1st off, you really want 800+kg payload for a 400l tank. As for battery, I wouldn't bother with a separate one, I have been using a 500l diy system in my transit connect for over 3.5 years now, with the pump controller wired to the van battery. Replaced the battery earlier this year as it was getting a bit tired, but otherwise, no problems whatsoever, and I have some places where I am parked up half the day cleaning.