There is no direct bus from Battambang to Sihanoukville; you have to go through Phnom Penh. You can probably do this in one day by taking an early morning bus to Phnom Penh and then the afternoon bus from PP to Sihanoukville, but it will be a long day and unless you can buy both tickets in advance for one company (i.e. the company runs both routes as Mekong Express does from Siem Reap through to Sihanoukville) you are not guarenteed to make your connection.
I wouldn't want to be on buses for this long, but if you are planning on seeing Phnom Penh seperately after Sihanoukville and are short on days then it is worth the push.
I spoke with someone I know living in PP, their advice was overnight in PP at the halfway point and continue early the next morning down to Sihanoukville.
#5 Ashe88 has been a member since 19/11/2007. Posts: 24
Yes, defo worth breaking the trip in P Penh, otherwise makes for a VERY long day on not the best of buses ;-)
#6 steveatkins has been a member since 15/6/2007. Posts: 33