You should never leave the battery fully charged for more than a few days. It's better to even run a little bit of a discharge on it on your charger just to take off the pressure on the cells even if you only take them down to 4.0. Ideally taking them to storage charge.

The battery puffing up may not necessarily have been due to its sitting at full charge you could've overtaxed it causing it to puff up, I've done that with batteries in my MonsterTruck. Do you have a picture of how much it puffed up? A little or a lot?

Regarding the batteries in the off-season you should really cycle the batteries at least every 2 to 3 months. Do a full charge, a full discharge, and then a storage charge. It helps their lifetime. It's a bit of a tedious task to have to do but I do it on my batteries and it has extended the lifetime of many of them longer than I thought.

There are lots of good chargers to choose from. If you're willing to spend $200 spend just a tad more and get this one, I have the earlier version of it ( that did 7 A in and 2 A out ) and it is a phenomenal charger. With that much power to charge and discharge and storage charge and four ports that gives you a lot of flexibility and saves you a lot of time ... Well worth the money in my opinion. Particularly if you have a lot of batteries and don't want to waste a lot of time sitting around. And realistically what's another 50 bucks over the course of 5 to 10 years?

https://www.amainhobbies.com/protek-...k-8517/p488513