Yes OperaHouse. This is an extension of the current limiter circuit. In the boat, I have a VSR that kicks in at 13.2v, that in turn allows the battery to battery charging.
What I am doing with this version, is to eliminate the VSR, but combine it's voltage trigger function with the current limiter circuit to keep a house battery in my truck. I have the circuit working like I want. There are a lot of smart people out there, and I was hoping some one would know the formula for Zener with Voltage Divider. I can get close with R1/R2 + Zener Voltage when R2 less than 1K, but that isn't totally right.
My Truck battery at rest is around 12.8v. When I start the truck, the alternator kicks in at around 14.3v, and winds down shortly afterwords to 14.1v. So I set a trigger voltage of 13.4v, which will kick in when I start my truck. This will charge my house battery. When the truck is off, the start battery voltage will drop off below 13.4v and turn off the circuit. I don't want there to be a connection between the start battery and the house battery unless the truck is running. I don't want to drain the start battery while the truck is not running. And since I am not constantly using the house battery, I want to limit current and voltage to the house battery to keep from destroying it. The house battery is more like a standby battery. Little used but always charged and ready.
There is a commercial towed battery to battery charger for $55 that I could buy:
https://rvibrake.com/products/towed-battery-charger?variant=8804388741&_vsrefdom=adwords&gclid=EAIaIQobChMItK7vvIDY3gIVxbXACh0WqAdmEAQYASABEgJ1YfD_BwEBut it is more fun to make one that works.