This is the thing I’m worried about from a flying perspective. Obviously, I want the BMS to keep me from hurting anything, but it would be a shame to have one parasitic cell dragging down my flight time when there’s still juice left in every other cell group.
Sure it would be nice to maintain level flight, but even if it helps you hobble to a better LZ I’d prefer that to just shutting off altogether. Interesting to know that that’s what it’s supposed to do; I wouldn’t have known otherwise! This suggests that there is a genuine issue with that cell group. Good suggestion on flying with the app. I’m going to give that a try on my next flight and see what I find!
Thanks @GliderPilot and @evan for your guys’ expertise and help! I thought that coming to the world of electric would make for a simpler time, but I’m finding out that it’s a lot more complicated (or at least different) than just rebuilding a carb or replacing parts on a 2 stroke!
Basically correct, I think. It would be good to have a benchmark from a known good pack to compare against - you won’t get exactly the rated capacity at higher C rates but it shouldn’t be far off.
You do have a kWh reading but that includes the voltage which is heavily affected by resistance, load, temperature, etc, so it will give a less clear comparison.
On closer reading of the info you’ve posted so far, I’d probably agree with Gliderpilot that this isn’t a balance problem, but first thing to do is establish if you’re really lacking capacity or not, and depending on that we could try to determine the cause.
My guess is that you actually are getting close to full usable capacity.
Another clue will be, if you can watch that cell voltage throughout the run - if it’s consistently sagging below the rest that would indicate a problem too.
In the end, the electric system definitely can be fly and forget, and I’m sure it will get there. I think at the moment it’s still slightly early days so it’s a bit more hands on, and more like the R/C flying experience.
before i told you about my point of view regarding error analysis. now i would like to say something about batteries in general. batteries in this performance class have a relatively high value and also danger potential in general. which of course can be very well mastered. but what surprises me in comparison to european batteries from companies is:
during the performance check that a new battery usually has to pass under full load and thermo check before delivery, a higher cell drift would be noticed immediately.
a bms programmed in this performance class would have to immediately go to fail with such a high drift of a cell group and prohibit a restart. only the manufacturer can “unlock” the battery again. or a service company that checks the battery.
since this is of course an opensource project where customers are allowed to control the bms themselves, it will probably not be possible for the manufacturer to give a warranty because he does not know what the customer is doing with his product. so of course i understand that the manufacturer does not immediately write with such a posting as here: please send immediately, we must analyze the battery for safety reasons and warranty immediately in the factory.
but i think that openppg will surely exchange the battery if indeed a mistake happened during the manufacturing process. this is always possible, no matter which manufacturer.
Yesterday I did some data collection. As suggested, the bad cell seems to be what’s driving the cutout, not the total pack voltage. As soon as the lowest cell hit 2.5V, the system completely shut down. I’m guessing that this bad cell cost me 5-10 minutes of flight time, but not really sure of all the factors at play for that. It was also suggested to measure Ah instead whatever the percentage scale measures or kWh displayed to determine what capacity I’m actually getting out of the battery. Taking an amperage measure every minute, I came up with a consumption of 35 Ah. I have no idea how close to 40 Ah I should actually expect to get, but from the first chart it looks like I’m missing at least a little bit of juice due to the low cell.
Methodology:
My flight profile was a reduced power climb to 500’, then set the cruise control to maintain level flight until battery cutout. I screen recorded the Smart BMS app for the duration of the flight and then manually recorded that data post-flight every minute from the screen recorded video.
Known data anomalies:
The zero time data point is with no load, immediately after plugging in the battery before flight.
I didn’t realize I wasn’t actually screen recording until in flight, so the first real data point starts at 4 minutes.
At the 53 minute mark, a battery fault message pops up on the Smart BMS app, moving voltage data for half of the cells out of view, so no data for those cells for the last 2 minutes.
Great work Scott. Glad you’re getting a decent flight time first of all!
Seems that the low cell group has the lowest voltage for the full duration, so it does look like it’s got a lightly higher resistance as well as lower capacity.
However. Supposing you shut off at 3.0V which would have been a bit easier on the cell, you can see the other cells aren’t far behind at all - really only a couple of minutes in it, or like 3% of duration.
It might be more apparent because due to that difference you’re getting a BMS lock out instead of an ESC power reduction or restartable cut-off, which would feel less dramatic.
I was skeptical, so I extrapolated the data from the next 5 lowest cells and you’re 100% right. My “5-10 minutes” is actually closer to 2-3 minutes. I still want those 2-3 minutes, but it’s not nearly as big of a loss as I thought due to how rapid the drop-off is.
So my next question is: is there anything I can do to get the ESC power reduction before the BMS lockout in this case? Some setting to tweak?
Also, now that I have a good idea what’s going on at the end, I don’t really feel the need to run all the way to cutout. However, I still need to know what the limit is to maximize flight time (which was the point of running to cutout on these early flights in the first place). So suppose I want to end the flight by the time the low cell hits 3V. Could I monitor the BMS app at the end of the flight and see what controller voltage corresponds to the low cell reaching 3V? Does that correlate? Would the value wander over time? It would be great to be able to see the parameter closest to causing cutout (in this case the lowest cell voltage) on the controller; as it is now I feel like I’m kind of just guessing.
Yes you can definitely do that. You can also see from your chart that when the total reaches (3.3V * 23 = 75V) you only have a few minutes left. At that point it would be great to get a warning, and you can look at the voltage on the handle (reported by the ESC) at least.
Will that change over time? Yes, somewhat, and it will vary with temperature and of course load. And if your weakest cell gets worse, it could still catch you out.
My intention would be to get an individual cell voltage based “elegant shut down” - i.e. a vibration or sound warning followed by a gradual cut of motor power. But also not need to hit that limit, instead plan to be back in your LZ based on an accurate % readout.
As I mention in that other thread that Allex linked, that is how other EVs work, so it’s a reasonable aim.
