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Betaflight F3 Inaccurate Current Sensor / mAh Readout
🕑 Added 2017-03-27 03:56:18 +0000 UTC Some people are reporting that their Betaflight F3 boards are reading really low mAh and amps. I did some testing tonight and found that my older BFF3 that's in the Coyote seems to read close to correct with the default current scale of 400. But two boards that just arrived this week read way off. I calibrated them and found that a current scale of 260 for one board and 275 for the other gave correct readings. No change to the default offset of zero was needed.If your BFF3 is reading way off, try setting scale to between 260 and 275. Checking that the charger puts back in approximately as many mAh as the OSD reports came out is a rough (but not perfect!) way of confirming that the current sensor is correct. For increased accuracy, do a non-balance charge or calibrated using a battery that is already very well balanced and doesn't need a balance charge.
Comments
Incredible. I logged onto patron specifically to get Joshua's opinion on the validity of what I though was my idea about turning props upside down to test current for my BF F3 FC current sensor,
265 is working pretty well for me - showing about 1100maH drawn when I am landing at 13.8-14V (1300mAH Tattu)
Joshua Bardwell
I have a video I'm going to make about that but the short version is: Method 1. Flip your props upside down then swap them left to right. Now your copter pushes down instead of up. Strap it down anyway just for good measure and then use the Motors tab to spin the props to achieve a given amp draw. Use a clamp meter or other means of measuring current and compare to BF OSD. Adjust Offset and Scale (this is a bit complicated so wait for the video). Method 2: With a naked board, solder an XT60 to the ESC pad closest to the vBat pad. Connect the ESC pad to a load such as a bank of halogen lights or a big bucket of water with a steel fence wire in it (like I use for discharging batteries). Pull load through the board and compare measured to OSD current. The advantage of Method 2 is no terrifying spinning blades of death. The disadvantage is that you don't have an 80 amp load.
dumb question -- how do you calibrate it?
Joshua Bardwell
My testing suggests that the base of the curve is roughly correct, so if you calibrate the scale at some large-ish value, it will probably be correct throughout most of the range. When I calibrated 70 amps correctly, it was very close to right all the way down to 5 amps.
Matthew Miller (Scrub Zero)
Mine is definitely way off. Thanks for the tip!
I tested my new bff3 on the bench using a current meter from hobby king as well as a true rms current meter from home depot, and as far as amp pull goes the hobby long and home depot meters were not in agreement but close. At 1800 throttle one showed 37.6 while the other 34.4. So I set mine at 255 to get something around 36a at 1800 throttle. Also, I hear people saying that no matter the setting the amp pull and mAh used are not in agreement in betaflight. Haven't fly tested that roumor so I can confirm though.
Joshua Bardwell
There is some serious nonlinearity in the curve at very low amps. Especially at idle. I would say, if you want to calibrate, calibrate to be correct at 5 amps and 20 amps and that will probably get you pretty close for most of the curve. You will still be off by a factor of maybe 2 or 3 below 1 amp, but that won't throw your mAh reading off by very much.
Wow, I was just messing with this today on my BF F3 with DYS Fire 2100kv + 6045 Gemfan twin blade props. I had my Fluke 87V in current mode (10A limit) inline with my battery - I had it set to 110mV/A to make the reading match at "idle". However flying showed serious inaccuracies at the higher currents - overshoot! I will try the 260 setting...
