Purpose of battery in perc5/i controller

[Dmitriy Beloslyudtsev]

What is the purpose of battery in perc5/i scsi raid controller? Let's
imagine we have a write cache in write-back mode and during disk write
operation we lost the power. If the battery is needed to flush the cache
content to the physical disks during this power outage, who will produce the
power to spin the physical disks in internal bays of pe2950? Am I missing
some key concept in here?
Thanks!
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