After modding my Sega Saturn to use AA Panasonic Eneloops, I found a guide where someone modded their Dreamcast with rechargeable AAA batteries. The Dreamcast uses a rechargeable 3V lithium battery; I’ve seen conflicting sources on the exact type of battery. I’ve read it can either an ML2430, ML2032, or LIR2032. Since my Dreamcast battery seemed to be dead, I decided to do the same mod but with AAA Panasonic Eneloops. They have a minimum capacity of 750mAh which is 7.5x the capacity of the ML2430 mentioned above. It will be interesting to see if the Eneloops stay topped off in the Dreamcast; assuming the Dreamcast uses the ML2430 and charges at a rate of 1C, it would take about 7.5 hours for the Dreamcast to fully charge the Eneloops.
You can see below I used a 2 AAA battery holder instead of taping and soldering them together like in the guide.