I have an hexacopter with the 3DR PX4 autopilot. Today I tried to fly it with a Nano-Tech 5000 mA battery 25-50C discharge and after less than one minute fly, voltage lowered to near 7 V obviously the drone came down, fortunately with no major damage. The only change is that I attached a BaseCam Simple BCG 32 bit controller for the gimbal (2 axis). 

The data log does not report any issue (se attach) except the low voltage. Is this a failure of the battery?

Thanks for help

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I forgot, attached is a photo with telemetry readings. I checked the battery before the flight and it had 16.6 V and after the crash it recovered to 16.2 V by itself

The voltage is only relevant if you are measuring it under load, Run the motors (on the ground for a few minutes then note the voltage not just with the telemetry but with a digital meter

That way you get a reading that is close to the truth

I did this and I discovered that battery lowers in similar way (my telemetry is well calibrated), and then after load it recovers. I never have seen something like that, the battery under load downs to 7.0 or 6.7. Then it recovers! I did this with other batteries and they worked quite well. 

I conclude that the battery is bad, but in a strange way that i have not seen before

Thanks for your help

If this is a 16v battery and its going to 6-7 volts then you have over discharged it and permanently damaged it .
3.2 volts / cell is a minimum voltage

Yes it downloaded to 6-7 volts, then with NO recharging, the battery reads are about 3.7-3.8 volts per cell, giing a total of 15.1-15.2 volts. And it happened again with another battery! I do not understand this. The same with the other battery no cell reading below 3.7 and it seems perfect, but in the readings of current it dropped to 6-7 volts and the hexacopter crashed, fortunately from very low altitude. This battery that failed had 25C discharge rate.

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