Battery Tests

Discussion in 'ALTA' started by JoshuaDunn, Sep 4, 2015.

  1. Gary Haynes

    Gary Haynes Administrator
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    It has a push/spring lock like any micro sd card. I use a small flat blade screw driver if a finger nail won't depress it enough to release or lock. Also pay attention to putting the rubber boot back on. It is 'tabbed' at each end. Don't get it hooked under each end and it will fall off. I was lucky. Found it next to where my truck was parked when I went back to the launch field.
     
  2. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Err....On my unit I just push it in slightly and then it pushes back (to make me feel wanted, I presume) -- but it pushes back out far enough that I can pull it the rest of the way. Sounds like the microSD card holder might be an issue, I'm afraid. Might want to email support@freeflysystems.com before you make things worse.

    Andy.
     
  3. alex ryan

    alex ryan Member

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    Nah its all good, just inadequate fingernail length. I used a jewellers screwdriver and it popped right out.
    On a side note I just bought a pair of Movi m10 batteries brand new to replace my well used batteries.
    The weird thing is I just put them on charge and they have an internal resistance of around 22 milli ohms per cell? This does not seem good for brand spanking new batteries. Some of my other movi batteries which have had hundreds of charge cycles are only measuring about 18 milli ohms per cell. Is this a first charge thing? does it take them a while to start working more efficiently, or are they duds?
     
  4. alex ryan

    alex ryan Member

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    and just so you know its 22C (71.6F) where I am charging them.
     
  5. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    The conventional wisdom is that you need to charge/discharge Lipos three times to condition them -- however, like much "wisdom" one finds on the Internet I'm really not sure whether or not it is true.​
    If in doubt, open up a ticket with Freefly via email at support@freeflysystems.com and ask about this -- 22 milliohms per cell does seem a tad higher than I would expect. I would have expected around, meh, 5 milliohms per cell -- but with new batteries I'm not so sure.​
    You're obviously sure it's per cell, so, I'd say it's a tad high...​
    Andy.​
     
  6. alex ryan

    alex ryan Member

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    yeah thanks for the confirmation, just wanted to hear some thoughts from you guys before I opened a ticket.

    Cheers
     
  7. John Dennings

    John Dennings New Member

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    "With dual batteries, the current from each battery is halved and, because the batteries are connected in parallel, the internal resistance of the two battery combo is also halved -- this means the current can be provided without as much voltage sag across the internal resistance of the batteries."

    Isn't this true only if the single battery has half the capacity of the two combo batteries combined? If you replace two 10ah batteries with a single 20ah battery (same "manufacturer", same internal cells), the 20ah battery will have half the internal resistance (and double the discharge capacity) of each of the two 10ah batteries wired in parallel. So the 20ah battery will have the same voltage sag under load and same discharge capacity as the two 10ah combo, and there shouldn't be any difference. Except possibly at the wire leads and connectors (if those are the same size) and at the PDB.
     
  8. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Yes. The general practice is to fly with two "identical" (same vendor, same life-cycle, same charge level) Lipos.

    If you have a 6S 10 Ah battery, there will be, by definition six cells in series (stunning, no? :) ), so the internal resistance of each cell is in series and the total resistance = resistanceCell1 + resistance Cell2 + ... resistanceCell6

    I'm not sure from your example whether the 20 Ah battery is also 6S? If it is, then it, too, will have six cells in series with similar internal resistance.

    That said, it *sounds* like the 20 Ah battery you are imagining might internally be the functional equivalent of two parallel 6S because you say "will have half the internal resistance (and double the discharge capability)."

    But to continue the calculation for the 6S 10 Ah batteries, if you have dual 6S 10 Ah batteries arranged in parallel, then the combined resistance of the two batteries is:

    Total resistance = (1 / (6 * individual cell resistance)) + (1/(6 * individual cell resistance)) ​

    (From Ohm' law, resistors in parallel -- sorry about the programming-like formula -- easier to do that than draw pretty pictures but see http://www.sengpielaudio.com/calculator-paralresist.htm for snazzy calculator and formulae).

    Given that the 6S Lipos have 30 milliOhms total internal resistance (a hypothetical but not unreasonable number), that will result in a total internal resistance of 15 milliohms when you have two in parallel.

    I'll stop here to let you clarify what the internal cell geometry of the 20 Ah LiPo in your example is. :)
    Andy.
     
