My RAID card does not currently have a battery installed and its write mode is set to write-through. Data is rarely written to the RAID - large batches of data come in maybe every other week. Daily use is read-only. The card hosts 4 different RAID volumes, each being around 12 TB large.
I'm trying to assess the risk of my RAID being unrecoverable if the card fails. Have you ever seen a RAID card turn bad and render the RAID unusable when the card was replaced?
With write-through and a modern file system your drive should always be left in a consistant state on power failure as data is written both to the cache and the disk. It is write-back that requires a BBU because data is written to cache and then to disk.
Obviously it can still be nonsensical or not what you had open depending on the functionality of any open apps.
As people have said in the comments RAID is not a backup, drives do fail in groups under the added load of a rebuild, users do delete data by mistake, etc.