I have two blue rope lights that run at 290-400 mA @ 12Vdc. How long could they run for on a 12v 45aH car battery?

 

Something like 56 to 77 hours depending on how much current you're drawing.

 

It depends upon how long you want the battery to last.

Automotive batteries don't last long at all if you discharge them deeply without recharging them right away.

You could probably get away with a fair number of cycles if you only discharged it 10%, and then promptly charged it back up.

45AH / 400mA * 10% = 11.25 hours, or around 112.5 hours if you weren't planning on using the battery again.

 

i think ill just wire it to the battery in my car