Topic: battery on raid card

I'm researching raid cards for a postgres server and have noticed that some have a battery backup.

if the power goes out; is that little battery going to spin all 8 drives to finish writing? 
or does it hold the memory till the system boots again?

All I could find where high level answers such as:
"In the event of a power failure the RAID Card will automatically switch over to the battery back-up and complete any unfinished tasks."

Re: battery on raid card

The battery powers the RAID controller's write cache; once power is restored, the contents of the cache are then written to the disk. The battery can usually power the cache for a day or two, so if extended power outages are expected, it'd be a good idea to just disable the write cache (which will of course lower performance, but safety is often more important than performance).