Phantasy Star for Sega Master System on Mega Drive with adapter
After the incident with the corrupted saves a few months ago I did a few tests by erasing the corrupt save slots, clean the contacts again and try to save a few times. It seems the battery isn't the problem, it was probably a bad contact that corrupted the SRAM data. So, as I'm not currently playing anything else on the console, I've decided to not remove the cartridge when turning off the console until I've finished the game.
I've finally completed my party, the only annoying thing is that while my party was more ore less level 14/15 when I was finally able to get to Noah relatively unscathed, Noah was still level 1 (which translates to having an extremely weak mage with only one spell) so I'll have to grind leeches on Motavia for a bit before moving on.