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Feature Request: Automatic Battery discharger.

Discussion in 'MōVI Pro' started by Rick Gerard, Oct 5, 2017.

  1. Rick Gerard

    Rick Gerard Active Member

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    I started a thread about my MoVI having battery problems and powering off at 45%. Lots of folks jumped in with similar battery problems. I received a suggestion, which is a bit scary, to possibly hack together a discharger hooked up to an alarm so I don't brick my batteries. This makes me a bit nervous because there are no balancing leads in the MoVI batteries, only a positive and negative terminal. All of the balancing is supposed to be internal. I know it is supposed to work for charging if you leave the batteries on the charger long enough but I don't know if it works for discharge.

    Here's the request. Freefly should build a new charger with a discharge cycle to protect our expensive batteries or they should build some kind of discharge hub that you could hook at least two batteries, better yet four and have them discharge to the optimum storage voltage and then disconnect.

    It's a real pain to discharge batteries when you have been keeping a few of them fully charged during the day so you don't hold up production and then you end up home with 4 fully charged batteries and 2 at 80% like I did today. I'll spend several hours discharging batteries tomorrow because my next gig does not come up for more than a week.
     
    jim mundell likes this.
  2. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Sorry I scared you with my suggestion, Rick. My motivation was to try and help you solve the problem of how to discharge LiPo batteries.

    The hopefully-less-scary way, is to put them on the MoVI Pro, power it up, and set a timer and just let the MoVI Pro discharge it over a period of a time. If you check back on it every 15 minutes or so, you can get a good idea of the discharge rate and adjust the timer setting accordingly.

    Hope that helps
    Andy

    Forensic Software & sUAV / Drone Analyst : Photographer : Videographer : Pilot (Portland, Oregon, USA): Trees=2, Ground=1, Props=11. :(
    The Ground Is The Limit™
    ---------- Forensic Drone Analyst : Forensic sUAV Analyst : Forensic Unmanned Aircraft Analyst : Forensic Drone Expert
     
  3. Rick Gerard

    Rick Gerard Active Member

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    I ordered some parts and am going to try and build an automatic discharger that will switch it's self off. I ran some tests last night and the only reasonable discharge time I could get was when I powered up the Shogun Inferno, the Bolt 500 and the camera. 45 to 50 minutes gave me 50% on the MoVI display and about 22.97 volts when checked on the app. Just turning on the MoVI and letting it sit was taking way too long.

    Do you have any idea of the target voltage and best current draw for the MoVI batteries? It's going to be pretty easy to cobble together a draw of "x" amps, measure the voltage and turn the buzzer into a switch.

    My biggest challenge is going to be coming up with a connector to attach to the batteries.
     
  4. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    The MoVI Pro battery's label says "1.8Ah 22.6v."

    From that we can determine that the capacity (otherwise known as its C-rating) is 1.8 too.
    The 22.6 volts also indicates that internally there are six LiPo cells -- each cell has a nominal voltage of 3.7 volts (fully discharged it would be 3.0 volts, fully charged it would be 4.2 volts so it's more or less the mid-point).

    As you can see, FF rates the nominal voltage at 22.6. (As a double check divide 22.6 by 6 and you get 3.7666...)

    Thus you can tell that the storage voltage should be 22.6 volts -- the mid-voltage for all the cells. So you need to charge or discharge the battery to 22.6 volts for mid- to long-term storage.

    The C-rating also indicates the optimal charge/discharge rate of 1.8 Amps. Bear in mind that drawing 1.8 Amps at 22.6 volts is going to mean that you need to dissipate 40 watts (watts are volts times amps, so 1.8 times 22.6), so your dummy load must be capable of handling that -- most of the power will be dissipated in the form of heat.

    Oh....I forgot to add that you can calculate the resistance you need to get that current by dividing the voltage (22.6) by the current (1.8). That shows you need a resistance of around 12.5 ohms.
    Companies like Mouser.com or Digikey.com will sell those kinds of resistors. If in down, go for the next highest value (e.g. 20 ohms) and the next highest power rating (e.g. 50 watts), just to be on the safe size.

    For what it's worth, I built a test rig to discharge ALTA and other multi-rotor flight batteries so I use honking great 1,000 watt resistors. See http://rathergoodguides.com/suas-battery-testing.html -- these can be configured to pull 120 amps out of a typical flight battery.

