Wire it directly to the battery and you should not have a problem..Just make sure that you use an inline fuse.
I've got the pigtail for the battery tender that connects directly to the battery. Just not positive how many amps on that inline fuse. Pretty sure it is 7.5 amps.
There is a pretty big range of gear out there as far as how many amps they pull. Just have to be careful or you run the risk of blowing a fuse.