Sudden Potential drop when taking off

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.

have you calculated here ?

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Please provide a few more details regarding you build. Frame/Motors/ESCs/Propeller Size and power module for sensing voltage/current.

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And a .bin file from the flight please, otherwise it’s just guessing.

Have you calibrated the voltage and current sensors?
Have you calibrated them under load, not just sitting idle?

Without a log from the FC we don’t know what parameters you have setup.

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.

Here is the link for .bin file from the flight.

i flew the system with 20,000 mah battery and i got the flight time of 20 minutes but during testing one propeller came off and crashed, i somehow fixed the frame and after that I tried to fly but getting this problem now.

To start off with I do not think your current measurement is correct, you are sucking way more current than indicated looking at the voltage drop on that battery.
How long do you leave it balancing?
What is the internal cell resistance? (if your charger shows that)

Looking through your parameters you have the FS voltage set at 21.5, which is a bit high, we usually have it at 21v, although the voltage went to 21.29 when the FS tripped.
We use the Tattu 20Ah and you would need to be sucking 80A or more to bring that voltage down so quickly.
Have you retested with the 20Ah?
It could just be a sick battery.
How well balanced are the cells when you put it back on the charger after a flight?

The usual props used on the 400kV motors is 15”, so I do think your way over propped if they are 400kV.
And just checking the Xoar titan 6008 on their website they have it down as a 285kV motor, so which is it?
Their data is a bit strange in that they call it a 6S-12S motor but only give 6S data, and their weights are calculated at 60% hover rather than 50%.
But your rig, at 8.5kg is hovering at just under 50%

I always take the copter to hover throttle for the current calibration as the sensors are rarely linear.
(not an exercise for the faint of heart)


Also, what charger are you using? Does it indicate how many mA went back in to the battery when completely charged?

I have found the Mauch data provided with each sensor to be very accurate. Have you bridged the PL sensor for 6S systems? You should be using a V Divider setting of 10, and the A/V number supplied with the module.

Agree with @mboland - 19" props with 400Kv motors is a bit unusual, the 285Kv listed on the web site seems more likely with a 60 mm motor. Were the motors hot to the touch after you landed?

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How long do you leave it balancing? - It took a long time to balance the 14Ah battery (each cell balanced at 4.18, 4.20, 4.18, 4.18, 4.18, 4.18) the charger showed that battery is balanced but it did not equal each cell with 4.2V.

What is the internal cell resistance? (if your charger shows that) - right now I checked it is showing 6 m Ohm for 14Ah battery. ( ** I am using Hyperion Eos 720i charger**) whereas 10 Ah battery is of 8 m Ohm.

Yes, indeed the voltage drop will be more whenever the current draw is high. But I tested the same system with tethered which shows that the system is consuming 1500Watt with a voltage supply of 25V that is equivalent to 60A current but in tether, the weight is around 10Kg.

Have you retested with the 20Ah? - After getting a flight time of 20 minutes we tested it again but we ended up with a flight time of 7 minutes but I guess it’s because I did not put lipos at storage mode causing it to deteriorate. but I will be testing it again with 20 Ah batteries today.

How well balanced are the cells when you put it back on the charger after a flight? - with 20Ah they were well balanced(used two 10Ah battery in parallel for 20Ah cells were balanced for both)

Well, we asked Xoar for a custom solution due to some reasons so they custom made the 400Kv motors(not to be found on their website), I am using 19-inch prop as per their recommendation.

I did 3 flights to get to know the behavior.
In the first flight (without payload) I set the failsafe to 19V and 2000mah on reserved just to drain out the battery. I used two 10Ah battery with a total capacity of 20Ah.
When the first flight started the initial voltage was 25V but when the system took off the voltage dropped to 23 volts. battery behavior was normal and gave a flight time of 32 minutes.
at the end when motors disarmed the battery voltage recovered to 21.22 volt.

flight 2( with payload and Takeoff weight of 8.5Kg)
I kept the same failsafe as above with same configuration of battery.
at the time of hovering Voltage dropped to 22.5 V from 25V and gave a flight time of 22 minutes.
After the flight, the battery voltage recovered to 22.2 volts.

