During the battery learn cycle, the virtual disk controller cache setting is set to write-through on a low battery charge. Additionally, if the battery is detected to be in a low charge state, write-through is employed. http://support.dell.com/support/edocs/storage/RAID/PERC6/en/PDF/en_ug.pdf => pages 31 and 33.

If transitioned from write-back to write-through because of a low battery charge, the controller should flush the cache and write data that had not been committed to disk yet. If a power outage occurs during this time, data not flushed from the controller cache could be lost since it has not have been committed to disk while under write-back.

Based on the above condition, the cache policy should revert back to write-back once the controller battery is detected as present and good.

The following is a response from a Dell PG RAID engineer:
"The controller actively monitors the battery charge and state. If the battery fails or is detected not to have enough charge to backup cache (upon a power failure scenario) the controller will switch VDs running in Write-Back mode to Write-Through. Upon detection of the condition, the controller flushes any data in cache to the disk as it performs the transition. If the fail condition is detected by the controller to be resolved later, it will automatically restore the VDs to Write-Back."