Only way is using pandora's battery and memory stick
If you create the pandora's battery with another homebrew able psp (soft mod) you can change it back into a normal battery
If you open up the battery and lift the pin, I'm not really sure if you can change it back (you'll need to solder but I don't know if it will be ok)
Your best choice is try to find someone with a pandora's battery and memory stick
Or at least someone with a psp that has custom firmware so you can create one.
Another posibility is to buy an unbricking kit out of ebay or a datel tool battery