Right now you cannot downgrade your psp. Your firmware requires either a 1.5 firmware psp or any other homebrew enabled psp in order to downgrade. 3.71 official firmware requires a pandora battery to downgrade, so unless you know somebody in the "scene" so to say or buy a special Datel battery, there is absolutely nothing you can do.

Here is the battery

Datel MAX Power TOOL Battery (Downgrade your PSP) on Sony PSP from Gameseek.co.uk - The Webs Favourite Gamestore

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Hey stang dude.

Gosh confusing stuff ha ha, thanks for the link
copied info from site link :
:chef: Special TOOL version of our PSP battery works in exactly the same way as a standard SONY PSP battery but has the ability to put your PSP into service mode for custom firmware use.

TOOL functionality is hard-coded into the battery.
Same 1800mAh Lithium Polymer cell as our standard battery.

Please note: The TOOL functionality is hard-coded to put the PSP into service mode. If i did buy this item stang, would it enable me to fully downgrade my 3.71 PSP ?

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http://xsgfx.com/images/cqs1201750771j.jpg

You would have to have whats called a magic memory stick as well. The battery just puts the psp into service mode so it can boot from a memory stick. This also allows a firmware to be flashed onto the PSP as well. Since the battery doesn't have the storage space needed to hold the files needed to flash with, you need a memory card setup specifically for this purpose.

I also believe that if your already on official 3.71, you can't downgrade to 1.50, but you would have to use the latest files for the magic memory stick which would install 3.71 m33 (they may have extensions available to where 1.50 could be installed on "phat" psp's though). You can use any pro duo card to do this with, even as small as 256 MB. The bad part is, you can only make the magic memory stick from a PSP thats already modded.

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You can downgrade any FAT to 1.50 firmware even if your on 3.71 Official firmware with a Magic memory Stick Pandora Battery/DatelTool battery.

If you have a PSP SLIM then all you can do is add the custom firmware to it.