I have zippy 4000mah 20C 3S cell for my quad. Gives a good 12 mins or around flight time with full charge. Its been 2 weeks, since i have been nicely flying my quad. Before 2 weeks it was more or less testing. It was never a flight of more than 2 or 3 mins.

Lately i have noticed that after using my battery it gets puffed up and pretty hot too. But this happens only when the the low voltage alarm starts beeping. I check the voltage and its around 3.60 or 3.63 per cell. The puff stays on for sometime but after 5 mins or so the puff goes away. Then i charge it and use it again. Now i want to know is why my battery is getting puffed up?

My quad weights 1500 grams

i am using DT750 with F20A ESC combo with 11 inc props. My quad lifts off at 25%-30% of throttle

Looks like over discharge. See this thread, about the 80% rule.

http://www.rcindia.org/batteries-and-chargers/taking-care-of-lipos/msg194/#msg194

Seems like you already were part of this discussion earlier :

http://www.rcindia.org/electric-power/how-to-calculate-fly-time/

The thing to note is, once the alarm goes off, you may still take some time to land the craft, and the rate of discharge is sort of quick after you hit the 3.6 volts per cell etc.  If you over discharged it EVEN ONCE, then you will most likely see the exact symptoms that you are seeing.

Hmm yes 2 times when i was flying, the alarm started beeping and i continued to fly it still for 10-15 seconds but then i landed and stopped it. And i always after my flying check the voltage. And i am pretty sure that it has never gone below 3.6V, still the the symptoms are exactly same as you have pointed so i think i might have ignored it sometime. I will take care of that in future flights.

Now the other question: "Using this battery is dangerous now? can it be harmful in future?"

Today while flying my quad crashed, not major damage. Just one of the landing leg was broken and landed on one corner of the battery. Please check the attached image and tell me is that battery good? i would put some tape around it and re-use it, i hope there i no problem with this?

Anwar bhai, in this post http://www.rcindia.org/batteries-and-chargers/taking-care-of-lipos/msg194/ you have said

Quote from: anwar on March 23, 2009, 09:32:34 PMUse of the timers in your tx radios

How do you use timers?? can you help me with some articles?

I read the manuals and got to know how to use the timers, thanks Anwar for pointing at them

Please somebody help me with this

Quote from: satyagupta on February 24, 2013, 03:10:03 PM
Today while flying my quad crashed, not major damage. Just one of the landing leg was broken and landed on one corner of the battery. Please check the attached image and tell me is that battery good? i would put some tape around it and re-use it, i hope there i no problem with this?

Satya,

is there leakage? if no then i guess you can apply good tape and it will work fine.

I tried it on one of my battery.

No leakage..