Doesn't work on batteries. Works pluged in.

Unfortunately, the written instructions are in Japanese and the English section of the manual doesn’t show how to open it etc but the pictures are self explanatory.

You will need a soldering iron to remove the batteries from the board.

I found an old Ebay listing which has an image of the batteries where the specifications and dimensions of the battery can be seen and they appear to be standard NiMH 1.2V AAA 800mAH batteries but they have solder tabs. Check the specifications on the batteries that are there to confirm that they are the same

(click on image to enlarge fro better viewing)

When searching for replacements -example only ensure that you get batteries with solder tabs as you would have to heat the battery far too much to solder them back in if you tried to solder a standard AAA battery without the solder tabs. This could be dangerous as the battery may catch fire or explode if heated too much.

Just search for NiMH AAA 800mAH battery with solder tab to find suppliers that suit you. You only need two of then.

Note: Check the polarity of the old batteries before removing them i.e. which is the +ve end and the -ve end - it will be marked on the battery itself, as the two batteries are not connected on the board in the same direction. The last image in the instructions show the the +ve ends are at different ends to each other. Reconnect the replacement batteries back in the same way. Take pictures before removing the old batteries to help remind you when you go to solder the new ones in.

Hopefully this is of some help