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What could cause sudden voltage drop?

#1
While at hover after initial takeoff on a full battery, what types of situations could cause a sudden ~1 second voltage drop? Enough to kill the motors and set off the battery alarm for only a moment (but was long enough to crash it).
I know that batteries can fade suddenly. Are Turnigy nano-techs bad about this?
Are there any other possible causes?
 

Craftydan

Hostage Taker of Quads
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#3
under C-rated batteries can do this. what size battery (mah and C) and what's the motor's current draw?
 

Craftydan

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#6
C rating looks fine.

Is your wiring harness big enough to supply 200A?

(and *what* are you flying with this Haus of a power system?!?)
 

xuzme720

Dedicated foam bender
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#7
C rating looks fine.

Is your wiring harness big enough to supply 200A?

(and *what* are you flying with this Haus of a power system?!?)
It's sounds like the Bigfoot of multi's! If the packs are still charging, he just throws a small genny and a rectifier on board to fly longer!
 

xuzme720

Dedicated foam bender
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#12
200amps at maximum throttle.
Its a 1meter hexacopter carrying a canon 5d so its a pretty hefty bird
Yeah, your batts should be well inside the safe "C" rating. Are they very old or have many cycles on them? Or, have they ever been discharged very much below 3v per cell or damaged in any way?? You are pulling a pretty hefty amperage, so some of the common issues might not really apply...
 
#14
4100w!?!?!?! Good lawrd! This is beyond me but will say, with that much juice the wire used becomes critical. What gage you running from batteries to ESC(s)? I had a few brownout situations and they all were addressed with added capacitors along battery leads to tame the spikes. What kind of connectors are you using? they too have amp ratings and cant just assume one will work as well as the other.
 

vk2dxn

Senior Member
#15
Underrated C cells, bad cell in a battery and conductor size (power cables).
As an electrician I get a real giggle out of the RC world how they can have huge current draw on shoe string cables. As a comparison a house point of attachment rated for 100amp has a conductor size of 16mm minimum depending on the wiring installation. I see many copters including the first tri I built pulling in excess of 100amp on supply cables that are smaller than 2.5mm building wire (rated for 18amp). I'm surprised that the wire insulation has not melted off a few copters :)
 
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Craftydan

Hostage Taker of Quads
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#16
Underrated C cells, bad cell in a battery and conductor size (power cables).
As an electrician I get a real giggle out of the RC world how they can have huge current draw on shoe string cables. As a comparison a house point of attachment rated for 100amp has a conductor size of 16mm minimum depending on the wiring installation. I see many copters including the first tri I built pulling in excess of 100amp on supply cables that are smaller than 2.5mm building wire (rated for 18amp). I'm surprised that the wire insulation has not melted off a few copters :)
Agreed! And if we didn't keep the flight times down, we'd have more trouble.

Chatting with a guy last weekend who owned a dual ducted fan ME-202 -- he said the plane pulled anywhere between 80-120 A, and was grounded until it got a badly needed rewire. The stock plane came with deans(the nice ones are rated to 70A burst), and though he didn't mention the AWG run for the wires, he said they too were badly undersized.

The point it dawned on him when he opened the canopy after a long flight and saw the foam had melted away from the power lines and the connector was too hot to touch. quick IR reading showed he was about to reflow the solder on the connectors. He felt lucky -- If they had, at best he'd have a ball of foam to pick up off the ground . . . at worst a flying lipo fire.

At 40-60A per motor, each separate harness may have been fine for short flight times, but the stock design had one pack and a really weak link, right at the Y.
 
#17
Aside from pulling specs for a given conductor ga or connector a cheap ir sensor will help track down choke points after landing.... They will be the "glowy" red bits that smell like burnt rubber.
 
#18
Was not even the battery. The real culprit was found to be the weak PMU of the Wookong. Figures. Hopefully the client will take my advice and add a secondary switching BEC just in case.
 

vk2dxn

Senior Member
#19
Was not even the battery. The real culprit was found to be the weak PMU of the Wookong. Figures. Hopefully the client will take my advice and add a secondary switching BEC just in case.
Not really sure what a pmu or wookong is but why suggest a secondary BEC?
Isn't it a hex?
Does the esc's have bec's to power the RX & maybe gimbal or whatever for the camera?.
To trigger a low voltage alarm suggests that it may be major voltage drop due to either bad cell etc.
Can you describe where the pick up is in the circuit for the low voltage alarm, or if possabe post some pics?
I will try to help you the best I can
 
#20
its a heavy lift. the escs internal BECs would fail if we were to use them for our application. the secondary bec is to make sure we do not have another PMU failure and the flight board/rx shut off.