First let me introduce myself. My name is Jerry Coffman. I'm been flying RC since 1990 and tearin up earth with and old traxais buggy and my old Blackfoot truck. (kinda dating myself there) Fist airplane was a cox fairchild 24 with a wind up .049 glow engine. I could go on but I wont.

Current issue. I Have 2 quads. an X525 running an APM 2.6 FC and an HJ MWC X-Mode Alien, again running the APM 2.6 FC Each one flies great. Very stable and predictable. Both running Flourion 3S 5000 mah batteries.The last flight on the X-Mode Alien was 17 min before the battery balance port buzzer went off so I put it in loiter and waited a good 60 sec and fail safe kicked in and landed her. I have the fail safe set to kick in at 10.5 volts per Arducopters page. But those are for 3S batteries. There is no info on 4 S or even larger that I could find. I've spent many hours going through the subjects and threads here to no avail. Some even having similar problems as mine.

I'm currently testing an H550 with the lighted lighted arms from HK.I have the lights unhooked for now and will install a remote switch to turn them on as needed. I've tried 2 test flights. Battery is a Turnigy Graphine 5000 Mah. 45C. Fail safe set at 14.2 v In both test flights I was only testing loiter mode. She did well when about 2 minutes in I heard a sharp rapid beeping and noticed the main indicator light was flashing yellow. Both times I checked the voltage with a multimeter I still had 15.6 and the other was 15,8 volts. No where near the fail safe threshold.At 3.5v per cell that'll give me 14v so I'm going to rest and try that today and will report my findings. It still boggles my mind how its going into fail safe at half what it should be.

any comments or advice would be greatly appreciated.

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You don't want your battery level to go below 3 volts on each cell.  So 4 X 3 = 12.  So the failsafe would be set to 12.5 to give you enough time to land the copter before you damage the battery.  If you copter is heavy you would set a higher value to give you more time.  Also as you use the battery the internal resistance of the battery goes up so the battery voltage will sage as you apply power causing early failsafe.

Failsafe's could be caused by other things not just the battery.  You would need to download a flight log to determine what happened.

Mike

Micheal is correct, its important to consider how much energy is needed to return to home and actually land. The heavier the copter the more you need!

The RTL settings include choosing the Warning voltage  (I use 3.25volts / cell so 13.0 v for a 4s. I use 300mah for the reserve capacity for "close " flights. and 500mah for longer flights so the battery capacity "in reserve" for the landing process is sufficient, If you have defined a RTL height then the copter may have to climb to that height before it comes home taking more power, If you set the RTL height to 0 it will maintain the current height and save that extra energy but be aware it may not be a suitable path for your mission. you could have trees, people or buildings in the path. 

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