Near crash after Warning:Propulsions output is limited

AlexSP · Mar 11, 2016

I still believe that the battery percentage isn't really flawed, but rather limited. It's different, and understanding that difference is important to make better use and judgement of the P3 battery status at a certain point and along the battery's lifetime. That's standard procedure with LiPOs in general, nothing really new here.

Voltage fluctuation isn't linear, but it can be useful to have some sort of linear display about "capacity drop" to give the pilot an instant, intuitive glimpse on overall remaining capacity. Thus, battery percentage. Based on voltage alone, I'd land my P3 every time voltage reaches 3.6V or thereabouts. But that depends on current draw at the moment, among other factors, so it isn't perfect in itself.

It's really hard to get a perfect calculation of percentage, and even more so on "real time battery remaining X flying time" which ultimately is what matters, for LiPOs. But compared with most other initiatives DJI pulled this off quite OK on the P3. Again, that's just my opinion. Either way, I believe it's best to look at the overall picture and keep an eye on our battery's behavior along the time. These things are pretty reliable but can get moody at times, so to speak, and conditions vary quite a lot too.

John Locke · Mar 11, 2016

AlexSP said:
I still believe that the battery percentage isn't really flawed, but rather limited..

If you use msinger's log viewer on all your flights and compare voltage vs percentage, you'll see flaws. Especially if you start a new flight with a partially charged battery (which isn't recommended, ever). Keep in mind, when you turn on the craft the motors aren't running, however your system is measuring the voltage with no load and making judgement about how much battery is left, as a starting point. Hassan's flight is an example. His percentage led him to believe he had 40% left in the battery, but it's clearly over 1 volt lower (T volts 13.481V) than my flight at 40% (Tvolts 14.618V), a flight that started with a full charge. This clearly shows a flaw in the algorithm of the battery percent gauge IMO. Yes, we should all know to NEVER start up the craft on a partially charged battery because of this flaw, but some pilots still do it, thinking logically, that if the percent bar shows 40 or 60% left in a battery it should be OK, but not realizing that 40 or 60% reading is inaccurate.

Try comparing your logs with msingers reader. It's pretty revealing, and it will quickly show how vulnerable the uninformed can be. You'll start ignoring the percent bar and use ONLY voltage level as you're indicator when to start heading home, which should be around 3.6V (lowest cell reading) IMO. When the battery voltage font color goes from orange to RED, you need to land immediately. I don't remember what voltage level triggers red font, but I think it's around 3.3V, I'm not sure.

AlexSP · Mar 11, 2016

I agree it´s not accurate John, but that doesn´t mean it´s flawed. Not if you use as instructed, it should be noted! Battery percentage isn´t even meant for takeoff decisions, as DJI explicitly recommends 100% for doing that. That said, I insist (my opinion, based on my experience with Phantoms) percentage is still useful and decently reliable for "normal" flight if you´re following the instructions (again, taking off at 100%), AND you need to take decisions during flight based on provided battery information (like RTH and such).

Pilots taking off at less than 100% are going against DJI´s recommendations to start with. So it doesn´t really matter percentage or voltage, as not only it means making poor judgement decisions, based on inaccurate information, but also going against specific manufacturer´s recommendations. That´s why I don´t call percentage as flawed.

I wouldn´t take off on a 40% battery with either 13.48V or 14.61V as both are too close or bellow to the 3.6V limit per cell (UL) and takeoff puts high loads on batt. AC can fly alright but there´s still the chance of damaging cells if you go at or bellow 3.3V, though it´s OK to get ~3.6V if you´re flying under normal load because that means usually more than 3.7V after landing, etc. and that means SAFE. Of course that implies some risk: if one of the cells are slightly out of balance that could bring problems, even during normal flight, depending on which FW one´s P3 is. So it´s better play on the safe side, unless you really need to fly as in RTH or something.

On the other hand, if you know your batteries, if you have some knowledge on LiPOs and RC aircrafts then you´re able to make good judgement on battery capacity and take measured risks like multiple flights on a single charge, etc. I do multiple flights out of a single charge all the time, but only on the same day. Not a single issue during almost 300 flights, and all my 3 batteries are fine. I have a background in RC and LiPO, I screwed up a lot to learn a little and I still do, but I know the level of risk I´m taking most of the time. If you´re flying, can´t eliminate risks completely, only decrease and/or manage.

To be honest, I´m not even sure the P3 voltage is accurate, or how accurate. I´m used to measure voltage of my LiPOs right upon landing directly from a plug-in voltmeter, but I never did this with my P3 (and I confess i find it relieving LOL). I also time my flights based... well, on TIME, rather than percentage or voltage, since most RC ACs don´t have DJI real-time telemetry. Can´t go much wrong with that really. DJI GO and most other apps have a timer, so it´s one more info to base flights on, and learn about batteries.

But that comes from my RC and I am pretty sure most of Phantom pilots aren´t even remotely familiar with such procedures. Nor should they, P3s are aimed at consumers, beginners really. But they still need some info on battery status, untill they learn better, and that comes instantly and easily from percentage and voltage (provided) and time (provided and estimated). As long as they follow the instructions, they should be OK most if not all the times.

Sorry for the long post and thanks for your insights!

RunningWithScissors · Mar 24, 2018

msinger said:
The battery was nearly depleted when you took off.

I get it that you're new to this but, as msinger pointed out, taking off with a nearly depleted battery (41%) is an accident just waiting to happen - literally. At 41%, I'd better be approaching home or well on my way there. Hypothetically speaking, it's real easy to get caught up in the moment, get over-confident, and start a second flight with a battery at 50% or less, and then white-knuckle it all the way home when low battery starts sounding. Hypothetically speaking, of course.

RunningWithScissors · Mar 25, 2018

After reading the op's post again, I kind of read between the lines and picked up on something that could explain why he attempted three flights on a nearly depleted battery in the first place. Being that he's new to flying a Phantom (only been at it for ~5 months), he may have only one battery. If that's the case, I'd strongly suggest buying a second (and third) battery for a number of very sound reasons. Besides the drone itself, multiple batteries are (IMHO) the single best investment you can make in this sport. Some (including myself) would call it a necessity.

Neon Euc · Mar 26, 2018

To be honest I had battery propulsions quite regularly on my flights.... Even if I have over 90% battery. I just assumed it was the battery drawing a lot of power going against the wind so it slowed it down for me to stop any risk of burning out the battery or motors. Thinking about it now.... Yeah... It does happen on windy days lol