In both flights, I checked the throttle in stabilized mode the throttle stick was at 50% throttle that means the battery was working fine ( wheres, when the battery about to die the system require more throttle to maintain the same altitude but in my case, the throttle was nearly same throughout the flight)

Flight 3(with payload and single 14Ah battery)
with all the parameters same as above the flight time was 17 minutes)

The battery cells were almost at a similar voltage.
My question is that is it safe to push the batteries up to 19 volts?
The motors were normal, but the battery was a little heated.

In my opinion, absolutely not. At 19 volts you are barely over 3 volts per cell! For an aircraft that large I would land the aircraft at well over 3.5 volts per cell, which is consistent with Tattu’s guidelines. From a safety perspective landing on a nearly empty tank is a bad idea. There is no error margin.

Here are Tattu’s discharge guidlines:

Personally, I make sure I am already on the ground at 3.6 volts per cell or 21.6 volts on the pack, and the alarms on my radio are alerting me at a little below 22 volts so I can make arrangements complete the mission and return to the LZ for a safe landing. This usually equates to just a little under 30% of capacity remaining on the battery at 18 minutes of flight. I run 10,000 mAh Tattu batteries on a 5Kg+ Tarot Hexa. Typically my batteries rebound about .6 volts and I end up with a resting voltage around 22.4 to 22.7 volts on the pack. Recharging the battery typically results in about 7,500 to a little under 8,000 mAh going back into the pack.

I am curious to hear the opinion of other pilots on “how low they will go”.


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I won’t enter into the battery debate as everyone has their own opinion.
But be warned, at 3v per cell there is no usable current left in the cell. So that equates to EMPTY.

Balancing is critical for LiPo’s and ALL cells should be showing 4.2xx v per cell otherwise I would not consider it balanced.
In this respect not all chargers are equal.
A good quality charger will continue to top up the cells while keeping the charged cells at 4.2xx v.
Cheap chargers only use resistive load to even up the cells once a full charge is indicated.
So you can see that although a cheap charger might show it put Xamount of Ah into a battery you can lose quite a bit of that in their method of balancing.

All LiPo’s deteriorate with use and age (especially if they are stored charged).
Their voltage under load will be lower but they will still perform.

It’s always a personal choice on how hard you push them.


@mboland I had not thought about energy used in the balancing process as a resistive load counting towards the mAh balance but that makes sense. @zabbie which charger are you using? My everyday charger is an iCharger 406 Duo.

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@wn0x @mboland
Thanks for your time and suggestions.

@wn0x I am using Hyperion eos0720i net3 ad charger.

Yes, indeed 3 volt is a dead zone for cells. the thing is that the initial drop in voltage is killing me. I should be able to achieve a flight time within the voltage range of 21-25 Volts.
but in my case, it is happening in voltage range of 19-22.5 volt.

After the flight, I charged the 10000 mah battery, the charger charged it with a capacity of 9055 mAh. That means the capacity of the battery is being utilized within the range of 22.5 - 19 V.

@mboland as you are emphasizing that each cell should be balanced at 4.20 volt, I charged my batteries and ended up with each voltage level of 4.19, 4.19, 4.19, 4.19, 4.19, 4.19.

I am afraid that one day I might see the drone free-falling if I use this voltage range for flight time.

So, how can I eliminate this voltage drop so that I can achieve the endurance within the normal voltage range that is 25 to 21 volts?

Could It be because of the wiring or could it be because of the motors or is it the batteries? as I don’t see any other reason to account for the potential falling down that much in a very early stage of flight.

Check the PDB for heat under load. (Thermal camera) The copper traces on those boards are feeble and quite frankly I’m surprised it even works at those current ratings.