The factory battery is a 3.6vdc Varta NiCad soldered into the bottom side of the daughterboard on the CPU. They have a real memory problem, losing the ability to retain programming info after even the shortest power failure after a few years.
The OEM batteries are available at most electronics stores (even Radio Shack) for $7-12 each. You can find the newer style batteries with Nickle-Metal Hydride cells for about the same price. They get no memory problems.
The soldering process is not particularly difficult, just use a little heat. A 35 watt soldering pencil is more than enough to get the 3 solder points taken care of.