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Electrical current measurements

Discussion in 'Inspire 1 Discussion' started by ajohnsonlaird, Jul 16, 2015.

  1. ajohnsonlaird

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    I know that Bob Marley over at rcgroups.com has done some measurements of the current flowing between the DJI power supply brick and the smart battery while it is charging.

    It's easy to measure that in our lab, but measuring the inflight current drain on the battery is another matter.
    I've decoded some flight logs using some on-line web sites, the most notable of which is http://www.djilogs.com/ which outputs a very comprehensive CSV file.

    If the data it reads from the flight log is to be believed (and I have no reason to disbelieve it), it shows the battery voltage but does not report the electrical current in flight.

    Just curious whether anyone's got approximate data on, say, the current draw in Amps at hover?

    Thanks in advance.
     
  2. The Editor

    The Editor Moderator
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    Hi Andy,

    I'm going to have to say as a very rough and ready back of a beer mat calculation just under 15amps at hover.

    If you take average hover time of TB47 of 15 mins down to 20% capacity you are utilising 3600mAh. Multiply that by 4 (since you are depleting the pack in 15 mins or quarter of an hour and you get 14.4amps draw.
    If you do exactly the same calc with a TB48 and call in 18 mins at hover the figures come out at around 15.1amps. Average the two and you get circa 14.75amps.
    I THINK my maths is correct (?) and I did say it was rough and ready calc. :p
     
  3. ajohnsonlaird

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    Thanks, Editor.
    That was pretty much what I came up with (I used a Fuller's beer mat -- that could explain the slight difference). I was thinking of plotting out some discharge curves just to see if there was anything unusual in the way the voltage decayed compared to the other measurements I've done, especially given the reports of from 40 to 4% in 40 seconds (I'm exaggerating for conversational effect).

    I think I'll need to make something around 1.7 ohms at about 500 watts as a dummy load.
    I might have to settle for 2 ohms in which case the current will be a little light: 13.1 Amps. Alternatively 1.5 ohms, but then that's 17.46 amps....