Internal resistance and voltage drops

ArduCopter Copter 4.2
1

I have been chasing down false voltage drops in the readout on my quad with a 30Ah 6S battery and MAUCH PL 200A current monitor and BEC. This turned out to be a sag in the 5V input to the Pixhawk Cube Orange. I rearranged my power wiring and now the reported voltage is significantly better, but not perfect.

The estimated battery internal resistance is 30 milliohms (previously it was around 70-80) but my Hyperion EOS 0840i charger says it’s 12 milliohms. Has anyone compared these readings before? Is this a symptom of additional voltage problems or is my charger being optimistic with its readout? Any other tips on reducing these voltage drops?

2

You could do a manual calibration of the voltage, usually the Mauch-supplied value would be close.
There’s a procedure to calibrate current draw without using mega-expensive high current tools:

Here’s how to adjust you’re Amps Per Volt

  • Plug in a fresh battery and do a hover flight for 5 minutes or until you’ve used a signifcant amount of battery capacity
  • Immediately unplug the battery and charge it, note how many mah’s it took to charge. Let’s say that was 1,500 mah
  • Download your log and look at maximum CurrTot. Let’s say it was 1,300 mah
  • Get your existing BATT_AMP_PERVOLT setting from Mission Planner
  • NEW_AMPS_PER_VOLT = OLD_AMPS_PER_VOLT * CHARGED_MAH / LOGGED_MAH
    With my example numbers, you’d get
    NEW_AMPS_PER_VOLT = 17 * 1,500 / 1,300 = 19.61
  • Set the new value in the BATT_AMP_PERVOLT parameter and save to your copter
  • Fly to check, repeat until you’re happy the new value gives consistent readings
1 Like
3

Hi, I’ve done all this but it doesn’t seem to be related to the problem. My problem is that as the current drawn from the battery increases, the monitored voltage decreases severely and illogically. For example, when I draw 40 amps from the battery, the reported voltage goes from 25 to 20 volts, which I have verified is not true with a multimeter.

4

Then there’s a wiring issue, or the ground wires to/from the voltage/current sensor are suffering a voltage drop.

5

Yes, that’s what I noticed and already fixed to a significant degree. But I’m not sure if it’s fully fixed and what is acceptable voltage drop for my battery and current. That is a pretty subjective question, so I’m wondering if the battery internal resistance estimate by Ardupilot is evidence that it’s fixed or not. How does this value relate to that measured by a battery charger? Should they be equal?