If you've got the newer ones, you won't need it mate. They've got a voltage regulator in them, and if the voltage drops a bit they go into a power saving mode (tends to activate if you're sitting in traffic for example) - if the voltage gets below a certain point (can't remember the exact number off the top of my head but its in the manual I think) then they just turn off, so you should be okay going straight from the battery!