SmartRTH setting: mystery solved

[DanMan32]

So I came to a point in my flight where my battery was getting low to not be worth flying very far, but not low enough in my opinion to quit or trigger RTH.
So I turned off SmartRTH where it warned me of the potential dangers. Sure enough, it never triggered RTH when battery level was in the teens. I don't recall even seeing the predicted point of no return.

 

[alokbhargava]

I m not sure if it's worth taking the risk. No doubt you will get few more minutes of controlled flying but it's too risky to fly with low juice on the battery. Good luck.

 

[quaddamage]

you will get few more minutes of controlled flying but it's too risky to fly with low juice on the battery.

Agreed.
I'm doing it only if there is no way to charge the battery without expensive trips.

Also, the more you discharge the battery, the less cycles it will fly. One discharge to 10% counts to its lifespan as 3 discharges to 30%.
(several studies on this are online)

 

[Pharm]

...(several studies on this are online)

Can you link to any of those? I don't disbelieve you, I'd just like to read them.

 

[quaddamage]

Can you link to any of those? I don't disbelieve you, I'd just like to read them.

Yeah, I knew it will go this way.

Here you go:
https://www.google.pl/search?q=battery+discharge+lifespan

Example direct link:
http://www.power-thru.com/documents/The Truth About Batteries - POWERTHRU White Paper.pdf

And more vague but on video:

Finally, note that Li-Po and Li-Ion have very similar properties, besides Li-Po releasing hydrogen gas when on prolonged charge above 80%.

 

[Pharm]

Yeah, I knew it will go this way.

Here you go:
https://www.google.pl/search?q=battery+discharge+lifespan

Example direct link:
http://www.power-thru.com/documents/The Truth About Batteries - POWERTHRU White Paper.pdf

And more vague but on video:

Finally, note that Li-Po and Li-Ion have very similar properties, besides Li-Po releasing hydrogen gas when on prolonged charge above 80%.

Thank you!

 

[Oso]

So I came to a point in my flight where my battery was getting low to not be worth flying very far, but not low enough in my opinion to quit or trigger RTH.
So I turned off SmartRTH where it warned me of the potential dangers. Sure enough, it never triggered RTH when battery level was in the teens. I don't recall even seeing the predicted point of no return.

I'm a bit curious. What was the mystery?

 

[DanMan32]

Many have argued what "smart RTH" meant in the documentation and exactly what that setting did. The documentation is inconsistent in the use of the term. Some say it meant RTH in general. Others say it is the manual engagement by use of the button on the RTH. I hardly see how having to manually engage RTH as "smart" when it can also engage automatically when signal is lost and when battery is at a point calculated to be the point of no return.
So I thought I'd try it in a controlled, safe setting in a ball field.

 

[msinger]

Many have argued what "smart RTH" meant in the documentation and exactly what that setting did.

The "smart RTH" that you're referring to is called "Low Battery RTH" in the Phantom manual.

 

[DanMan32]

Now about not being a good idea getting down to 10%. A percentage is relative and you have to ask 'relative to what?'
I agree that one should not go down to 3.2v very often and definitely avoid 3.0v. However DJI's scale is conservative, where 0% is closer to 3.4v so ending near 10% should be just fine. Toyota did the same thing with their Prius batteries (though those were NiMH) where there was an unusable margin of 10% high and low of battery's absolute capacity and even then the electronics avoided 20% range on either side during cruising Operations.

Anyhow, if you are flying distance, best not to ignore call for RTH on low battery or else she might not make it home for a safe landing. That point will vary based on how far it is when battery is getting low. If you are within 100 ft, RTH might not be called for until under 20%. But if you are a mile out, RTH might be called on as high as 40%.
The low battery warning level is just that. It does not affect RTH at all. Actually no action is taken other than letting you know battery has reached that threshold.

 

[DanMan32]

Which makes my point about the manual being inconsistent.
Manual calls the feature of intelligently returning home on low battery "Low Battery RTH", but the setting that turns this function off is "Smart RTH"
If you ask me, the low battery RTH is the smartest trigger of RTH.

