this is an interesting part. I need to learn more about these. They are pretty expensive, but so are 4 decent solenoids. I will read more on them. My first concern is whether or not they can handle continuous duty. They may only be built for 10 secs or less at a time. (just like lawnmower solenoids)#1 I didn't look over your schematic but I have an idea about an off the shelf piece. Some diesel-powered trucks have starters that run off 24volt, to accomplish this they use a "series/parallel" switch. Check it out maybe it will help.