[NSRCA-discussion] "Sick" Lipo Packs

[Ron Van Putte]

Those of you who use lithium polymer battery packs to power their  
competition airplanes are familiar with "puffed" packs.  I recently  
had four elderly 5S packs "puff".  We all know that's not good, but  
what I'd like to know is what's actually happening.

I know it's probably not wise for consumers to take lithium polymer  
packs apart, but that's exactly what I did with four packs.  I  
discovered that in three of the "puffed" packs, only a single cell  
was "puffed".  In the last pack, there were two "puffed cells.  I did  
a little arithmetic and quickly discovered that I could make three  
"unpuffed" packs from the good cells I had.  So, I unsoldered the  
"puffed" cells from the four packs and cannibalized one pack to make  
three 5S packs from what I had left.  This process is obviously for  
the timid or the careless.  I was careful and had no mishaps.   
However, I would suggest that anyone who says "Oops" a lot should not  
attempt doing this.

The three 5S packs I have left are "rock solid".  Experienced  
electric-pilots will know just what I mean.

I have flown these packs and they seem to perform just as they did in  
their "youth".

My questions are:  Why do lithium polymer cells "puff"?  What is the  
likely future of my recovered 5S packs?

Ron VP