John is spot on, and I have been guilty of this in the past.
"leaving whats left after a race" might not be good either, as this might still be too much charge.
Your LiPo battery charger should have a STORAGE level on it, which takes the voltage per cell down to about 3.8v.
Use this if your batteries are going to sit for anything more than about 7 days.
Also, both my VAMPIRE saddle pack pairs swelled up ever so slightly under normal conditions and after only a handful of times racing.
Contacting Vampire about this, they told me that this is completely normal because the cells are big ones (5400mAh). Their advice was to continue to use them, but watch the swelling and if it gets any bigger at all, to contact them for replacement.
...that was over a year ago and they are still going great and never swelled up anymore.
Hope it helps mate