  9. John Dennings

    John Dennings New Member

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    Hi Andy, what I meant in my example is comparing the use of two batteries of the same manufacturer, same "series" and discharge rate, same voltage. So for instance Tattu 16AH 6S 25C and two Tattu 8AH 6S 25C in parallel. In this case the IR of the 16AH should roughly be the same as the total IR of the two 8AH in parallel, because the IR of the 16AH is 1/2 of the IR of a 8AH.

    Your calculation of total resistance if of course right but it does not take into account the fact that the "individual cell resistance" is not the same in a, say, 8AH battery and a 16AH battery. For the same manufacturer and Lipo chemistry, the 16AH battery will typically have a total IR of 1/2 the total IR of a 8AH.

    To give a real world example. Take two Tattu 10AH 6S 25C and a 22AH Tattu 6S 25C battery. If you measure the six IR values for the 22AH battery, you should find that they are a bit less than 1/2 of the six 10AH ones. (A bit less, because we are comparing 22AH vs 2 x 10AH). Which makes sense, since the 22AH battery, at 25C, can deliver a bit more than twice the amount of current as the 10AH one). If a 20AH Tattu 6S 25C battery existed, it's IR would be 1/2 of the 10AH 6S 25C one.

    I've actually measured this on cheap Multistar 16AH and 8AH ones. 16AH Multistar: 9 MilliOhm. 8AH Multistar: 17 Milli Ohm. I also use 22AH 6S Yabos, and their IR is proportionally even lower, 4MilliOhm, I suspect both because it's a different chemistry but also because it is not at the same point in its life cycle.

    So I'd guess that a Tattu 22AH (given it's more than twice the capacity of two Tattu 10AH) will actually have lower IR that two Tattus 10AH in parallel, be able to deliver a bit more current and have less voltage sag that two 10AH ones in parallel.
     
  10. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    That's interesting. Do you think they achieve that by actually putting cells in parallel, although externally presenting the battery as 6S?

    Or is it just intrinsic in the size of the cells, or different electrolyte?

    I'd like to read up more about this, but I couldn't find any references that correlate internal resistance to cell capacity -- do you happen to know of any?

    Andy.
     
  11. John Dennings

    John Dennings New Member

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    I think it's just intrinsic in the size of the cells, but frankly I am just guessing. The larger the cell the less resistance to internal ion exchange as there is more surface between the anode and cathode material, sort of the same principle as electrical resistance inversely proportional to the cross section of conductive wires, or resistance to liquid flow in larger diameter pipes vs smaller ones. But again, just guessing. That said in a way it has to be, if it were not you could not have C ratings that are the same for smaller and larger batteries from the same manufacturer and "series", as discharge rating has got to be proportional to IR and a larger battery will discharge more than a smaller one if both are rated the same.

    It has to depend on the manufacturing process and how they package individual cells, obviously. I wouldn't be surprised if some manufacturers for a given series of batteries just double capacity by wiring in parallel two batteries of half capacity and repackaging them, in which case IR of the resulting battery is exactly 1/2 of the IR of the two batteries. But it's definitely not the case for the few retired Lipos I've opened up, you just end up with six rectangular cells in the case of 6S and it's just their size that changes, proportionally to the capacity of the batteries.

    I do know that some similar chemistry batteries like Li-Ion batteries (e.g. Tesla batteries) are made from the same small individual cells, 3400 mah, 4.2V (cylindrical, non prismatic 18650's I think) wired in series and parallel (Tesla batteries have thousands) so in that case it's obvious, the more capacity (for a given voltage) the less overall internal resistance since there are more individual cells in parallel. Since the main difference between Lipos and Li-Ion is that the electrolyte is not liquid in the case of Lipos, one would think that the same principle applies. Also with 12v sealed lead acid or gel batteries, their IR are usually published and are inversely proportional to their capacity.

    I've been quite curious about this also, but short of my observations and own measurements I have not been able to find references. I suspect there are quite a few, but probably buried somewhere in research papers or chemistry textbooks :)
     
  12. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Thanks, John....I was hoping that you'd hit the Mother Lode for LiPo technology.

    I would have thought that the internal resistance related to capacity but probably not in a linear way -- some power function related to anode/cathode area surface area -- perhaps some square law?

    But it's just conceivable they pack the higher capacity as 6S6P (or however one would designate that).

    Let's hope another forum member is steeped in Lipo tech and can clarify..

    Andy.
     

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