Two ways to calibrate the ammeter on a flight controller:
Which would you say is better ?
Been using the 1st method for > 10yrs on countless craft. It’s fine for all useful purposes.
Option 1. It will give you a good sense of what’s going on without the safety issues that option 2 can present.
I don’t see why you’d account for the “charge loss”. Just disable balancing and there should be none.
Charging generates heat so there is a loss. If you charge 1000mAh into a cell, only 900-950mAh is added to the cell. And when the cell is used there’s another loss again.
I think you’re overestimating the impact of inefficiencies, potentially significantly, especially with lithium chemistry cells.
I’ve gotten impressively repeatable and dependable results by averaging a few trials using option 1 without doing any guesswork adjustment to account for inefficiencies involved.
I don’t think either method is better, as both can introduce inaccuracies (mitigated to some extent by using good equipment), but option 1 is generally safer and requires no test setup.
I’ve been using method 1, check what it took to charge the battery, and check that matches with the flight reading.
The method works very well until I change battery size or flight style. I was wondering if the issue is that method 1 calculates the factor very accurately, but doesnt look at offset at all. Hence why I considered a bench test.
Which setup do you use? Sounds like you having non linear current measurement issues.
Maybe you need to switch to a better current sensor. I’am quite happy with these INA2xx based “digital” current sensors as they are a hell more accurate than the analog ones.
Both methods are adjusting the same parameters, so if something impacts one it will equally impact the other.
I use a Matek H743 Slim. Maybe it’s too small to be accurate.
@allister, they dont quite use the same method, as only the bench test calculates offset. Flight test only calculates error factor.
Get this one:
https://www.mateksys.com/?portfolio=i2c-ina-bm
And don’t waste you time with the onboard current sensor if you really need accuracy.
(There is also a dronecan variant of this)
Okay, I see where you’re going. I was thinking this process Power Monitor/Module Configuration in Mission Planner — Copter documentation where the video only shows adjusting the BATT_AMP_PRVLT, but now I see you’re referring to Analog Current Monitor Calibration — Copter documentation . Makes sense. My bad.
Personally, on all the shunt based amp-meters I’ve used recently I’ve never needed to adjust the offset. I tried a few times in the “old” days, but in the end I ended up back at the default value (or close to it) for the board. Just using the flight method has given me acceptable results that I haven’t needed to mess with the offset. YMMV.
It seems you’ve convinced yourself that option 2 is better, regardless of any expert advice presented here. What’s stopping you from trying it?
If the answer to that is a need to purchase test equipment (an ammeter accurate enough to compete with option 1), I’d suggest your money is better spent on a board with a better current sensor, as already suggested.
Otherwise, I fear we are all just wasting words futilely arguing on the internet with someone whose mind has already been made up.
Heat is generated, agreed, but that’s due to voltage drop due to IR.
Charge is conserved, so there cannot be any offset between charge and discharge Ah (aside from the battery wear)
I understand offset as the measurement error when the current should be 0? This should be pretty small so practically, unless you spend a long time on the bench just powering the FC and the peripherals (such that the total current is less than the minimum current draw that the analog sensor is reliably able to read), then it shouldn’t matter as “Option 1” should average the error out after 2 or 3 attempts.
The effect depends on vehicle and flying characteristics. On a copter which hovers at 40Amps this doesn’t make a difference but if you have a glider for example, that flies a very long time with just some FC and Servo power consumption and has some short high current use to gain altitude this might be a problem if you want to have an exact measurement.
But anyway. Its very hard to get the analog inputs on the STM32 MCUs accurate. I’ve wasted some time with this and wouldn’t suggest others to do the same as there are plenty of better options.
What he is referring to is actually correct though. I think a small fraction of the charge pumped from the charger is used up in internal chemical process inefficiencies which I think also generates a bit of heat.
But yes, practically this doesn’t matter. @RCX749 just try option 1 and see the results
Okay that makes sense, but in this use case, you shouldn’t be using an analog in the first place.
Sounds like bench test won’t do any better then and the issue is down to hardware inaccuracies.
I’ll give the Matek external sensor a go, it won’t fit on my small quad, but will fit on my large one where its needed more.
Very useful thread, thank you.
You might be able to find a small one that fits.