Cell puffing

At first glance, this seems like a simple question.  And on one level it is.

Probably lots (all?) of us have "squeezed" a pack that looked a little "puffed" when it came out of the heli/plane.   When the pack cools off, it doesn't seem nearly as "puffy" so we hope no damage was done.

I spent some time on the 'net today trying to get an accurate explanation of just what causes "puffing".

What I found was a lot of contradictory information, especially in old posts, i.e. more than 2-3 years ago.  For example, one claim is that puffing is caused by the release of hydrogen gas.  Another claims is it oxygen.

But the simple answer seems to be that puffing is caused by the abnormal release of some kind of "gas" inside the sealed pouch that surrounds each lipo cell.  As with any gas inside in sealed container, as the gas heats up the pressure on the container will increase.  If the container is a flexible polymer as in plane/heli lipos, it will expand.  But the cell can't puff up without also "expanding" the tough heatshrink that surrounds the whole lipo.

Once released, the "gas" is never reabsorbed.  But when the cells cools off, the pressure from the gas decreases and the pouch also contracts.  But the heatshrink doesn't contract, or at least not as much.  So when we sqeeze the cooled down battery it feels more like the heatshrink has "puffed" not the cell.  However, once gas has been released into the pouch,  it will expand as soon as the cell heats up, whether due to an increase an ambient temp or use of the battery.

There did seem to be a consistent view that lipo puffing has four basic causes:

improper manufacturing that lets water or metal contaminants into the cells before they are sealed;

physical damage to the cell which results in some parts of the cell not working, causing other parts to heat up too much;

over-discharging the cell, which causes permanent damage either by drawing too much current, even for a very short time,  or causing the voltage in the cell to get too low;

over-charging the cell, leading either to immediate explosion or fire, or damage that causes the cell to get too hot when it is used again.

The actual chemistry involved in puffing would seem logically to depend on the manufacturing methods and chemical formulations used for a given cell.  These change often.  For example, now we are seeing the "nano" cell structure.

Is is safe to use a puffed lipo?  The "safe" answer is "No".  But the relative degree of "unsafeness" (i.e. danger) may depend on how the battery was manufactured.  For example, does it use a "gelled" electrolyte (safer) ?  The danger is that the pressure of the gas will get so great the surrounding pouch will rupture.  Then the gas will come out.  What happens then?  That is not clear.  Will the pouch fail "explosively" like a balloon popping or will it just be a slow leak?  Will the  cell catch on fire spontaneously or does there have to be something to get a fire started?  It may be relatively safe  to use a slightly puffed lipo in conditions that don't demand much power, e.g. hovering around vs. 3D.

I hope this information is of some use.

Happy flying!

Storeing LiPo's (for longer times), is best at 60% charge, and keept cold.

I keep the lipos I'm not going to be using soon in the refrigerator inside a plastic bag.

I should  make a clarification (or correction) -

A lipo cell has a "natural" voltage of 3.7 volts.  Thus the voltage for a 3S pack, i.e. 3 cells wired in series, is 11.1 volts. 

A lipo cell can be charged to a "do not exceed" voltage of 4.2 volts per cell.  So, again for a 3S pack, the maximum voltage is 12.6.

Lipo cells should never be discharged below 3 volts per cell. 

I use a TURNIGY Accucell-8150 charger.  It has a "LIPO Storage" function which will either charge or discharge (whichever is necessary) to reach a nominal voltage of 3.85 volts per cell.  Thus 3.85 volts per cell is the value Turnigy uses as best for long term lipo storage.

Using 4.2 volts per cell as "fully charged", then the 3.85 volt "storage charge" is about 91.7% of full charge.

I don't know how Blind Vision calculated 60% of charge as a good storage value.  I just repeated that value in my post without checking it.

But 60% of a full charge of 4.2 volts per cell gives 2.52 volts per cell.  This is way too low and below the "absolute" minimum of 3 volts per cell.

Now, finally, to your question:

What you could do is charge your packs to about 3.85 volts per cell and then fully charge them just before you go flying.

If you put the packs in the refrigerator one problem will be that you have to let the packs fully warm up to ambient temperature before charging them to 4.2 volts per cell.  If the batteries are not up to room temperature then fully charging them could cause damage because a cold lipo cannot take as much charge as a warm lipo.  Having said that, I don't know just how "warm" a pack has to be to accept a 4.2 volt per cell charge without damage.

I don't think you would gain much in overall battery life by keeping the packs in the refrigerator if you are going to be using them every week.