www.span.com :D
there 2 ways that the power losses.. 1 way i dont know how to say it in english... but i will try to express it in my own way... the power goes up and then dives down like eg if ur power socket ensure in stady way 220volts for miliseconds goes to 260volts then to 175Volts and then 220volts that power goes in to ur psu and after psu inside ur pc. the second way is when the power goes from 220 stady volts to 175volts and then goes up to the 260 volts. the second way for me is more more more dangerous couse the loss of the power reduse from minimum to max and that gap is very painfull to electronics.
ALL serius RAID controllers have bbu's. when u work ur data for seconds are on the cache of the controller those 512mb or more that the controller have. if the power loss hit u in that moment ur data stays in the controllers cache for 72 hours top. thats what u pay for the bbu. For a home user maybe this bbu is useless but for enteprises is gold.
Now for home users the only benefit that the bbu stands is for Write Cache Back. if u dont have the bbu u cant enable it. ;)