the new p410 has flash based backup - no battery.
the battery is good for 0 to 72 hours depending on age and charge state. there are many features that can not be done (sanely) without a (flash/battery) backed write cache. like restriping (block size), raid migration, raid expansion. you can convert raid-10 to raid-5 while running safely, including running out of power with the BBWC/FBWC. the flash based is brand new technology. up to 1gb. wonder if its a bunch of slc chips with a capacitor.
A nice 750 to 1000 watt apc with apcupsd can shutdown your file server/htpc gracefully for sure. it can command many pc's to shutdown via network as well (apcupsd is open source and runs on *nix/osx/windows*).
I can't imagine why you'd skimp on a $50-100 battery back up when building a nice raid system that is insane.
Benefit of the hp smartarray controllers if they work with your mobo is they use the same CCISSM driver for like that last ??? 10 years ??? and will continue to do so. e200,p400,p410,etc. stable - linux (think vmware esx) especially.
nothing sucks more than to upgrade to a new generation pc/raid controller and not have the option of just swapping the drives over.
i've had dell do this trick on me because of a failed 1U rack server. they brought out a whole new server - fancy - new raid controller- not compatible with old raid controller format - had to rebuild the freebsd box. most people that have used freebsd know you could stuff the image/disks in any hardware and it would just boot and work.
BBWC - for sure
APC backup with usb cable - for sure
small costs to do it right