Charging concerns/questions with NMP Lithium Ion batteries

[Keith Black]

Jim, you actually addressed a question that I had been pondering, if current from the charger will go to the maxed out cell, however that isn't the question I had intended to ask. 

What I was trying to ask is if a two cell pack had one cell at say 4 volts and another cell at 3.7 volts, and this pack was left sitting by itself off of a charger, would the higher voltage cell cause a trickle effect and bring the lower cell into parity? 

In regards to your question below, I think this is what Troy was trying to suggest (the two balloon example), however without the cell being designed to prevent itself from exceeding 4.2 volts I don't buy into this theory. Here's my reasoning. Let's assume you have one cell that's at 4.2 volts and another at 3.8 volts. Given that you'd have to apply voltage in excess of 4.2 volts to charge both cells I can't see why the higher cell at 4.2 volts wouldn't continue to accept the charge and eventually exceed its safe limit. After all, the charger doesn't know that one cell is at 4.2 volts and the other is at 3.8, it just knows that the total is 8 volts, which means it needs to continue charging until they reach 8.4 volts. Without special circuitry what's going to stop the 4.2 cell from receiving some of the charge? 

BTW, this is what Duralite is promoting in their cells, the circuitry to limit the voltage on each cell to 4.2 volts (or actually 4.35 volts which is supposedly "really" the limit, though industry standard seems to be to only charge to 4.2 volts). I've heard that Duralite cells can still catch fire even with this circuitry, but I'm not sure how much abuse they require before doing so.

If you do any experiments it will be interesting to hear your results, just make sure to do them in a fire proof area ;-)

Thanks,
Keith

  ----- Original Message ----- 
  From: JOddino 
  To: discussion at nsrca.org 
  Sent: Saturday, May 08, 2004 12:23 AM
  Subject: Re: Charging concerns/questions with NMP Lithium Ion batteries


  You wouldn't want to trickle a good Lithium pack because you would need to put more than 8.4 volts on it to get any trickle current.  If somehow one cell got partially discharged it would seem that charge current would flow through both cells and overcharge the one that hadn't got discharged.  However that cell might set at 4.2 while the other cell comes up.  I guess it is time to purposely unbalance a pair of cells and put them on the charger and see what happens.  I will report back.
  Jim
    ----- Original Message ----- 
    From: Keith Black 
    To: discussion at nsrca.org 
    Sent: Friday, May 07, 2004 6:18 PM
    Subject: Re: Charging concerns/questions with NMP Lithium Ion batteries


    Thanks for your reply Jim. 

    I do like design of the NMP charger and the way it shows how much current is being feed to the battery. This is clearly an excellent design feature. 

    What I gather from your response is it is "possible" for a cell to be charged beyond the safe limit if another cell is either out of balance or defective. Also, I understand your point that one may be able to spot a bad cell after becoming familiar with the charger. 

    As it relates to the cells being unbalanced, will lithium batteries equalized each other via a "trickle" as is the case with NiCad batteries?

    Jim, I really appreciate your always candid responses and willingness to assist others on the list.

    Thanks,
    Keith
      ----- Original Message ----- 
      From: JOddino 
      To: discussion at nsrca.org 
      Sent: Friday, May 07, 2004 10:30 AM
      Subject: Re: Charging concerns/questions with NMP Lithium Ion batteries


      Hi Kieth,
      I believe the NMP solves most of the problems we have all read about.  1) it is the right type of charger for Li-Ions and is dedicated to that chemistry by limiting the current and the voltage. 2) it is set for two-cell packs and that cannot be changed by the operator or on its own. 3) it limits the current to less than the 1C rate (for 2000 mAh packs).
      Probably its best feature vs. the other chargers is the meter that tells the operator what is going on.  After you use it a few times you will be able to predict the current at the beginning and during the charge cycle depending on how many flights you had.  If something is wrong, like a cell way out of wack, it will give you an indication.  Of course it tells you when the pack is charged and if for some reason it was taking too long you would figure it out.  In other words you will learn what is normal proper charge current vs. time and could spot an unbalanced pack.  It cannot assure you a cell will not be overcharged if the cells are not matched but I believe you would spot it.
      I've been using it for over a year and always charge the packs in the plane.  The total charge time is relatively short so you can monitor it, but I have left them plugged in over night.  
      I have only seen one pack (a Duralite which was in a plane I bought) with a slight unbalance.  It was a 2S2P configuration and I don't have much experience with that configuration.  I have never seen a simple 2 cells in series pack unbalanced.
      Where you really need to worry about this is in long strings like those used in electric flight.  I talked to Fred Marks (FMA) about bringing out each node so each cell could be checked and charged individually if required.  He said they plan to do that in the future.
      In the mean time, I believe one could stick a pin through the heat shrink to check the cell balance.  I'll try that today and get back to the list.
      Regards, Jim
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