I have been using a Hobbyking 4S 4200mAH LiFePo4 battery for some time now, but in the last 3-4 months it has started swelling. I wanted to replace it with the same type as LiFePo4 are a more stable battery technology (compared to Li-Po) but I find the LiFePo4 hard to come by lately without spending a lot of money.
I have had the battery for a few years, and always looked after it, and never over discharged it. I did notice that the internal resistance of two of the cells had increased considerably during the last six months, so I thought it was time to retire it.
In theory the battery should have lasted longer. Batteries like to be well looked after in a stable environment, but we cart them around hills in a ruck sack, and through more temperature variations than is probably good for them.
Hobbyking is the cheap end of the market, where these batteries are concerned, and you generally get what you pay for, so this may have some impact.