They should be powered on if you want to retain data on them long-term. The controller should automatically check physical integrity and disable bad sections as needed.

I’m not sure if just connecting them to power would be enough for the controller to run error correction, or if they need to be connected to a computer. That might be model specific.

What server OS are you using? Are you already using some SSDs for cache drives?

Any backup is better than no backup, but SSDs are really not a good choice for long-term cold storage. You’ll probably get tired of manually plugging them in to check integrity and update the backups pretty fast.