  But that doesn't mean it won't go to critical auto land before 10% because it'll also auto land when any of your four cells drops below 3.3 volts.  That's why you want to have a battery with balanced cells.  And if you have a damaged battery, your battery could force an auto land much earlier without warning.  Swollen batteries are known to autoland early.  

Smart battery RTH is designed so the battery (hopefully) won't force critical auto-land before you make it home.  

It is well explained thank you very much.

If the critical low battery level is reached then the AC will land on the place within the time of it.

Hello and good day minilou. Thank you for reaching out and for the inquiry. The low battery level warning's and critically low battery level warning's thresholds can be set to 10% to 50%. In addition, When the battery power is low, the aircraft should land as soon as possible. Otherwise, the aircraft may drop directly.To avoid any risk caused by insufficient battery level, the flight controller of the Phantom series aircraft will judge whether the current battery level is sufficient based on the current position. If the aircraft can fly to the Home Point with the current battery charge, the DJI Go app will prompt the customers to decide whether to conduct RTH. If no action is taken within 10 seconds, the aircraft will automatically fly to the Home Point. During RTH, tapping the RTH button can terminate the RTH and the low battery level warning will not be shown again. If the aircraft can only land with the current battery charge, the aircraft will land automatically and cannot be stopped. During RTH and landing, the aircraft can be controlled by using the remote controller ( when the remote controller’s signal can be received ). Thank you.

Geebax Posted at 1-3 13:16
It can vary, but the simple answer is when the aircraft figures out it cannot get back home on the remaining battery capacity.

Yes!    But remember, it doesn't take in to consideration the wind conditions.  If you fly  down wind in a good breeze and hit Critical Battery, you may not be able to get back home before you hear that dreading announcement  "Landing".  If it is not home yet, or if you are over water, you will have a fight on your hands trying to keep it in the air and trying to find a safe place for it to come down.  And it is going to come down with the propellers spinning...or not!

djordan2 Posted at 1-6 08:09
Yes!    But remember, it doesn't take in to consideration the wind conditions.  If you fly  down wind in a good breeze and hit Critical Battery, you may not be able to get back home before you hear that dreading announcement  "Landing".  If it is not home yet, or if you are over water, you will have a fight on your hands trying to keep it in the air and trying to find a safe place for it to come down.  And it is going to come down with the propellers spinning...or not!

Correct. Which is why an experienced pilot will note the prevailing winds and head home while they have plenty of reserve battery capacity. IMO, too many pilots are watching the pretty pictures and the drone in the air and not paying attention to the telemetry on the screen.

Geebax Posted at 1-6 13:18
Correct. Which is why an experienced pilot will note the prevailing winds and head home while they have plenty of reserve battery capacity. IMO, too many pilots are watching the pretty pictures and the drone in the air and not paying attention to the telemetry on the screen.

Oh boy do I agree with your telemetry comment.  When I hit RTH, the first thing I look at is the Telemetry readout for the distance.  It better by gosh be decreasing!  My RTH altitude is set at 100 feet. But if I'm in a questionable area I will go up another 50 or so feet just to make sure.  And I'll point the camera full forward looking for any obstacles.

An interesting thing happened to me today while flying the P3Pro.  There was a light wind blowing maybe about 5 MPH.  Nothing to worry about.  I've flown in much stronger.  This time I went downwind.  At about 1,000 feet distance the wind suddenly picked up to about 20MPH at 25 feet.  It came on suddenly and actually blew my hat off.  I decided to bring the bird home.   So I hit RTH.  The drone went up to 100 feet as programmed and turned for home.  I looked at the telemetry to verify RTH.  To my surprise the distance was decreasing very slowly if at all.  The ground speed readout was about 4 MPH.  Oh boy, time to take action.

I cancelled RTH and dropped it down to about 25 feet AGL.  I then gave it full forward stick and was able to get the speed up to about 10 MPH.  It wasn't long before I got the dreaded "Low Battery" warning.  I pointed the camera straight down and began to scan the ground looking for a clear landing area.  I just wasn't sure I was going to make it back.  Fortunately, I did.

The wind just came up that strong while I was in flight.  It could have been a pilot error Fly Away if I had not taken action when I did.  Some folks might just hit RTH and then sit back and wait...until it is too late.  Watch the Telemetry!

djordan2 Posted at 1-6 17:51
Oh boy do I agree with your telemetry comment.  When I hit RTH, the first thing I look at is the Telemetry readout for the distance.  It better by gosh be decreasing!  My RTH altitude is set at 100 feet. But if I'm in a questionable area I will go up another 50 or so feet just to make sure.  And I'll point the camera full forward looking for any obstacles.

An interesting thing happened to me today while flying the P3Pro.  There was a light wind blowing maybe about 5 MPH.  Nothing to worry about.  I've flown in much stronger.  This time I went downwind.  At about 1,000 feet distance the wind suddenly picked up to about 20MPH at 25 feet.  It came on suddenly and actually blew my hat off.  I decided to bring the bird home.   So I hit RTH.  The drone went up to 100 feet as programmed and turned for home.  I looked at the telemetry to verify RTH.  To my surprise the distance was decreasing very slowly if at all.  The ground speed readout was about 4 MPH.  Oh boy, time to take action.

Yes, as someone else has said, RTH is a slow flyer. And just as well you were monitoring the speed.

Here is a good tip that I know most of you experienced pilots already know, but I'll post it here anyway.  It's another "watch the telemetry" statement.  A good way to measure the wind speed aloft before you go too far downwind is to take the drone up to about a hundred or so feet in the upwind direction.  Then turn off the GPS.  Look at the telemetry readout for the horizontal speed.  Give it a second or so to accelerate.  That will tell you what the wind speed is at that moment at that altitude.  Then decide if you can handle the wind if you go downwind.

I like www.uavforecast.com

followed by www.windy.com

Someone will correct me if wrong ..... but isn't critical landing also taking into account the height to descend ? I realise it will not normally add much to the power demand ... but we are talking about very low battery level and its fast running out !
Before anyone says it ... I am not confusing with the power to ASCEND to RTH height before actually starting home.

solentlife Posted at 1-17 13:21
Someone will correct me if wrong ..... but isn't critical landing also taking into account the height to descend ? I realise it will not normally add much to the power demand ... but we are talking about very low battery level and its fast running out !
Before anyone says it ... I am not confusing with the power to ASCEND to RTH height before actually starting home.

Yes, regarding the smart battery function - it seems that is part of the calculation - you can see the data change if you study the smart battery columns vs altitude column in the verbose log.  So we could more accurately say, if smart battery has not been disabled, "...it varies depending on your distance from homepoint and your height from homepoint."  And I'm sure the RTH ascent calculation is in the equation too, and applied as needed...

HOWEVER, if you have the smart battery protection disabled, then it'll keep flying to 10% or when any cell dips below 3.3 volts.  

When any cell dips below 3.0 volts, the battery shuts itself off.  

Mark The Droner Posted at 1-16 10:52
I like www.uavforecast.com

followed by www.windy.com

I use UAV forecast a lot too. I had never heard about windy, thank you for mentioning that, I will have to try it.