    Hope that helps.
    Andy

    Forensic Software & sUAV / Drone Analyst : Photographer : Videographer : Pilot (Portland, Oregon, USA): Trees=2, Ground=1, Props=11. :(
    The Ground Is The Limit™
    ---------- Forensic Drone Analyst : Forensic sUAV Analyst : Forensic Unmanned Aircraft Analyst : Forensic Drone Expert
     
    #4 Andy Johnson-Laird, Oct 6, 2017
    Last edited: Oct 6, 2017
  5. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Oh, I forgot to mention this, Rick, but I *think* the connector this cable will work: https://store.dji.com/product/inspire-1-remote-controller-cable-kit although you will have to cut back the plastic shroud to get it to fit into the MoVI Pro battery -- and do a "cablectomy" to put the correct connector in place of the present barrel connector.

    I have one of these cables and the spacing between to the two power connectors is identical to that on the MoVI Pro battery -- but I would still proceed cautiously and only purchase one cable and try it.

    I should also point out that, if the MoVI Pro battery has a warranty (and I'm not sure whether it does), then you're probably going to ensure that it does NOT have a warranty anymore -- the risk of over-discharging is ever present.

    Cheers
    Andy

    Forensic Software & sUAV / Drone Analyst : Photographer : Videographer : Pilot (Portland, Oregon, USA): Trees=2, Ground=1, Props=11. :(
    The Ground Is The Limit™
    ---------- Forensic Drone Analyst : Forensic sUAV Analyst : Forensic Unmanned Aircraft Analyst : Forensic Drone Expert
     
  6. Rick Gerard

    Rick Gerard Active Member

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    Thanks Andy. I learned a lot about how to read the data on Lipo from you. If Freefly does not come up with a solution to this problem then I'm going to build a discharging device that has a cut of so that the load goes away when the voltage reaches about 22.6. I already have parts coming and it should be easy and a lot safer than just soldering together a bunch of light bulbs and putting a volt meter with an alarm in the circuit.

    I had no idea that all limo's should be at about 50% charge when stored. That explains why I've had a couple of batteries for other gear all of a sudden stop taking a charge.

    Thanks again.
     
  7. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Glad it helped, Rick.
    Yeah, LiPos are temperamental critters. They turn into paper weights or fire starters if you over charge them, over discharge them, or store them fully charged. On the other hand, they provide pretty good energy density in a lightweight package. But never turn your back on them!

    The MoVI Pro batteries have their own built in battery management circuit (BMC) so the over charge/under charge problem is not present, but the Lithium Polymer battery chemistry still means that they're happiest stored at 50%.

    If you're not going to use them for a while and you take them down to storage voltage, it's probably wise to fully charge them and then take them back down to storage once every few months. I've not seen any hard science that measures what happens if you don't, so this is probably me starting an urban myth! :)

    Andy

    Forensic Software & sUAV / Drone Analyst : Photographer : Videographer : Pilot (Portland, Oregon, USA): Trees=2, Ground=1, Props=11. :(
    The Ground Is The Limit™
    ---------- Forensic Drone Analyst : Forensic sUAV Analyst : Forensic Unmanned Aircraft Analyst : Forensic Drone Expert
     
  8. Rick Gerard

    Rick Gerard Active Member

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    I'm working with a national champion RC airplane guy that really knows LiPos and he says anything from about 75% to 50% is good but that I need to check the voltage with a voltmeter/watt meter and a load. He commonly uses a 100-watt load to test his 3 cell batteries because that is approximately the load at half throttle. So my question is how many watts does the MoVI pro draw at what would be the equivalent of half throttle. I know what the draw is from the Shogun Inferno that I commonly use, and I have the numbers for my Teradek Bolt and camera. Any idea what the Movi Pro's current draw is supposed to be? I'm sure that 100W will be sufficient but I'd like to have some numbers. This will also give me the numbers I need to start charting the internal resistance of the MoVI batteries.

    This is the meter he recommended: https://www.amazon.com/Keenstone-Pr...8088605&sr=8-3&keywords=RC+battery+watt+meter

    I used to have to load test my Atton Bauer Nicad bricks that powered my Arri 35 all the time to make sure the NiCads were healthy. It looks like I need to start keeping internal resistance charts on my MoVI batteries too.

