There are many threads on this.... and a few new ones right now, I just responded to another thread:
I had terrible luck with the P2 batteries... and I've been using LiPo's for over 10 years. I am hoping the P3's are better... so far so good for me, but I take extra precautions like:
1) I set the auto discharge to 3 or 5 days max. the default 10 days is too long in my opinion.
2) If available after a flight I put the battery in a cool place until I can recharge it.
3) When I store batteries for longer than a week or so I keep them at 50% and in a cool place.
4) I take them from my cool place after a discharge and charge them when cool.
I will add to this post:
5) I do not go below 20% discharge during flight unless an emergency.
Above I keep referring to "cool"..... meaning my refrigerator or if remote situation my igloo ice chest. I'm not sure I recommend my home refrigerator method 100% as I have read that normal fridge temps (37 to 40 degrees) can cause condensation within the battery pack.... but so far so good for me. I have considered a $99 wine cooler fridge from Home Depot to store my batteries in.... at about 50 degrees.
If you have let your batteries in full charge for more than a month and they did not discharge they could already be toast.
My general perception is that the P2 batteries were total crap... the P3's seem to be better... so far.
This is all from experience, I have no absolute solid data to back up my methods
only the apparent better longevity so far on my P3 batteries. It could be better quality control or some of my methods or both.
Hope this helps.