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"Smart" batteries not so smart

Discussion in 'Inspire 1 Discussion' started by Rhino Schmidt, Jun 8, 2015.

  1. Rhino Schmidt

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    Browsing through http://batteryuniversity.com/ is highly recommended reading for all of us who use LiPo batteries to power our flying machines. One extract of interest is presented below.

    Specifically, there have been several alarming reports of batteries seeming to suddenly lose all power near the end of their discharge cycle, ie.: dropping from 30% to 3% in a matter of seconds.

    One of the two major reasons why LiPo battery remaining capacity can appear to suddenly drop near the end of its discharge cycle is an inherent property of LiPo batteries:
    The discharge curve of a LiPo is anything but linear. Once the voltage gets down to about 3.6V per cell it drops off a cliff, this capacity vs. resting voltage graph shows just how fast it plummets:

    [​IMG]

    So basically you put a battery in that was virtually fully discharged, so just pulling a little bit more power out of it would have inevitably cause the voltage to collapse, it's exactly what you would expect to happen.

    http://www.rcgroups.com/forums/showthread.php?t=2157055 post #7 elaborates this point.


    The other major reasons why this apparent battery failure occurs lies neither with the battery nor with the % fuel gauge, but rather mis-understanding of what the fuel gauge is *really* telling us.

    http://batteryuniversity.com/learn/article/the_battery_fuel_gauge notes that

    While the State of Charge information {based on battery cell voltage} displayed on a battery or a display screen is helpful to the user, the readout does not guarantee the expected runtime. The fuel gauge resets to 100 percent on a full recharge regardless of how much capacity the battery can store. A serious miscount occurs if an aged battery shows 100 percent SoC while the battery’s ability to hold charge has dropped to 50 percent or less. We ask, “100 percent of what?” If, for example, 100 percent of a good battery results in a four-hour runtime, a battery holding half the capacity would run for only two hours. The user should know that the fuel gauge only shows SoC; capacity, the leading health indicator, remains unknown.

    Prior to the advent of smart batteries & their displays, R/C Heli flyers successfully guesstimated battery capacity, ie.: flight time remaining, by noting how much juice was pumped (charged) into the battery and how much had been consumed. Some of those calculations have been done for us in the DJI Pilot app (battery sub-menu) that displays battery "Remaining Power" in mAh. THAT reading tells us how much fuel is left in the tank, and *not* the % displayed top right of the Pilot app!

    Using the stock TB47 4500 mAh battery as an example, if we want to follow the golden 20% rule then our bird MUST be back on the ground (not on the return home) when there are 4500 x 20% = 900 mAh remaining. If we also factor in Murphy's 10% Law that means we should be planning on safe engine shutdown with 1350mAh remaining.
     

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  2. The Editor

    The Editor Moderator
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    A lot of information on that site is good info but doesn't really apply to the Inspire batteries. (Well it does AND it doesn't....if that makes sense :p)
    The 'smart' logic in the I1' batteries take into account their aging and that site is more geared tandard Lipos and fuel gauge technology.
    The DJI packs take into account increasing internal resistance with age/use/number of charge cycles etc and also adjust percentage 'life' of the battery dynamically.
    Unfortunately, some of that logic is flawed when people fly with less than a fully charged pack so that part hold true and is what many of us have been trying to educate people to do on this forum......ALWAYS start off with a fully charged pack. :)
     
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  3. Carlsberg

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    Please, for the love of God, post a link to the source of this info you post.
    PLEASE, I'm serious, I want a link, I beg you.
     
  4. Kilrah

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    Which is why the manual tells you to fully discharge the battery every 10-20 cycles so the circuitry can measure the battery's exact capacity.

    Wrong, both are of course linked, with the the % being that remaining power in mAh / the current total capacity that is displayed next to it and is determined using the procedure above. So percentage is perfectly valid in normal circumstances, and is simpler and quicker to understand.
     
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  5. sirnikolas

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    Absolutely concur!
     
  6. Luis Morales

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    a lo of what that site says can be used with the inspire, voltage drop, how capacity works etc.

    most people don't know that the more depleted a battery is the more rapidly the voltage will drop, also its important to know that the less voltage there is the slower the motors works (amount of turns and voltage is related) so if you let it drop too much its harder for the controller to maintain stability and speed.

    its also a good idea to use ATTI mode instead of GPS when on low battery (the controller does less corrections and the motor works less)

    that site is full of useful information everyone working with LiPo batteries should know, no matter how smart the battery is!