The battery is empty if your Mac tells you at boot time that the clock is not set to the right time or the Finder occupies all available RAM. (About this Macintosh) A replacement of the standard logic board battery is quite costly. Rayovac batteries are around 10 - 15 Euro plus shipping. I decided to get me a R6 (size AA) battery adaptor for four batteries ( ~ 1 Euro). The adaptor comes with connector like on a 9 V block battery. I simply disassembled an empty 9 V battery to get the appropriate pin plate and solder that to the original cut-off wires of the exhausted battery. Of course, one of the four batteries must be replaced by some metall piece of the same length because the required voltage is 4.5 VDC (equals three R6 batteries). The adaptor can be fixed to the board with the same same velcro pad that can be retrieved from the old battery cube. The entire thing is trivial but works good for me. Picture below. Stay tuned.
PRAM battery replacement: