You can perform an upgrade on the SRAM and battery module, upgrading it to an FRAM chip, meaning you will no longer require the coin cell battery to installed to maintain saves.

The original SRAM is located by the coin cell and labelled typically MB8464A. It is a 64kb Static RAM requiring battery backup.

The process is simple:

The new FRAM module pinout allows for bank-switching and 256kb of memory.

You can see the pins are identical except A14 is N/C on original SRAM, and A13 is CS2 (chip select 2) on original SRAM.

Lift pins 1 (A14) and 26 (A13) and solder to GND (VSS) or 5V (VDD) to fix it to one bank if you have no plans to do bank switching.

You can wire these 2 pins to switches that go between ground or 5V and have 4 full banks of memory to choose from, or if you want to just have a fixed single 64kb bank of FRAM like the original, lift pin 1 (A14) and pin 26 (A13) in the air on the FRAM and solder them to a ground pad.

The best way I found to test FRAM is with the Mega CD's own Internal RAM tool, and Sonic CD.

Turn on the console without a game in and go to Options.

You should be presenting with at least the first option (Format Internal RAM).

In order the menu goes

The Mega CD 2, unlike the Mega CD, will not allow you to format the memory without a game in. But also, you need to first start the console without a game in.

Once in the CD player menu, then insert a game. This will unlock the CD-ROM and Memory options. Now you can click Memory.

Go to the Options menu and select the first option. Confirm to format the memory, and then click Exit.

To confirm to format, unplug the power, leave it 5 minutes, then plug it all back in and turn on again.

Go back to the Options and confirm it states the format is sega_cd_rom.

Another test either before or after this is to use Sonic CD, Road Rash or Lunar which all use the internal RAM and will complain if it is not formatted or working.

If you get this message, format the RAM as mentioned above. If you still get it, your SRAM/FRAM is bad.

After this, start Sonic CD and play a game through a full zone (3 levels for the first Zone).

Die by losing all your lives or restart the console to get back to the main menu. You should now see Continue as an option.

Turn off the console, unplug all power, wait 5 minutes and see if the Continue option is still there. If so, the FRAM is working.

If you keep pressing left or right, you should also see RAM data.

Inside RAM data it will appear blank, as by default Sonic saves in position 0 for your current game.

If you want to copy the current game and back it up to load back in at a later date, go to RAM Data, select DATA, then Copy, then give it a number above 0.

Turn off the console, unplug all power and then turn it back on 5 minutes later and confirm your saves are still there.

To play a copy of the game again just copy the game file and change its number to 0.