I have an Adaptec 3805 RAID controller and with two arrays without write cache. Now that I have bought the battery module for this controller, can I now enable the write cache on the existing arrays? Or do I need to delete and re-create new arrays with write cache on?
You enable/disable the write cache without any other modifications. Just use the same management software you use to create/delete arrays.
But I think you make a mistake about
The purpose of battery backed-up write cache is to allow the cache to be written on disk next time you power on the system.