What's going on inside a puffed cell ?

[properchopper]

OK, I admit I really don't know the answer to this seemingly obvious question. I know that too much heat from over-discharging can cause this, but what electro-chemical process goes on inside ? I have several packs that seem to get the shrink somewhat inflated when warm - usually around 130-135 degrees, then go back to [close to] normal dimensions with the shrink appearing to be a little stretched. Just curious.

[sjslhill]

The seal was broke and it's air inside there......LiPoly cells are under a vacuum. There must also be a reaction with the air that is sucked inside.

If they are puffing, they will fail in time and should be considered very dangerous, take even extra precaution if you are still using them.

[Bill-SOCAL]

I have heard the explanation of a broken seal allowing air inside the cell before. What I do not understand is that if that is the case why do some cells puff, and stay that way?? For instance I have a little 2S packs here that was used for RX power in an airplane. Both cells are puffed significantly and have been that way now for months. I have them in a glass dish sitting in the shop just to see if they ever come down.

If the seal has been lost why are they still puffed??

[AntronX]

Poked around on Google, found best explanation so far:

Lithium cells in general are composed of 20 various chemicals, the formulation of which will vary between each manufacturer. This formulation is of importance for the performance on capacity and the ability to deliver current. I mentioned previously the ability of the cell to deliver current but with any high energy source there has to be a maximum current that it can deliver safely on a continuous basis. Commercially this is a safety rating and it is related to the fact that the cell will not get so hot that the lowest boiling point chemical used in the formulation starts to boil and give off gas ( puffing of the cell pouch). Commercially the maximum current rating of the cell is related to the capacity of the cell and recently it appears that everybody is marketing 20C rated cells .What this rating should mean is that the cell can be discharged at this maximum current from fully charged to fully discharged state without ever getting hotter than 140F and pulling below 3Volts cell voltage.

For example a 2000mah 20C rated pack should be able to be fully discharged at 40amps continuously without ever getting hotter than 140F and pulling below 3V per cell .A 3000mah pack can be discharged at 60amps on the same basis.

If you were to charge a pack and discharge it at its continuous rating for the whole of its lifetime you will get about 25 to 50 cycles of use from it but it will be safe to do so. This temperature of 140F is derived from the lowest boiling point chemical used in the cell in that we assume that if the outside temperature of the pack is 140F then the inside temperature of the pack will be at least 150F, some more margin of 10 to 15 F is allowed to ensure we are well away from that boiling point.

Link: http://www.gbrcaa.org/ntoa/Li%20Poly%20Review%201.htm

[sailr]

Interesting. Thanks! I have found that sometimes my packs appear puffed but it's really just the shrink wrap that has expanded. If you CAREFULLY* punch a small pinhole in the shrinkwrap and squeeze it, it will come back down. It's not the cell that's puffed, it's the wrap.

*Don't punch a hole in the cell!

[properchopper]

Thanks Antron, this was the information I was hoping to find. I read the whole discourse on the link and stand much better informed. Interesting ! The puffing is caused by gasses released by the boiling of the chemicals inside if I've read it correctly. The number of fulll discharge uses was said to be 25 to 50 ; most of us likely don't fully discharge; at least I haven't lately, and many of my packs have lots more "runs" on them than the figures quoted. I have batteries from several different makers, and one brand seems to suffer more from premature mild puffing than others, although they weren't cheapos. I have some low-buck Zippys in use and so far they're holding up fine, and just ordered some Turnigys [ 30/40 C 4S 5000's at $ 62.50 ea. ] for sport running apps. It's interesting that the better batteries have their burst rating at twice the continuous rating, as in : 30/60 C. The Zippys and Turnigys have ratings of 30/40 C. The full article points out that "burst rating" is not an empirically standardized measurement. I wonder if these lower dollar batteries are more honestly rated, or [more likely] are not constructed with better materials. Time will tell I guess.

[sjslhill]

Good question

Maybe it's both Bill, that's just what I was told. However, I read that other information on here and maybe that's the case as well.

I have heard the explanation of a broken seal allowing air inside the cell before. What I do not understand is that if that is the case why do some cells puff, and stay that way?? For instance I have a little 2S packs here that was used for RX power in an airplane. Both cells are puffed significantly and have been that way now for months. I have them in a glass dish sitting in the shop just to see if they ever come down.

If the seal has been lost why are they still puffed??

[sailr]

One way to tell if a cell has been 'breached' or the 'seal' being broken, is the smell. The pack will have a very sweet smell almost like peaches! If you smell this, keep a very close eye on the pack or dispose of it.

[AntronX]

One way to tell if a cell has been 'breached' or the 'seal' being broken, is the smell. The pack will have a very sweet smell almost like peaches! If you smell this, keep a very close eye on the pack or dispose of it.

Great advice. I took one cell apart and that smell filled whole garage. It may be a little toxic because within 30 minutes I had a headache.

[Gary]

I just recently cooked two cells in my saw boat..like on fire cooked LOL...and that smell will not come off your hands or inside the boat , and since it was cooking out on the lake for a few minutes...it was pretty cool by the way.... the chemicals from the batteries stained everything. Even my GPS turned grey . Pretty nasty indeed.