This document explains how to change the lithium battery on a Korg EX-800. I believe it also applies to the Poly-800.
The battery is a 20 mm, 3 V lithium model that's soldered onto the PCB. Since the model number is written on the side that faces the PCB, you can't read it until you've unsoldered the battery. It's a CR2032, a.k.a. DL2032 :
The piece of metal at the bottom is the + tag. It was removed because it masked the part number.
Assuming you want to install a battery holder (which is a good idea anyway), a model that has worked for me is Keystone 106. Here's how it was installed on my unit :
There isn't enough room between the two chips on either side for the battery holder to fit in, so it had to be raised above them. I also had to bend the leads, as the EX-800 PCB has solder pads for the battery, not through-holes.
The nick you see along the bottom edge of the battery holder on the side view is where the tip of the soldering iron got a little too close to the plastic.
This is http://www.teaser.fr/~amajorel/ex800bat/, last updated 2003-10-13.