I have a psp slim currently running 3.90 M33-3. I installed Dark_Alex's time machine and made my psp have 1.50 by inserting my pandoras battery. Then i fully turned off my psp slim by holding the power button for about 5-6 seconds. Then the psp went back to my 3.90 M33-3 firmware while still running on my pandoras battery. No need for normal battery anymore! (if u just turn off the psp but not fully, it will stay on 1.50) So now, i could run off my pandoras battery. Also, if u don't know and if u have a slim, u hold circle while inserting the pandoras to get 1.50 and u must have 3.90 M33-2 or higher. You must also install the time machine, have 1.50 and 3.40 EBOOTs as well as 360 PSAR.