I really got an appreciation of how strong a LiPo battery cell is when LiPo puffing occurs. The above picture shows a mAh cell that had shorted internally during a flight. This one cell experienced thermal runaway, completely puffed out and got very hot! Too hot to touch in fact, and I was sure the entire LiPo pack was going to blow after I landed. That was not the case however and the plastic pouch cell contained all the vaporized electrolyte process called gassing. It was tight as a drum mind you, but no venting or fire occurred.
As was just mentioned, a puffed LiPo cell is caused by gassing of the electrolyte. If you really want to impress your buddies out at the flying field or race track, tell them this process is called "electrolyte decomposition. Electrolyte decomposition is the chemical breakdown of the electrolyte into its primary elements, mostly lithium and oxygen lithium oxide - Li2O. This compound will be deposited on the anodes or cathodes of the cell depending if you are over charging or over discharging.
The puffing happens when excess oxygen in this reaction is also released. If you want a deeper understanding of the chemistry involved in this complex process, here's a good article. It's actually somewhat normal as they age since electrolyte decomposition is occurring all the time. The speed at which it occurs, and if excess O2 or CO2 is released, is based on many factors including how hard you run discharge the packs, how fast you charge them, how you store them, their age, the quality of the packs, and of course how much they heat up during use.
What do think happens when the cell puffs up slightly? Yep, that strong "pressed" lamination bond between all those layers, can be compromised, not to mention the electrolyte has partially broken down. Which in turn increases the internal resistance, which creates more heat while used, which creates even more puffing and can lead to good old thermal runaway! If however the internal resistance has not increased much with that slightly puffed cell, then it's still likely okay to use with relative safely, if you are careful.
As packs age, the swelling can and usually will get worse. Simply put, a battery is made of three things: the anode, the cathode and the electrolyte. The cathode and the anode are the positive and negative terminals on your battery. The electrolyte is a chemical inside the battery that allows charged ions to flow from the anode to the cathode during discharge and the other way during charging.
Electrolyte decomposition is what happens when that electrolyte chemically breaks down. So in a lipo battery, as the electrolyte breaks down you end up with lithium and oxygen. This forms lithium oxide on the anode and cathode depending whether you are charging or discharging. This excess oxygen is part of what causes a battery swell. And oxygen likes to burn. See here for more details.
He also goes over some other reasons a battery might swell. Other gases that can be found in the battery during the normal chemical reactions of a battery are carbon dioxide CO2 and carbon monoxide CO.
For a technical overview of this, see this paper. The proper way to dispose of a swollen lipo battery is the same as what you would do when you throw out any old battery. You need to discharge it completely first.
The two main methods that people use to discharge a battery completely is to hook it up to a light bulb or to put it in a bucket of salt water. There are debates about which method is better but I will avoid that debate here for now. As the name suggests, the electrolyte solution decomposes into individual components.
In most cases, the components contain oxygen. For LiPo batteries, the polymer nature of the electrolyte means that carbon dioxide CO2 and carbon monoxide CO also tend to form as they decompose.
The generation of these gases causes the expansion and swelling of the LiPo battery. In many cases, LiPo batteries have caught fire when connected.
The probability of getting caught up with fire for a LiPo battery is even greater after the onset of electrolytic decomposition because the oxygen-rich gases are highly flammable. One of the things you must remember is that the swelling of batteries cannot be avoided. Decomposition of electrolytes is a natural process, but it should be a slow process. However, improper battery maintenance can speed up this decomposition process.
A swollen battery is likely to occur for various reasons, whatever the cause, it is very much important that you dispose of the battery properly to avoid explosion and any other hazard. LiPo batteries can be disposed of safely with the local waste or battery disposal service, or they may even be thrown into the normal waste bin, provided that they are completely discharged.
The first thing you must do is to discharge the battery to 0V because lithium polymer batteries are safe when they carry no voltage. You must, therefore, ensure that charges cannot buildup subsequently. Leave it connected for about a whole day after the light has stopped glowing. Some people nail or shoot their battery. While it sounds like a fun way to dispose of the battery and even know the level of charge in it, this method is very dangerous and we are not going to discuss it.
Whatever the disposal method you have chosen, ensure you do it outside of your home. This ensures a certain level of safety and is a better precaution. Industrial Battery.
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