Hello, I was thinking of doing a little project with a arduino wifi board since I already have all the needed components laying around. I was going to put a silicone heating pad on my car's oil pan to warm up the oil for cold starts in order to put less wear on engine components.
I have a small 8ah 12v agm battery laying around so I wanted this to supply the power rather than the car's battery. However, I want the car's battery to charge this battery when the engine is running. It would be easy for me to make a connection from a key on relay to the 2nd battery but obviously it's not good to connect 2 batteries directly with different voltages and the smaller battery max charging current is rather low as well. I'm guessing a diode can limit the power in one direction but how can I limit the current and make this work?