I live completely off grid, and have managed to get my house connected to a battery bank through an inverter, get DC water pumps on a separate circuit, get everything on two separate solar set ups, and now have a slightly more technical project I need to figure out how to get started on.

I need to make a voltage sensing cut-out circuit.

Background, I have two battery power systems, one at 12VDC, the other at 48VDC. The 48VDC system only has solar and runs the "mains" power, while the 12VDC has a small solar and wind combo (gifted wind, else I'd have bought 48VDC wind and saved myself a LOT of headache). Given the sun only shines about half the time, but the wind blows around here nearly constantly I bought myself a step-up converter, connected it all in, and promptly drained my 12VDC nearly dry. Nearly constantly wasn't constantly enough...

Hokay, next idea, hook in a relay to cut the converter out when the 12VDC batteries get too low, and connect the relay electromagnet to the solar charge and load controller. And it works! No more over-drained 12VDC batteries! But since it also controls the rest of the loads it cuts off my water pump if the charge gets to the cut off point, and there appears to be a lockout that won't allow ANY load until the controller senses input on the solar line... Less than ideal, especially if the wind gets the batteries back up above cut-off at say 0200 and it's still very much dark out...

So, I need a new cut-out sensor. It has to cut the relay off at a higher voltage than the original controller does, also lock-out to avoid on-load/off-load voltage drop/spiking causing bouncing on-off cycles, but not be reliant on actual inputs to determine if it should unlock, just absolute voltage.

Which is where resources come in, and my question. What is the best way to learn circuit design fundamentals online? A YT channel, a site for the guy with "Wimpy Pockets", books I might be able to get from a library, anything that teaches more than rote "this circuit does that" type stuff.

Also, what's the best source for components like breadboard and the various resistors, diodes, capacitors, and inductors I'm likely to need for this project? It'll be tight, but a couple bucks on this and I can save myself a couple hundred on more panels and batteries that still won't charge at night.