Weird cell degradation
 

Can anyone explain this -

http://www.rc-monster.com/forum/pict...&pictureid=986

The pack is a few years old, beaten and abused (by me:lol:) it used to be a 6s but one cell got punctured after skidding across the road. So it's been used as a 5s pack for a while. Now it has lost a lot of mah, has poor punch and I could see one of the cells was deformed so I unwrapped it to take a look. The cell at the bottom has general mild puffing but all the others are swollen at the positive end. The swollen bit is rock hard, just back from the swollen ends it feels rather normal. Plus one or more of the cells has a slow leak 'cause I can smell it.

Any theories as to what's going on?

That doesn't look like your typical puffing from internal gas expansion where it would expand the whole pack and be squishy in a more uniform manner. I wonder if that's from the sudden stops during crashes or front-heavy landings causing the "stuff" inside the cell to bunch up against the front?

I think not. Each cell is swolen at the positive end ie- top cell big on left side next cell big on right side.

if you puncture a lipo it will let air in....then it will start to react and oxidize.
if its punctured its as good as dead...and really smelly too :yes:
chris

So you think they were poorly manufactured? That's why 4 of the cells have puffed in the same way. The pack is as good as dead but I was just curious about the deformation.

i did read up about lipos a few years ago, as a child i was into electronics....im more into mechanics and engineering rather than chemistry, so i didnt understand all of the text.

basically in certain situations like over discharge and over draw, oxygens are created inside the pack....puffing, the oxygene will start to oxydize the internal tabs on the lipo which pickup the voltage and take it out the pack.

this accelerates the degredation of the pack...once it starts there is no stopping it, basically rots the tabs...which creates more heat...

no i dont think they where poorly made...just well used
chris

I can soughta understand that:lol:

Thanks for the insight Chris:smile:

one more fact i just thought of while i was just repairing a bouncy castle blower...DC voltage allways flows in one direction...this would explain why the packs are swollen at one end only..now i cant remember if its + to - or - to +.
durr....:lol: the old grey matter is not as good as it used to be :no:
chris

Most explanations of electric theory kinda assumes the flow goes from + to -, which is technically "hole flow". But electrons actually travel from - to +.

As to the OP; I still think it's more of a physical deformation than electrical. Maybe relative tightness of shrinkwrap in the center vs the ends? Maybe the straps slowly over time squeeze the middle more? Maybe an inner cell puffs under usage, and since they are all stacked up, the tightness of the heatshrink causes that bulge to squeeze the material of adjacent cells around? Since lipo cells are long/wide layers of material, any gas expansion would affect the entire surface area of the cell, and would tend to bulge in the middle.

I was thinking that.

That may be true and a contibuting factor but I don't think it is the sole reason for the 'one ended bulge'.

sounds and looks like what chris said to me, the internal tabs on the positive end are oxidizing first.

cut 'em open and see what's inside:lol:

disclaimer: don't really cut them open, you will die:whistle:

hmmm...lipo surgery :intello: sounds like fun.

but a word of caution...lithium is alkali...and can cause burns to exposed skin.
not to mention the chance of still shorting out the pack (it will still have some charge in there).
and the fumes inside wont do the lungs much favours too...:whistle:

you should see the corrosion on the lithuim...looks dull grey, turning black eventually...if you do decide to do an autopsy :intello:

it is rather odd i agree usually lipos puff the whole cell up...or at least all the ones i have seen so far...
chris

If I get time this weekend I will operate.

I took the top cell off today. Before doing anything I put the multimeter on it, the voltage bounced up and down like a yoyo. At first I didn't know what to think but after fiddling with the +tab (swollen end) I realized there was a poor connection inside the cell. Pressure it one way and the voltage was zero, pressure it the other and the v=~3.7. The movement within the cell seemed to better the connection cause it then held a steady 3.74v. So it is not quite as dead as I was anticipating.

How do I drain it? From memory I think your surposed to dunk it in salt water for a day or so.

yer..i read that too.
i had a 3s align 2200 heli lipo with a low cell...i took off the low cell and used the remaining 2s for my t8fg.

i put the low cell in fresh water for 2 days and it was still charged...so i stabbed it loads of times and put it back in for another 2 days...still charged :gasp:.

in the end i had some fun by shorting it out, it still had enough charge to spark and make the tabs glow :intello:....disclaimer....dont do it...it can be dangerous and cause burns...:diablo:...but its fun :yes:
chris