I recently found my Turnigy 1000mAh 3 Cell 11.1V Li-Po battery is inflated / swollen, as the picture attached shown.

Left: normal battery
Right: swollen battery

They are the same kind!

By touching the surface, it feels resilient...It might be some gas trapped in there, might be something else. I noticed it after I took it off from my quad, so it was actually discharging normally. And the voltmeter reads normal value. Is this a normal phenomenon or a dangerous indication?

If you have any clue or know any old discussion over this, please instruct/direct me! Thank you a lot!

This is usually caused by discharging at too high a rate and it is evidence of battery damage. If you keep using it, keep an eye on the performance. This condition is not reversible.

Watch it especially during charging. Don't go away and leave it charging.

That advice goes for ALL Lipos. Never charge unattended - and invest in a fire resistant container.

Swelling is sometimes called puffing. A Search for puffing in this Forum gives about 600 threads. Very common and something you should become educated on. A good read on Lipo battery care is here: http://www.tjinguytech.com/charging-how-tos

As has been said, it's reasonably common. Puffing indicates that the battery has sustained some damage but does not necessarily mean you shouldn't keep using it. I know guys who have kept flying mildly puffed packs for years.

Even I do so, tho, in lower current draw models to make most of them. I still use few slightly puffed up packs which are 6-7 years old, still serving the purpose.
I also know someone who got his way of treating it, "tho I definitely wouldn't recommend anyone doing it!!!!" He said to me that he punched a little hole in the swollen pack and put it in the vice to squize it out! Oughch!!!!

Originally Posted by jofro

Even I do so, tho, in lower current draw models to make most of them. I still use few slightly puffed up packs which are 6-7 years old, still serving the purpose.
I also know someone who got his way of treating it, "tho I definitely wouldn't recommend anyone doing it!!!!" He said to me that he punched a little hole in the swollen pack and put it in the vice to squize it out! Oughch!!!!

Sounds like a recipie for disaster, risks shorting internally i would think.

I agree, most of local people I know using them even keep them inside their houses and garages thinking they are just little toys.