Now that I have an extra backup of my Mega-CD save files on the Sega CD Ultra Backup RAM cart I can replace the battery of my the old CD BackUp RAM Cart without worrying about data loss.
It is possible to do an open heart surgery to prevent the memory from being erased and replace the battery while the Mega-CD is turned on . I went for that procedure to avoid having to copy all files from one cart to another again.
Two screws are hidden behind the label on the back, try to be careful when lifting the edges of the label (I wasn’t):
The battery is a CR2450 3V Lithium cell. As space is very limited a cell holder won’t fit, probably. Use a cell with solder tabs instead.
To prevent cart from moving in the slot (e.g. when using a desoldering pump) secure it with a piece of polystyrene or rubber.
Old battery removed and system status checked: still running, save files are still there – it’s time to clean up the pcb holes and solder the new battery.
When the new battery is in place turn off the Mega-CD and inspect the three solder points, add more solder if required.
Final check and a reminder to be careful with the back label: