One Battery or Two?

Discussion in 'Cinestar Misc' started by Andrew Goodwin, Sep 25, 2012.

  1. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Dave:
    Also bear in mind that if there is a given probability of failure for one battery, flying a 4x battery configuration multiplies the probability of failure for your system by four -- it does not reduce the probability of failure for the system as a whole.

    There is also the question of what happens when a LiPo fails? Does it drag down the voltage of the other batteries? There is one reported case of this on the forum.

    Perhaps the way to approach this is to use a large diode on each battery so that when forward-biased, it allows current to flow out of the battery, but when the battery fails it will not allow the failed battery to drag the other batteries down.

    The issue there is going to be (a) finding a diode with the current carrying capacity and (b) dealing with the 0.7 volt drop across the diode when it's forward biased.

    Just some thoughts to ponder...
    Andy.
     
  2. Dave King

    Dave King Well-Known Member

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    Great idea however you would need a diode that would have a forward bias voltage of approx 12 to 13v. Most diodes that I know of turn on and allow current to flow at 0.7 or 0.3. I don't know if there are any industrial grade diodes that would have a higher voltage bias than 0.7. Also if you think about it what if you had a situation where both lipos were fine and discharging at the same rate but for whatever reason you ignore or don't hear the warnings and you don't realize that your battery voltage falls below the forward bias voltage? The diodes would stop current flow once they reach the forward bias threshold and the bird would fall out of the sky. At least in the situation with lower voltage (even 12) you have a "chance" to at least land safely even if it means the battery is destroyed. Plus even if the circuit worked in theory you would have a 0.7 voltage drop that there would be no way around it.

    Also if you had a diode that would turn on at say "12 volts" you would have 12 voltages across that diode now leaving 4.8 volts to the PDB at full charge
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  3. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Yeah....not a pretty solution. Must be solvable though....could you use a circuit with an SCR to control the battery and isolate it from the power "bus" if it fails? The problem is the amount of current you need to handle, I suspect.

    Andy.
     
  4. Dave King

    Dave King Well-Known Member

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    I'm thinking that you would need some sort of logic control like in the picture below. You could hook up something like a BEC so you can control the amount of current and voltage to the IC. The IC then could be programmed for any conditions you want. Output of the IC can go to the base of the high current transistor that can turn on and off depending on the output of the IC.


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  5. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Sounds like you have yourself a good product, there! :)
    When can I buy them?
    Andy.
     
  6. Dave King

    Dave King Well-Known Member

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    LOL!!! Maybe I can program an old 8088!! :D So you think the odds of having more than one bad battery at a time is high enough that the 4 batteries in parallel wouldn't work? Aren't there anyone here running 4 batteries? I thought there was????:confused:
     
  7. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Anyone running with 4S is running four batteries! 6S six batteries.
    It's just a question of packaging. :)

    Andy.
     
  8. Daniel garate

    Daniel garate Member

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    Hi,

    I had a bad experience with a Nanotech lipo 4s during a job a few weeks ago.
    I was doing a lot of flights with my old battery packs, and a friend offer me to use hes new never flown nanotechs,.....
    i used 2 packs from him in parallel and they were fresh and perfect, then when i was changing packs, i connected two more, new ones in parallel and one of the lipos exploded in a ball of fire and burned all my electronics so quickly that i couldnt safe my beloved MK !!

    one cell exploded out of one battery, maybe came bad from the store, i dont know what happened, ......
    After 3 years of flying with this MK and my Cinestar, in every conditions, countrys, temperatures, no Crashes, .....i was in love with this Rig!!!
    and it dies on the ground burned because of a F...ng NEW battery!!!!!

    well , i finished that job changing cinestar centerplates and booms for my back up wookong....

    heres a video of that day...
     
  9. Steve Maller

    Steve Maller UAV Grief Counselor

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    Wow...good for you for having a backup! Bummer about the MK electronics.
     
  10. Shaun Stanton

    Shaun Stanton Active Member

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    I might be imagining things, I thought I have seen someone on this forum that has a system that removes a bad battery from the circuit. I tried to find it on here but could not. I believe it was part of a discussion about using redundant batteries after a connector failed or something along that line.
     
  11. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Daniel: Sorry to see your MK boards toasted, both metaphorically and literally...
    Thanks for sharing your video and the story. I presume you're not flying with Nanotech's any more? :)

    Andy.
     
  12. Sam Slape

    Sam Slape Member

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    I've got 4 nanotechs 5AH 4S, there NEW too!......o_O
     
  13. Shaun Stanton

    Shaun Stanton Active Member

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    I wonder if this can be isolated to just nanotechs. I have 4 3S's that I use on my Flamewheel, and have not had an issue with them yet. Since reading about all of these battery failures in general, it has made me super cautious now. Every flight I take off hover for 30 seconds or so, land then I check the battery with a Hyperion tester. My hope is that I can find out if a battery is about to show its ugly head so to speak by comparing an disparity in the cells. I figure if I see a cell that is unbalanced then I am tossing that battery. I can speculate that if one cell starts discharging more than that will force a battery in parallel to start feeding current into that cell, and causing the chain reaction to a battery fire.

    There is a Brazil guy, Tiago that did some tests on some batteries where he charged and discharged some batteries 4 times to verify that they all evenly discharge. Coincidentally, one of the Turnigy batteries had an issue. Makes me nervous that such a single point of failure is the lifeblood of these systems.

    When I flew jets if we had a generator that was 5 volts less than the others the system kicked it offline. It seems like this is something that we need, a system that will kick off a faulty battery.
     
  14. Ozkan Erden

    Ozkan Erden Distributor

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    It is very important charge and discharge around 4 times before using the new batteries.

    One of the Turnigy Nano Tech batteries that I bought, had a dead cell; unfortunately it was dead in flight. Luckily, it didn't blow anything and I was flying with single battery.
     
  15. Daniel garate

    Daniel garate Member

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    yep, i used Nano techs for years,....honestly i dont thnk im gonna buy them anymore.
    Im not sure what im going to buy now at the same cheap price....

    turnigy 5800 maybe? i heard those are bullet proof and cheap...
     
  16. Oliver Cvetkovic

    Oliver Cvetkovic New Member

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    Hi,
    What about this solution? They seem to know their stuff well. Now what to think, batteries up or down???
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  17. Tabb Firchau

    Tabb Firchau Administrator
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    Hi Oliver, The CG on the vertical axis of that machine is WAY below where the optimal CG would be. This will not allow the Flight Controller to do as good a job as it could with a properly CG'd machine.
     
  18. Howard Dapp

    Howard Dapp Active Member

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    Daniel, I'm curious if you connected the lipos in parallel after they were fully charged?
     
  19. Tabb Firchau

    Tabb Firchau Administrator
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    Hi Howard,

    Connecting Lipos of the same nominal voltage but different charge states should not cause to much of a problem provided they are anywhere close to the same charge state. I always charge my packs separately and then connect in parallel and have never had an issue.

    Now if you connect 2 different nominal voltage packs in parallel lookout (say a 4s and 7s)

    Tabb
     
  20. Howard Dapp

    Howard Dapp Active Member

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    Hi Tabb, I think the problem in doing it that way is if you have a pack with bad or out of balance cells..you may have one or two cells that will not take a full charge and the other cells picking up the slack, causing one or two to over charge. This is the best explanation I can come u with after my incident!:eek:

    I've always read that you should never connect two fully charged lipos in parallel. Always let them build up a charge together. Is this old school thinking? Are we now using lipos with 'smart' chemistry? IDK
     

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