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Mavic 2 crasched

Lars_Tornquist

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My Mavic 2:s battery capacity dropped from 60% to 0% and the drone started landing over Water. I managed to switch to Map View on the Phone to quickly see the fastest way back to me. During the way back it's altitude decreesed and it ended with a crasch into a rock which you can se here:
I have the DJI Care Refresh which made DJI to send me a new drone, but they never explained what that had gone wrong. I am curious to find out why the battery syddenly went from 60% to 0%.
 

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Keule

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I am curious to find out why the battery syddenly went from 60% to 0%
You took off with a non fully charged battery. You thought the battery was at 62%, but in reality, it was at a voltage level not correctly reflecting the battery percentage.

A normal battery cell at ~ 60% should be around 3.8500V / cell.
Yours was roughly at 3.6000V / cell.

That led after a few seconds critical battery level was reached and the aircraft started landing.

NEVER take off without a fully charged battery.
NEVER trust the battery percent (%) display, always use the battery voltage levels!

Here an overview of your battery performance:

2020-04-02_15-27-56.png
 

sar104

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It looks like you started with a partially discharged battery. If the battery has been self-discharging then the initial estimate of remaining capacity is very unreliable. As soon as significant current is drawn the cell voltages collapse, triggering the autoland and the recalculated battery %.

battery.png

As for the crash - you got it back to dry land just fine, but flew it into the rock wall under full control:

Pitch_1.png
 

Photogad

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NEVER trust the battery percent (%) display, always use the battery voltage levels!
I am not arguing with your assessment but shouldn't DJI base their battery % indicator on voltage anyway (how else would it measure?), and furthermore what's the point in having a battery % indicator if it's not reliable. If it's really that unreliable where you should never use it, that seems like a very fatal flaw that should be addressed by DJI or at least always lead to a 100% warranty success rate.
 

sar104

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I am not arguing with your assessment but shouldn't DJI base their battery % indicator on voltage anyway (how else would it measure?), and furthermore what's the point in having a battery % indicator if it's not reliable. If it's really that unreliable where you should never use it, that seems like a very fatal flaw that should be addressed by DJI or at least always lead to a 100% warranty success rate.
It is based on voltage, but voltage is not a definitive indicator of remaining capacity.
 

swededrone

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It looks like you started with a partially discharged battery. If the battery has been self-discharging then the initial estimate of remaining capacity is very unreliable. As soon as significant current is drawn the cell voltages collapse, triggering the autoland and the recalculated battery %.

View attachment 97767

As for the crash - you got it back to dry land just fine, but flew it into the rock wall under full control:

View attachment 97768
Again @sar104 a flawless analysis. Impressive!
 

swededrone

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You took off with a non fully charged battery. You thought the battery was at 62%, but in reality, it was at a voltage level not correctly reflecting the battery percentage.

A normal battery cell at ~ 60% should be around 3.8500V / cell.
Yours was roughly at 3.6000V / cell.

That led after a few seconds critical battery level was reached and the aircraft started landing.

NEVER take off without a fully charged battery.
NEVER trust the battery percent (%) display, always use the battery voltage levels!

Here an overview of your battery performance:

View attachment 97763
Very good analysis!
 

DanMan32

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Basing a % strictly on voltage only works if remaining capacity is linear to voltage, but it isn't. Not only is it not linear, but it's not fixed either. Age and temps are among the other variables that determines remaining capacity. As the battery ages, the bottom out voltage goes higher as the cell internal resistance increases. More power demand will suddenly drop the voltage when the internal resistance increases.

There's also the thought that if the battery is in the middle of self-discharge mode, the discharge circuit is still connected during use if not placed on charge first to turn it off. This will distort the calculations that the BMS is attempting to make.
 

Lars_Tornquist

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You

You took off with a non fully charged battery. You thought the battery was at 62%, but in reality, it was at a voltage level not correctly reflecting the battery percentage.

A normal battery cell at ~ 60% should be around 3.8500V / cell.
Yours was roughly at 3.6000V / cell.

That led after a few seconds critical battery level was reached and the aircraft started landing.

NEVER take off without a fully charged battery.
NEVER trust the battery percent (%) display, always use the battery voltage levels!

Here an overview of your battery performance:

View attachment 97763
Thanks so much for your analysis!
Now I know I should never start with a partially discharged battery.
I had map mode enabled to be able to quickly steer the drone back to me, standing on a 25 m high ledge. This meant that I did not see that the drone had lost so much height.
Thanks again!
 
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Lars_Tornquist

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It looks like you started with a partially discharged battery. If the battery has been self-discharging then the initial estimate of remaining capacity is very unreliable. As soon as significant current is drawn the cell voltages collapse, triggering the autoland and the recalculated battery %.

View attachment 97767

As for the crash - you got it back to dry land just fine, but flew it into the rock wall under full control:

View attachment 97768
Thanks so much for your analysis!
Now I know I should never start with a partially discharged battery.
I had map mode enabled to be able to quickly steer the drone back to me, standing on a 25 m high ledge. This meant that I did not see that the drone had lost so much height.
Thanks again!
 
