Hi,
this is just a general question I would like to ask which is related to an issue that I have been dealing with lately.
I built a hexacopter with a weight of 8.5 Kg, I am using a 14000 mAh lipo battery with it. The batteries are brand new (Tattu 6s 25C lipo).
Before the flight, I charged my battery and took it for testing. The initial voltage was 25.1 Volts but when taking off the voltage dropped to 22.5 Volts and the average current consumed by the system was 46 amps. According to my calculation, I should have at least got 14 minutes of flight time but ended up with a flight time of 5 minutes only.
Somehow, the voltage is dropping really fast and the capacity of the battery is not being utilized completely(only 18% of the battery was utilized by the system).
My question is how can I identify what is causing this sudden voltage drop?

Your experienced suggestions are most welcome.
Thank you.

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you have not given much relevant information? your amp draw is 46 amps seems too much but we need to know more relevant information  like prop size and motors how meany battery's your using to determine what is your issue ?  are you running other equipment on the same battery ?

I was testing the propulsion system from Xoar titan 6008 (400kv) motors with 19x6 CF propeller, ESC(60A , model name P60 from Xoar) propellers, the frame used for testing was tarot X6.
I used the power distribution board of the tarot X6 to power up the motors. I used 14guage silicone wire in each arm.
The power module used is mauch PL 100. I calibrated the sensor with a load on it when the system reached 11 amps I calibrated the current reading in mission planner with the current shown by clamp meter.
the battery used was Tattu 6s 25C 14,000 mAh lipo.

25C is a bit low for a draw of 46amps regardless of what the maths would indicate. The battery would be getting quite hot and the internal resistance would increase greatly. On a copter that size I'd be using something more in the 60C range. I've run in to the same situation several times when building quadplanes. For a build like that I'd go with two 45C batteries in parallel. 

I am not sure if you are aware that a voltage drop is actually expected as you draw current, as the other posters point out. This is due to the battery's internal resistance and, as pointed out, your battery's nominal discharge rate is probably somewhat on the low side given your current draw.

You should bear in mind that the voltage you see when you are drawing current (especially lots and lots of current, the drop is proportional to the current) is masked by the voltage drop, which is why you are getting such low discharge. This is why, looking at voltage is such a poor way of figuring out remaining battery. To get a feel for your battery, fly a bit, notice the voltage while flying, land and then look at voltage without the props spinning (it will rise quite a bit as soon as you stop drawing current). This "resting" voltage is the voltage that tells you how much you have really taken out and this is how you can get a feel for how long your total flight time is. Once you have done a few tests like this, verify by checking how much charge your charger is putting in when charging after the flight.

Hello,

Actually I have done 2 flights of my Hexa with 20000mah lipo battery a few weeks before. and I was getting 20 mins of flight time(with take-off weight 8500gms). after a week again I tried to test the same. but to my surprise, the voltage was dropping drastically than the last time and UAV didn't even utilize 30% of the capacity and trigging low battery warning. I have checked the battery health and it is in good condition. 

I would like to know what is happing with the flight time and any other issues? 

What do you mean by dropping drastically?Please look at the LiPo discharge curves here. Notice the difference in measured voltages due to increased voltage drop caused by higher current being drawn.

Dropping drastically immediately the moment you draw current but then stabilising more or less is to be expected. Dropping continuously and increasingly rapidly while drawing constant current indicates you are falling off the edge beyond 80% discharge (very dangerous place to be).

If you are not even utilizing 30% of the capacity (how do you arrive at this conclusion? Charge input during recharge?), indicates that you are witnessing only current related voltage drop, which is to be expected. It is typical for voltage related failsafes to freak out due to this. I personally either set them low enough to account for my expected voltage drop at my typical current draw (around 1 volt for a 4S at around 20A) or just disable them and go for reserved mAh failsafe, which is a lot more accurate, assuming your current sensor is doing a decent job. Another reason to avoid voltage failsafes is that they can also freak out if you apply too much throttle for whatever reason.

A voltage drop when drawing current results from internal (battery) resistance (V = iR). The internal resistance of a battery is based on chemistry, design, and temperature. The "C" rating of a battery may be helpful to gauge your purchase from a known manufacturer, but it is mostly a marketing factor. A good quality lithium battery charger will have a mode for measuring internal resistance. Measure your battery at the temperature you will be operating, then calculate the voltage drop anticipated for the current you will be drawing. 

Based on the voltage drop you reported, most likely you need batteries than can deliver more current, or you need to lower the current draw of your aircraft, but you might try turning on battery voltage scaling in Arducopter. With this parameter active, Arducopter will attempt to measure the voltage drop based in internal resistance and scale the voltage reading. That will provide you with a better reading on the actual battery voltage. 

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