The Naomi DIMM has a 3 wire battery pack consisting of small PCB and 2 x Panasonic CGR18650H cells. The H is probably some rating, I used CGR18650CF's which have a bit better capacity. Search for CGR18650, or just 18650 and you should be able to source some locally.
The pack is connected like this:
![Image](http://www.sega-naomi.com/dimm/1.jpg)
Open the DIMM and disconnect the battery pack:
![Image](http://www.sega-naomi.com/dimm/2.jpg)
Strip off the outer plastic, and you should end up with this:
![Image](http://www.sega-naomi.com/dimm/3.jpg)
Desolder the PCB, be careful not to short the battery:
![Image](http://www.sega-naomi.com/dimm/4.jpg)
Assemble the new cells with the cell spacer and some electricians tape. Solder the metal part from the old battery pack to the new cells.
![Image](http://www.sega-naomi.com/dimm/5.jpg)
Add some more tape:
![Image](http://www.sega-naomi.com/dimm/6.jpg)
![Image](http://www.sega-naomi.com/dimm/7.jpg)
Solder jumper wires to B-, B+, and BC.
![Image](http://www.sega-naomi.com/dimm/8.jpg)
Tape it up, and you're done:
![Image](http://www.sega-naomi.com/dimm/9.jpg)
Thanks to InvZim of KC for this :smt007