The Real Jdbye
That seems unlikely. It won't swell just by being left in storage, it swells when you try to charge it up, or potentially at some point after charging it up (like when you're using it) as charging a faulty battery (when it goes below 2.5V it's considered faulty per the manufacturer's datasheet, they all say 2.5V) accelerates the damage. That's been my experience every time.I hope the OP isn't upset that I hijacked his thread, but I'd like to add something, for the sake of accuracy.
Most of the online testimonials report the case of the battery left inside the PSP unused for a long time, and then found with the lid broken and the battery already swollen.
I've never heard of it swelling afterwards, during charging: obviously in this case it was already swelling, but it wasn't visible yet...