I found a commercial product that also charges 12v SLAs from a vehicle (cigarette lighter) and was surprised to find no charging circuit at all! It has a direct connection from the cigarette lighter to the batteries, with a 20A fuse. Is this ok? I like the idea as it would certainly reduce the complexity of my design, but it ok to charge batteries this way? If not, what would be a preferred method?
That's fine. That's essentially how the automotive battery in the car itself gets a charge. The voltage in the car's electrical system will hover right around the correct voltage for keeping a 12v lead acid battery topped up. (typically around 14v).