Just out of interest, is the "at least 5A" strictly necessary for some reason, or will it just ensure that the battery is charged to a reasonable level before the natural end of your life?
I was told before that 1/10 of the battery's capacity is usually a suitable current to trickle-charge at (though that may specifically apply to NiMH batteries, which is what I was using, I don't know), but that the current could be reduced, it'd just cause slower charging.
Is there a minimum current for charging a car battery? Other than the leakage current of the battery of course