 

[Papa Tom]

Sorry if this is off subject but this discussion has either confused or enlightened me.

The "Battery Safety Guidelines" say drain to 8% every 20 cycles which I found unusual but did because it was a 'smart" battery. I have never taken any of my other lipos this low.

So which is it. By the manual or usual lipo wisdom.

Thanks

 

[quaddamage]

The "Battery Safety Guidelines" say drain to 8% every 20 cycles which I found unusual but did because it was a 'smart" battery. I have never taken any of my other lipos this low.

Li-Po cells don't like getting so low, and it's good you're not doing it for other batteries. What needs complete depletion is the "smart" part - otherwise it may incorrectly compute remaining energy.

So - complete discharge is only needed to calibrate counters in the internal ICs.

 

[crash1sttime]

I've done several long distance flights and had the so called smart RTH kick in, on the first 2 occasions I let it RTH and still had over 40% battery left at landing. However I have seen YouTube videos where the SRTH has been overriden and the quad has landed some distance away from the home point, even though they flew only a couple of minutes past the point.

To be fair DJI included the option as a fail safe measure for fliers who aren't sure of their quads capabilities.

In my humble opinion it should be left active, wether you then choose to override is up to you, but as many say it's better to be safe than sorry.

 

[DanMan32]

Not to mention if you lose inbound signal but don't realize you did not lose outbound signal, you'll get it back on low battery.
Otherwise it will land where ever it's been hovering.

 

[DanMan32]

I get starting on its way home at 40% if it is really far out, but landing at home at 40%? Something is wrong there.

 

[yorlik]

Agreed.
I'm doing it only if there is no way to charge the battery without expensive trips.

Also, the more you discharge the battery, the less cycles it will fly. One discharge to 10% counts to its lifespan as 3 discharges to 30%.
(several studies on this are online)

Not quite, but your concept is right. 1 run down to 10% uses 90% of 1 'cycle,' 3 runs down to 30% count as 70%*3=210% so a tad over 2 cycles.

 

[yorlik]

Yeah, I knew it will go this way.

Here you go:
https://www.google.pl/search?q=battery+discharge+lifespan

Example direct link:
http://www.power-thru.com/documents/The Truth About Batteries - POWERTHRU White Paper.pdf

And more vague but on video:

Finally, note that Li-Po and Li-Ion have very similar properties, besides Li-Po releasing hydrogen gas when on prolonged charge above 80%.

Well, one link is a google search with no data, other is data on lead acid battery - totally different animal. OK on not leaving lipos above about 60% for long periods, but that has nothing to do with discharge cycle info discussed above.

 

[yorlik]

Now about not being a good idea getting down to 10%. A percentage is relative and you have to ask 'relative to what?'
I agree that one should not go down to 3.2v very often and definitely avoid 3.0v. However DJI's scale is conservative, where 0% is closer to 3.4v so ending near 10% should be just fine. .

Mostly right on, but for flying down to 10% "should be just fine." IIRC 0% may even be closer to 3.5v, but the fact remains that if you fly down to 10% often, and compare your life to mine flying down to 50% often, you will find your battery life is less than 1/2 of mine.

 

[yorlik]

Sorry if this is off subject but this discussion has either confused or enlightened me.

The "Battery Safety Guidelines" say drain to 8% every 20 cycles which I found unusual but did because it was a 'smart" battery. I have never taken any of my other lipos this low.

So which is it. By the manual or usual lipo wisdom.

Thanks

I believe newer models of DJI no longer recommend that. And if you do that you defeat the purpose anyway as it is incorrect. The correct wording is "drain below 8% every 20 cycles. When the old (not sure of newer ones as I do not have one) hit 8%, the internal coulomb counter BEGAN calibration. It did not FINISH until after dropping BELOW 8%. So taking your batt to 8% and stopping actually is most likely detrimental to life; taking to BELOW 8% - just 1 digit - 7% - DOES re-calibrate the charge counter to the cells.