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Lars_Tornquist

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Basing a % strictly on voltage only works if remaining capacity is linear to voltage, but it isn't. Not only is it not linear, but it's not fixed either. Age and temps are among the other variables that determines remaining capacity. As the battery ages, the bottom out voltage goes higher as the cell internal resistance increases. More power demand will suddenly drop the voltage when the internal resistance increases.

There's also the thought that if the battery is in the middle of self-discharge mode, the discharge circuit is still connected during use if not placed on charge first to turn it off. This will distort the calculations that the BMS is attempting to make.
Thank you for your answer! Now I have learnt that I always shall use fully charged batteries. Normally I do.
 
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LukiDrone

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It is based on voltage, but voltage is not a definitive indicator of remaining capacity.
Fully Agree, apart to play with drone I'm sys. eng. were ups systems are used. @sar104 explain it very well. The voltage is the common method to calculate the capacity, there are systems to make the capacity measure more accure but normally are not found in consumer electronics products.
 

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Before you fly out over water you can do a voltage test by punching the elevator stick real quick and watch the main voltage. If it drops significantly than that battery is on its last leg. Not to mention you’re going to want to always start with a fresh battery which has been mentioned above.

Glad you have refresh!
 

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And the same would be the same if we were driving an electric car? I suppose in the case of a Tesla or similar, we would not have to start a trip with the battery fully charged. Technology is supposed to be much better in cars.

Anyway, we don't always fly a battery. That is, we assemble the fully charged battery and take off, fly 5+ minutes, land, and then find another place to fly. Could that battery be used then?

The correct thing would be to take off the battery, if it returns to the ground, better not to use it again until it has been fully recharged?
 

JORGE SM

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And something related to batteries. I present a case: fully charged battery equals 4.35.

We take off and it is consumed and the estimated time we have left to continue piloting and the amps to spend appear on the screen. There comes a moment that reaches 3.70 and in% appears 50% and + - 10 minutes of flight. This is correct ?? 3.70 is not the normal charge for a battery in order to recharge it. We already know that you can hurry a little more, but I do not understand that it marks 50%
 

sar104

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And the same would be the same if we were driving an electric car? I suppose in the case of a Tesla or similar, we would not have to start a trip with the battery fully charged. Technology is supposed to be much better in cars.

Anyway, we don't always fly a battery. That is, we assemble the fully charged battery and take off, fly 5+ minutes, land, and then find another place to fly. Could that battery be used then?

The correct thing would be to take off the battery, if it returns to the ground, better not to use it again until it has been fully recharged?
Tesla batteries don't enter self-discharge cycles as far as I'm aware, and that's the most common problem here. During self-discharge the cells become unbalanced, and the overall voltage ceases to be a good indication of available capacity. Making multiple flights on one battery is usually fine provided that the battery hasn't started self-discharge.
 

JORGE SM

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Tesla batteries don't enter self-discharge cycles as far as I'm aware, and that's the most common problem here. During self-discharge the cells become unbalanced, and the overall voltage ceases to be a good indication of available capacity. Making multiple flights on one battery is usually fine provided that the battery hasn't started self-discharge.

Right, I understand that. Maybe I missed something in the conversation. He did not know that it happened to his partner that he had the battery in self-discharge and without having charged it again, he took off.

It is also correct that when we come to pilot and the batteries come exhausted, better after they cool down charge them to 50-60% and leave them like this until the new full charge to go out to pilot. Others what they do is load them at 100% and then save them so that after the configured days, it self-downloads. But with this method what they get is 'to waste battery life'.
 

DanMan32

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Tesla batteries don't enter self-discharge cycles as far as I'm aware, and that's the most common problem here. During self-discharge the cells become unbalanced, and the overall voltage ceases to be a good indication of available capacity. Making multiple flights on one battery is usually fine provided that the battery hasn't started self-discharge.
But self discharge simply puts a single resistor across the main battery output. That shouldn't unbalance the cells any more than flying with them.
 

sar104

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But self discharge simply puts a single resistor across the main battery output. That shouldn't unbalance the cells any more than flying with them.
Are you sure about that? I was under the impression that the self-discharge is via the individual cells bleed resistors that are used for balancing during charging. Either way, it's pretty clear from battery data that the cells become unbalanced during self-discharge.
 

gnirtS

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Are you sure about that? I was under the impression that the self-discharge is via the individual cells bleed resistors that are used for balancing during charging. Either way, it's pretty clear from battery data that the cells become unbalanced during self-discharge.
Mavic 1 for sure is per cell and once that started to auto discharge cells became unbalanced almost immediately every time. You'd get cell warings, battery warnings, big drops in percentage all over the place.

Mavic 2 im unconvinced either way. I have powered up and flown a few times with discharging cells by accident (Admittedly only slightly discharged) but the cells have always looks a lot more balanced to me.
(I wish it would warn you when a battery has entered self discharge rather than just hoping you notice its warm assuming its still near full at the time)
 

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