Voltage plummeting at measly 30 A

Hi there. I have a 6S 8000 mAh battery that can send out 25C, up to 50C at burst. My hexacopter took off and the battery failsafe was quickly triggered because the voltage dropped dramatically fast, and below the failsafe threshold. The drone then RTL’d. I have 10 AWG wires and AS150 plugs to connect the battery to my Holybro PM07 PDB, which is rated for up to 140 A.

All in all, I see no limiting thing here that would cause my drone to RTL. Battery can supply current, wires are thick, failsafe threshold is not unreasonable, etc.

Anybody have any hunches on why the voltage is dropping? Important note: The battery is old. Would this be behavior of an old, deprecated battery? See attached current and voltage plot of the flight.

Thank you!

PS: The scales are weird, but the voltage on the left is 23 V - 20.5 V, and the current on the right is 0 A to 40 A.

A fully charged 6S battery should be 25.2 (4.2v * 6). At 23.1v the battery is at storage charge. So the battery wasn’t fully charged before take off. Properly charge the battery flight and see how it reacts.

Also, battery manufactures use of C ratings is a little bit weak. Some C ratings are more optimistic or wishful than others.

Ok, thanks, I’ll give it a major charge. Does over 2V sag seem expected? That’s a bit higher than I would have guessed but I also have absolutely no idea.

Lots of variables. Try with a fully charged battery and see how it goes.


I’m dealing with something similar and I learned that calibrating both voltage and current using one of those external voltage/current meters helps to learn what the values actually are. There’s also an option on the BATT_FS_VOLTSRC parameter. I’m still studying it. I know it’s worth looking at.

I highly recommend trying/testing/using Amilcar’s Methodic Configurator. It’s worth exploring it.


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Please use BATT_FS_VOLTSRC = 1
It makes your live easier, just set the voltages you want.
It will take internal resistance into consideration so that you do not need to do it yourself.

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Thank you everyone. I’ll implement all of these suggestions, but it is worth noting that through a 330 Ohm resistor at 23.23 V (70 mA), the voltage under load was 24.83, so 0.4 V drop. I am quite confident based on this the battery is just done, though I will fully charge them and then re-check. I talked to a colleague and it is potentially 8+ years old.

Update: I picked up a brand new 6S and that completely fixed the problem. As I suspected, the insane voltage sag on from my plot above was because the battery was toast. Once the battery depleted from full I took off at 23 V once again and didn’t see anything close to the same behavior for the same current. Regardless I’ll be flying with BATT_FS_VOLTSRC = 1 from now on; no reason not to. Thanks for the responses.