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LiPo Voltage Cutoff.

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LAN_403

Patrick

Wed Feb 20 2013, 08:15PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ive had some, what i think are false low voltage cutoffs on my drone, and need some advice on batteries.

first:
-- Battery is 248g, 4S, 30C, 14.8 V, 2200mAh, LiPo.
-- has gone through 15 charge and 15 disharge cycles.
-- seems to charge reliably to 4.19 - 4.20 V per cell.
-- I have a professional charger.

Prior history: Ive set the ESC for "Auto LiPo cutoff" and at 3.2V, but if often cut out at 3.7 -3.8V under load.
And i changed the setting down to 3.0V cutoff, but it appears during "high-load flight (14 amps)" that its voltage never falls below 3.65- 3.69 volts, but still cuts off.

A comment: i think the Auto voltage feature using a User programable Vref for the comparator, but it may be error prone under rapid throtle commands of the multirotor circumstance. Next there is a fixed feature, that will cut off at 6, 9, 12 and so on volts, all multiples of 3.0v per cell, so for my case it would be 12V chosen. Perhaps that fixed-value refeerecne would be more reliable?

Problem: what voltage should i set the protection circuit for? i need many flights from this battery, so i dont want to deep discharge it below 3.0V per cell, but it apears im never even close to it.

And is the 3 v per cell deischarge an ok value? voltage/charge graphs seem to indicate so.

this isnt my brand, but the graphs are all similar. it appears one can go down to 3.1 or 3.0 volts safely, i beleive some have said the 2.5 to 3.0 range is reserved for long life and should never be entered.

Ash Small

Wed Feb 20 2013, 08:49PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Just guessing here, but could it be a 'resistance issue' (ie dirty connections, or something) in the voltage cutoff circuit?

If you are dropping a few tenths of a volt over a bad contact on full load it could explain your problem.

As I said, I'm only guessing here.

(I've a similar 'occasional' problem with my new android phone, I'm not sure if it has a temperature sensor on the battery though, which could also cause it to cut out.)

Patrick

Wed Feb 20 2013, 08:58PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash Small wrote ...

Just guessing here, but could it be a 'resistance issue' (ie dirty connections, or something) in the voltage cutoff circuit?

If you are dropping a few tenths of a volt over a bad contact on full load it could explain your problem.

As I said, I'm only guessing here.

(I've a similar 'occasional' problem with my new android phone, I'm not sure if it has a temperature sensor on the battery though, which could also cause it to cut out.)

thats a good question to raise, but i have 4 wires, each 14 awg, 2 black, 2 red, from the battery to the 2 ESCs. each pair of 14 Awg wires would source and sink 7 amps. the wires are 10 inches long.

we are talking about tenths of volts, so let me figure on that...

heres another graph:

this is not my battery size or brand, but the graphs are all similar. In the above graph look at the proportion of the graph, not the exact dimensions, and look at how much area (power) under the dashed lines im missing out on!

from the above graph, im thinking i should be able to go from the current flight time of 4.1 mins, to 6+ minutes.

Carbon_Rod

Thu Feb 21 2013, 08:32AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

@Patrick
LiPol do not necessarily correlate cell float voltage with remaining capacity.
Additionally, the discharge curve will unlikely appear that smooth with a variable intermittent load on the cells.
Many people use charge management chips with built in LiPol fuel-gauges, as they know many ESC will begin to give a warning-stutter before the cell voltage drops too low.

Gyroscopic stabilization is interesting, but is usually avoided as lagging control loops become difficult to predict. Note the stabilization bar on dual-blade helicopter rotors is often adjusting the blade pitch offset independently, and until a few years back these were very difficult to fly without one.

Perhaps a setup with thrust-vectoring flaps would have less mass, auto-center, and reduce demands of tandem torque cancellation balancing.

Patrick

Thu Feb 21 2013, 06:46PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Carbon-Rod, are you suggesting tiny flaps used for fore-aft translation, while the main gyro-props are used for rotational stability?

Carbon_Rod

Fri Feb 22 2013, 10:12AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

In general,
Wider diameter non-rigid plastic props will feel more stable with lag, but because the blades are fixed pitch they will track far worse with those "weights" and add additional problems.

The effects of counter-tilting your axial torque cancellation systems may create unintended gyroscopic torques.

Flaps are much easier to handle given they usually help stabilize prop wash too.

Ash Small

Fri Feb 22 2013, 11:07AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Carbon_Rod wrote ...

The effects of counter-tilting your axial torque cancellation systems may create unintended gyroscopic torques.

That's exactly what Big Bad and I were on about in the 'other thread'.

That should explain the tail stall problem. That's why I asked what manouvre you were attempting at the time of the tail stall.

If the torque reaction from the two props isn't balanced, it will tail stall, or the opposite.

It's not so much about inertial stability, but about torque reactions at 90 degrees.

EDIT: I posted this link once before:

Patrick

Fri Feb 22 2013, 06:18PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ok let me finish these mods...

ive doubled the for-aft rotation ability, so wee'l see if that changes the stall behavior.

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