JD-800 diagnostic mode:

here you can find out your ROM version

1. choose the MULTI-MODE 
2. press LEFT CURSOR, RIGHT CURSOR and EXIT at the same time

JD-800 test-mode:

to choose the different tests press EXIT and a BANK or NUMBER button

EXIT + BANK 1 = RAM test 
EXIT + BANK 2 = RAMCARD test 
EXIT + BANK 3 = test of the 3 PCM ROMS 
EXIT + BANK 4 = PCM CARD test 
EXIT + BANK 5 = test of all buttons
EXIT + BANK 6 = test of all LED's , move the pitch-bender to speed up or slow down 
EXIT + BANK 7 = test of the faders 
EXIT + BANK 8 = test of the waveform fader 
EXIT + Number 1 = test of the Keys, the pichtbender and the mod-wheel
EXIT + Number 2 = test of hold and expression pedal 
EXIT + Number 3 = test of the keys and velocity 
EXIT + Number 4 = test of the midi connection
EXIT + Number 6 = test of the effects 
EXIT + Number 7 = DAC test
EXIT + Number 8 = back to the normal MULTI-MODE

Initializing - Factory Reset

1. Press DATA TRANSFER

2. Use the cursor buttons to select (FACTORY RESET)
3. Choose YES

The battery:

CR2032 at the back middle of the main board