Charging concerns/questions with NMP Lithium Ion batteries

[JOddino]

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
  From: Keith Black 
  To: discussion at nsrca.org 
  Sent: Friday, May 07, 2004 12:30 AM
  Subject: Charging concerns/questions with NMP Lithium Ion batteries


  I'm not trying to start a "heated" debate about the safety of lithium batteries, but I purchased a couple of these packs just before the AMA warning and now I have some concerns and questions. 

  My question revolves around how the NMP batteries are built and how the NMP charger works.

  In the past I'd believed that fires with lithium batteries are primarily due to user error when charging, typically one of the following errors: 1) Using the wrong type of charger, 2) setting the charger for the wrong battery type/cell count, or 3) trying to charge cells too quickly, in excess of 1C. 

  However, I did some research on E-Zone and found that these packs can catch fire even on a slow charger if they are charged beyond 4.2 volts per cell. This means if your charging a multiple cell pack and cells differ in voltage by more than 1 volt and the charger keeps charging until (total cells x 4.2 volts) is reached then even slow chargers can cause a fire.

  Therefore, my question is does circuitry exist either in the NMP battery pack or in the NMP charger to assure that a single cell will never be charged in excess of 4.2 volts? If so, how does this work?

  If this circuitry exists then we can probably feel quite safe leaving the NMP batteries inside our planes while charging just as we do with our laptops, cell phones, cameras etc. If not then we're taking a big chance charging inside our planes, vehicles and houses (as has been pointed out on this list before).

  FYI, I was shocked when I visited E-Zone and found that the electric guys have known for a long time that fires are not only a possibility but have occurred many times. In fact, it's common knowledge in the electric community that precautions such as charging in a flame proof container, not charging in vehicles or homes, never leave the battery unattended, etc. must be observed when charging.  I found it very interesting that there are even numerous people working on various types of fire-proof charge bags made from things like ceramic fibers.

  Obviously the electric guys push things to the extreme with their batteries, but if such a simple thing as exceeding 4.2 volts can catch a cell on fire then we should make sure our batteries/chargers have circuitry to prevent this or we must take the necessary precautions when charging.


  Here's a link to a good Lithium safety primer on E-Zone:
  http://www.rcgroups.com/forums/showthread.php?t=209187

  Thanks to anyone who can share the details of the NMP lithium system.

  Keith Black
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