downgrading a slim with a fat pandora

ok so my little cousin got a white slim psp. its version 3.72 and uses 3.6v 1200mAh battery (which is thinner and seems smaller?) and im trying to downgrade it for him using the pandora's battery made by my own psp (already on a custom fw, black fat psp) i just tried to put my fat 3.6v 1800mAh battery into his slim psp battery slot and it kinda got stuck and so i had to force it out. im just worried that if using a fat battery on a slim psp might ruin it somehow (i dont know much about electronics stuff but he worked pretty hard to pay for it so i dont wanna screw him over)
i just need some response as to if its ok to use a fat battery on a slim one, if 3.72 slim can be downgraded and have custom fw installed on it. thanks for your time and sorry for not using caps..

You can use a phat battery in a slim psp, but the cover won't close on it. And a Slim can be downgraded and hav CFW installed. It is about the same technique as the Pandora's Battery thing you have tried I'm guessing. You just need different files on the memory stick to change the PSP directly from 3.72 to 3.71 M33. Try this tutorial http://www.pspbrew.com/forum/viewtopic.php?t=10137

thanks for the quick response there and the link but i think ill just stick to pandora's battery method since ive done it twice without screwing up

On a slim psp? The slim psp requires different files to downgrade than that of the phat psp pandora. The Battery is the same, but they files you need are different.

Any chance of homebrewing on a slim 3.72/3.73 without a fat battery in the near future? Do you need it for the Lumines bug?

The Lumines just works on 3.50 . And I'm not sure about the slim battery.