This morning I took a beautiful flight from the highest point around where I live.  Looking at the hourly weather forecasts, I knew that this morning the wind would rise from calm to 6 mph as the humidity went down from 90% to 60%.  
For a long flight, I chose the lower wind - higher humidity option at 8 AM.  By 10 for example, the humidity was 75% and the wind 6 mph.
The flight out was a bit over two miles when low battery/ return home popped up.  EIGHT minutes into the flight.  
Beautiful flight and videos.
But eight minutes?  My hope is that the high humidity reduced the flight time significantly.  
BY the way, it was in the high 50's F and sunny when I flew.
Thanks for any help.
The other option is an unhealthy battery, I guess.

It's hard to guess why your flight was cut short just from that description but no, the humidity wouldn't explain it. If you upload the log from the flight to this web site and provide a link to it someone can probably tell you why your flight wound up being so short.

Unhealthy battery sounds about right, or flying around at high speed the whole time.  Sport mode will eat your battery fast.

Edit:  Check to make sure all three cells are outputting similar voltage.

Example of what they should look like.  They are all almost exactly the same voltage.  If you have one cell reporting more than say 0.05v difference, then I would not fly that battery:

Recharge and then try the same battery again. Check the status anfd the output of the cells and if happened again you have sis month warranty on the batteries!

One additional note - don't charge a hot battery.  Let it cool.

Well... humid air IS less dense than dry air.  So the props will need to work harder to provide lift.

The battery is recharged.  (I always let it good down.)  And later this afternoon I will fly again, this time checking the battery cells etc.  Thanks for all the help.

Yesterday afternoon I repeated the morning's flight.  The air was much less humid 45% and the wind was a bit higher, around 6-8 mph at my flight attitude (per Uavadvisor).  This time in regular mode the battery allowed me to meander out about 2.8 miles and 12:55 minutes before low battery warning.  It returned  in 8 minutes, for a total time of 22 minutes.  I think everything is all right.

Awesome.  Good to hear as that is a normal flight time.  

Griffith Posted at 2017-4-25 10:23
Well... humid air IS less dense than dry air.  So the props will need to work harder to provide lift.

Really? I'd have thought it would be more dense, laden with water as it is.

Ex Machina Posted at 2017-4-26 08:52
Really? I'd have thought it would be more dense, laden with water as it is.

Wow, learn something new every day!

At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become. You may be familiar with the concept that moist air is less dense than dry air.

Griffith Posted at 2017-4-25 10:23
Well... humid air IS less dense than dry air.  So the props will need to work harder to provide lift.

Wrong.  Humid is more dense, just as cold is more dense.  Two effects - More dense lets the props grab air better, but it also means more resistance flying through it.

AG0N-Gary Posted at 2017-4-26 09:52
Wrong.  Humid is more dense, just as cold is more dense.  Two effects - More dense lets the props grab air better, but it also means more resistance flying through it.

Wrong ???

From Pilot's Handbook of Aeronautical Knowledge:

Water vapor is lighter than air; consequently, moist air is lighter than dry air. Therefore, as the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance.

As a non-current pilot, I'll defer on that.  I may be remembering that wrong, but I don't think so.  It will make an interesting conversation this morning with some pilot friends.  

So it seems to be true, then, that higher humidity = harder work for motors = shorter battery time.

Roll Tide Posted at 2017-4-27 08:27
So it seems to be true, then, that higher humidity = harder work for motors = shorter battery time.

I've flown in a variety of humidity levels and it doesn't have a large impact on the flight time of a Mavic. If you really want to know why the flight in your original post was so short then upload the log from the flight and we'll take a look.

Roll Tide Posted at 2017-4-27 08:27
So it seems to be true, then, that higher humidity = harder work for motors = shorter battery time.

Yes!               

Roll Tide Posted at 2017-4-27 08:27
So it seems to be true, then, that higher humidity = harder work for motors = shorter battery time.

Yes.  Higher humidity = lighter air.   However, as DroneFlying eluded, humidity has a much smaller effect on air density than temperature and altitude.
BTW, RTR

Here is the link to the flight with a RTH low battery at 8 minutes.  Maybe it will help.  http://www.phantomhelp.com/logviewer/4I1Q0B9BZTVFZ1T2KQIU/

Well ... from the log, I can see that you flew 6 minutes out in Sport Mode which consumed about 40 % battery. You're return (at RTH speed) consumed only 24 %.  I don't have a lot of confidence in the  % battery values - I believe they are only approximate.  Sport Mode consumes much more power/mile than normal mode and I suspect that you were encountering a head wind  (at your flying altitude) on your outbound leg.  Still 15 minutes flying time (with 22 % battery reserve) isn't that bad.  You can see from your log how rapidly the battery capacity dropped once you entered sport mode.  I suspect humidity had very little effect.

You still had 46% battery capacity at you're max distance. so I suspect the RTH warning at that point was based on the energy required to get to that point, assuming it would be approximately the same for the return leg.  Many pilots panic with battery warnings and switch to Sport Mode for return, thinking that it's the safer thing to do.  Completely wrong! Better efficiency at 20-25 mph.  

Griffith Posted at 2017-4-28 05:57
Well ... from the log, I can see that you flew 6 minutes out in Sport Mode which consumed about 40 % battery. You're return (at RTH speed) consumed only 24 %.  I don't have a lot of confidence in the  % battery values - I believe they are only approximate.  Sport Mode consumes much more power/mile than normal mode and I suspect that you were encountering a head wind  (at your flying altitude) on your outbound leg.  Still 15 minutes flying time (with 22 % battery reserve) isn't that bad.  You can see from your log how rapidly the battery capacity dropped once you entered sport mode.  I suspect humidity had very little effect.

You still had 46% battery capacity at you're max distance. so I suspect the RTH warning at that point was based on the energy required to get to that point, assuming it would be approximately the same for the return leg.  Many pilots panic with battery warnings and switch to Sport Mode for return, thinking that it's the safer thing to do.  Completely wrong! Better efficiency at 20-25 mph.

What you say makes sense.  I also noticed the big difference in out & back.  Thanks.

Roll Tide Posted at 2017-4-28 08:37
What you say makes sense.  I also noticed the big difference in out & back.  Thanks.

Sport mode uses serious juice, but I also just saw that I started the flight at 95%.  I had charged it the night before, or maybe the night before that.  
I just ordered another battery.  But only because this thing deserves two.  I think the original one is good.  And I really appreciate all of your thoughtful comments.   And look forward to being able to help others in the same way.

Griffith Posted at 2017-4-28 05:57
Well ... from the log, I can see that you flew 6 minutes out in Sport Mode which consumed about 40 % battery. You're return (at RTH speed) consumed only 24 %.  I don't have a lot of confidence in the  % battery values - I believe they are only approximate.  Sport Mode consumes much more power/mile than normal mode and I suspect that you were encountering a head wind  (at your flying altitude) on your outbound leg.  Still 15 minutes flying time (with 22 % battery reserve) isn't that bad.  You can see from your log how rapidly the battery capacity dropped once you entered sport mode.  I suspect humidity had very little effect.

You still had 46% battery capacity at you're max distance. so I suspect the RTH warning at that point was based on the energy required to get to that point, assuming it would be approximately the same for the return leg.  Many pilots panic with battery warnings and switch to Sport Mode for return, thinking that it's the safer thing to do.  Completely wrong! Better efficiency at 20-25 mph.

That was a good analysis and a good point about switching to Sport mode.