    This is a YouTube video he recommended that I watch.
     
  9. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Rick: I don't know what the MoVI Pro current draw is likely to be -- the problem being that it all depends on the mass-in-motion (which determines how hard the motors have to work to stabilize the gimbal).

    You might be able to approximate a number for your particular setup by starting with a fully charged battery on the MoVI Pro. Then use the MoVI Pro as you would normally, but noting how long it takes to drop the batteries down to 50% charge.

    We know that each battery is capable of delivering 1.8 Amp hours at a nominal voltage of 22.6 volts. That means the battery is capable of delivering 1.8 x 22.6 Watts --- which is 40 Watts over a period of one hour. With two batteries that is 1.8 x 22.6 x 2 or 81.36 Watts for one hour.

    But let's say you find that when you use the MoVI Pro your particular setup allows you to run for 2 hours before the MoVI Pro's batteries both hit 50%. So that tells us that the wattage used is 81.36 divided by 2 hours, so 40.68 Watts. And we have to divide by 2 again as we have two batteries, so that means each battery delivered 20.34 Watts of power per hour for 2 hours. Given that the voltage is nominally 22.6 volts, and that Watts are volts x amps, that means we can divide the Watts by 22.6 to get the current, so that's 20.34 / 22.6, which gives a result of 0.9 amps.

    Hope that's not too confusing. (And I hope I didn't screw up the math either! I invite any other forum members to correct my thinking here -- I confess to not being very good at power calculations!)
    Andy


    Forensic Software & sUAV / Drone Analyst : Photographer : Videographer : Pilot (Portland, Oregon, USA): Trees=2, Ground=1, Props=11. :(
    The Ground Is The Limit™
    ---------- Forensic Drone Analyst : Forensic sUAV Analyst : Forensic Unmanned Aircraft Analyst : Forensic Drone Expert
     
  10. Rick Gerard

    Rick Gerard Active Member

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    I picked up a watt/voltage meter and wired it up to a d-tap so I could just plug in my Shogun Inferno or my Teradec Bolt and check the load.
    View attachment 11501 testing.jpg
    Plugged the Shogun into the D-tap port and it's drawing about 25 watts as it should according to the paperwork. The Bolt is also drawing about what it should. The #2 battery just came off the charger and is showing 24.45 volts.

    Now I just have to build a load that I can plug-in that simulates the normal operating load for the MoVI Pro with all accessories running. Unless I get some info from Freefly I'll back into the average draw by using the calculations that Andy suggested.

    Now I have a way to check my batteries that means something.

    The only question I have left is the 50% storage voltage. I'm assuming that I should be able to determine what that voltage is by pulling the batteries and checking the voltage when the batteries show 50% under load using the App or the screen on the MoVI. That should at least give me a baseline I can use so that the batteries are being stored somewhere between 50% and 70% and not below.

    This is the meter I bought and it seems to be well built. The display is kind of hard to read in direct sunlight but it will do just fine.
    https://www.amazon.com/Keenstone-Pr...8088605&sr=8-3&keywords=RC+battery+watt+meter

    The connector that goes to the battery came from DJI as per Andy's suggestion but I had to modify it a bit so it would slide into the MoVI Batteries. Find it here https://store.dji.com/product/inspire-1-remote-controller-cable-kit
     
    #10 Rick Gerard, Oct 16, 2017
    Last edited: Oct 16, 2017
  11. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Rick:
    You mean my advice was right? Wow. Always a good day when I happen to be right..... <evil grin>

    Thanks for posting the power data too.

    Yes, measure the battery voltage under load -- they tend to bounce back up a volt when you remove the load and they recover. Don't fret about getting the batteries to exactly 50% -- it's not that critical. 40% to 60% seems to be fine too (albeit in my limited experience, but who knows, I might be right about that too. :) )


    Andy

    Forensic Software & sUAV / Drone Analyst : Photographer : Videographer : Pilot (Portland, Oregon, USA): Trees=2, Ground=1, Props=11. :(
    The Ground Is The Limit™
    ---------- Forensic Drone Analyst : Forensic sUAV Analyst : Forensic Unmanned Aircraft Analyst : Forensic Drone Expert
     

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