The 'trio loader' is a 'IPL' (initial Program Loader)
it is a bootblock that allows to boot 3 different 'systems'
That bootblock is written to your Memorystick and is activated when you boot the PSP with a pandora battery.
if your psp is in 'normal state' it will boot your PSP normal from the internal flash.
if you hold the left trigger during the boot it will take you to the Pandora menu. (for fat psp's)
if 'HOLD' is on and the left trigger is held during boot it should take you to the Despertar Cementerio V3. (for slim PSP's)
thanks to the first option you should be able to go to the UPMS installer and make your battery normal again.
(i reccomend to write only the serial!, the backed up one preferably

)