Capcom programmed CPS-2 arcade boards to “suicide” when their original battery failed. The original decrypt key would be lost forever and the boards would die if their original battery died. The technique for decrypting the CPS-2 bios was first researched and documented at the Arcadehacker Blog and made open-source by the team that perfected it. Following this publication, CPS-2 boards can now be restored using a Darksoft CPS-2 kit or various custom solutions, although it is a relatively complex operation.
Based on this same research, new preventative measures for working boards have started to appear, including the new Infinikey-CPS2, which is a simple board that attaches to a CPS-2 B-Board (game ROM board) and writes a decrypt key at boot. This bypasses the battery-backed key and protects boards forever from data loss due to a low or dead battery.
There are two other similar methods, but Infinikey-CPS2 is the simplest.
Arduino Nano Solution: https://shmups.system11.org/viewtopic.php?f=6&t=63389
Undamned Solution: https://twitter.com/therealundamned/status/1050599127929696262
konosuke solution: https://twitter.com/konosuke/status/1050062876617129984