OK, I know the subject of battery swelling or puffiness is controversial and there are different opinions about whether it indicates a bad battery or not. This is my opinion, based on several years of using LiPo batteries in my drones as well as other RC models - feel free to have a different opinion.
First of all, the term "swelling" is very subjective and doesn't necessarily indicate reduced performance. A slight "soft" puffing of the wrap is what I am talking about, however, if the swelling is very pronounced so that it protrudes beyond the case dimensions and/or is firm, that may be cause for concern.
I currently have 4 original Yuneec Typhoon H batteries and each has a minor degree of "swelling", and all are performing basically as new. One of these batteries, actually #1 was unfortunately in my first H480 when in April 2017, it had what I call a UFIT or Uncontrolled Flight Into Terrain. It was caused by a corrupt ST16 with zero stick control and would not land, so it just kept flying until the battery had nothing left.
It did not go into auto land as the controller signal was locked in a full throttle toilet bowl up to 1008' AGL, but nearly in the same location above me. So, finally the props stopped and it came down very hard from about 300' into the corn field about 400' away with expected results. As I saw it gradually loose power and start to descend, I briefly thought I could catch it, but then remembering the gravitational constant, realized that would have been a fatal mistake.
Anyway, I ran to the crash site, the H beeping and dim lights flashing, pulled the battery. I sent in the telemetry to Yuneec and they said to send the remains of the H and the ST16 without H batteries to the repair centre. Yuneec covered the incident 100% and sent me a new H480 Pro complete kit. So, I had the depleted battery (#1) and the second battery in hand when I got the fully replaced H480 Pro kit.
Looking at the telemetry, the crash battery (#1) had been drawn down to 8.8volts, but bounced back to a measured 12.5v. The battery had minor swelling and I thought I would try to "revive" it. (I am not going to say how this is done because I don't recommend it and it doesn't always work). In this case it worked and was brought back to about the original performance.
That battery has been flown for 9 flights since that incident and the last flight was over 16 minutes a week ago. I constantly monitor the charge data on all batteries including each cell data. This battery is still good, but the key is monitoring the health of the battery. Other batteries of mine have slight puffiness but are still healthy. Minor swelling or puffiness is not necessarily an indication of a bad battery, you need to get the true battery health from a good charger.
As has been said many times on this forum, one of the most important thing you can do for your batteries is to always put them in "storage" mode after a flight, to do this get a good multi-chemistry, multi-function balance charger that has at a minimum; charge, discharge balance and storage functions.
There are many on the market, I use a few but my favorite is an older but very high function Hyperion 720i EOS Super Duo 3. It can report all the charge data of a battery, including mah input, peak volts, volts/cell, battery IR, and IR/cell. It has 20 customizable programs.
Sorry for the long post, and I realize this subject is controversial, but that is my experience. Here is a photo of the charger, #1 "crashed" H battery and a second #3 which has never been over discharged.
