Li-ion chargiing failures

[Keith Black]

I had an NMP failure today. Tuesday night I charged the battery with the NMP charger and tested it. Under no load it was showing 8.44 volts. 

Since I'm paranoid I run two batteries, two regulators and two independent switches each going into the RX. My primary pack is on a 5.7 voltage regulator and the other is on a 5.1 voltage regulator. This causes all power to be drawn from the primary pack unless it drops below 5.1 volts. Today I went flying and everything went well, however, upon returning home I put the primary battery on the NMP charger and the charge needle didn't move. After determining the pack was bad I tore it apart and found one cell at 3.87 volts and the other at 0.23 volts.

Based on the amount of voltage drop on the backup pack I believe the primary pack powered one or two of my flights today. Unfortunately I was lazy and didn't check the voltage between every flight so I don't know when it died.  Luckily though I did have a backup battery so I didn't loose my plane "IF" the pack did die during flight.

These packs were purchased March 2004 but weren't used until around May this year, they have 69 flights on them. Keep in mind that the only one of the two that has been repeatedly drawn down is the one that failed.

Other info, I store my plane in my garage which does get quite hot here in TX, but certainly not up to 140 deg.  I don't store my plane in my care other than during transporting to and from the field. Today was a rather mild day in the lower 90's to upper 80's when I was flying.

I post this for others' info and to see if anyone has any info on why this would have failed, I'm not posting to badmouth NMP. In fact, I've really liked the way the NMP batteries have performed up to this point. They are light, easy to charge, and provide a lot of flight time. However this failure is quite disconcerting.

Keith Black


----- Original Message ----- 
  From: Grow Pattern 
  To: discussion at nsrca.org 
  Sent: Thursday, August 04, 2005 10:53 AM
  Subject: Li-ion chargiing failures


  I have had a few (4) of my 2000 mAh Li-ions fail in the last couple of months.

  The failure only occurs during charging and when the outside temperature is very hot. 

  I was not sure what it was at first. I did read the warnings on some cell phone batteries that I use as set-up packs, and it says do not get hotter than 140F. Yesterday I actually caused it to happen at the field (and when I was taking notice.)

  I had a spare 2000 pack in my van, left over from my Nat's trip. It was on the tailgate area and caught some sun. It was showing 8.12 V so I  plugged it in to charge it up. The meter did not move, so I tested the voltage again. No volts. My laser-temp showed 143 F. After a teardown I found one cell as good as open with .05V. 

  Based on other feedback I believe that what happens is the cell blows a sort-of-fuse internally if hot (140+ F) and a charge is applied. Could be a safety feature for all that  know?

  It does not (or has not so far) failed if current is drained when this hot. I have one in a Radio South ni-starter that get warm and is mistreated , but does not fail.

  In practical terms what happens to you is that when you switch on there is no juice. So no juice, no fly, no plane loss....

  Please regard this as a "heads-up' to not charge your Li-ions if they are hot. Wait until they cool down.

  Regards,

  Eric.

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