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I am using lithium polymer batteries without much internal protection, as is common in RC planes and such (weight constraints). I have noticed that some of the older ones feel like they built up some slight pressure or like they have a bit of gas or liquid inside. If one presses rather gently (on the flat side) with a finger, they yield about 1mm (0.04 inches), then one can feel solid material again. The voltage seems fine (cells balanced) and there is at least no noticable loss of capacity (I haven't measured that though).

Is this dangerous? What is the cause for it (aging, abuse, …)? What is the cause from a chemical/physical perspective?

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    \$\begingroup\$ You might want to put them outside on something non-flammable and out of the reach of anyone else while you wait for answers. \$\endgroup\$ Jun 24, 2016 at 20:50

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The common cause of bloating is either age, or abuse (or a combo of the two.) As the cells are used, the anode degrades, producing oxygen and leaving the Lithium and other element in a useless state. In addition the cathode slowly starts to produce metallic lithium. The oxygen produced by the anode bonds to the metallic lithium produced by the cathode, again limiting cell usability, and creating lithium "rust." However, the anode produces oxygen faster then the cathode produces metallic lithium for it to bond to. This means that the oxygen has nowhere to go, and pressurizes the cell, making it bloat out. This is also one of the reasons an overcharged cell will ignite under water without the presence of oxygen and air. Also due to lithium being HIGHLY reactive with water.

I've been working with LiPo batteries on one of my former jobs and we have had quite some 'issues' with exploded or spontaneous ignition of LiPo batteries. Due to these experiences we replaced all bloated batteries directly and had fire and explosion proof containers for the batteries where we stored them in small amounts to prevent any issues or injuries.

I would recommend to stop using them as any current draw can increase the degradation and finally short the cells which is the main cause of the ignition or explosion of these batteries.

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  • \$\begingroup\$ Do you imply that there is a correlation (or even causation) between the presence of molecular oxigen and bad electrodes? If yes, could you elaborate a little, I don't see the connection. \$\endgroup\$
    – caconyrn
    Jun 24, 2016 at 22:00
  • \$\begingroup\$ During the discharging process of the battery the chemical process produces oxygen this is normally binded with lithium, but due to overcharging or to high currents the chemical process is influenced and doesn't bind the oxygen with the lithium but with cathode creating the 'rust'. And leaving free oxygen. Another cause can be contamination with water due to high humidity during the production. \$\endgroup\$ Jun 25, 2016 at 8:15
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A small amount of 'puffing' (1 or 2mm) is nothing to worry about, provided that it wasn't caused by over charging or discharging and the battery is otherwise in good health (cells stay in balance, has normal power and capacity, no physical damage etc.).

Some brands - even some batches of the same brand - may be more prone to puffing than others (I guess it's hard to get the electrolyte formula exactly right) and many will eventually do it as they age. I have a couple of batteries that I bought for a project several years ago and never used that are now slightly puffed.

Bad puffing will cause the plates to separate and reduce performance. It may also indicate that the battery has been seriously abused or has an internal fault, so you should discharge it completely (by eg. leaving it in a bucket of salt water for a few days) and discard it.

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  • \$\begingroup\$ "... and discard it." Surely you mean recycle it? \$\endgroup\$
    – Transistor
    Jun 24, 2016 at 23:00
  • \$\begingroup\$ Yeah, I 'recycle' them by cutting the plug and wires off, then the rest goes in the bin. Lipo batteries are not economic to recycle, and don't contain any toxic waste materials. \$\endgroup\$ Jun 24, 2016 at 23:34
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    \$\begingroup\$ I would recommend NOT to use the salt water bucket principle. You really need to know what you are doing. Even at my former company with more than 10000 LiPo battries a year we even didn't take the risk with the salt water solution. The cause is that the salt water causes corrosion and stops the discharging process before it's finished. \$\endgroup\$ Jun 25, 2016 at 8:23

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