I would like to add a 4.5V 1.5W solar panel to supplement an existing electrical setup in my vehicle. My current setup is an Arduino and other electronics wired up to an "add a fuse" in the car fuse box. Aim is for the solar panel to provide power to the electronics and maybe top up car battery in sunlight and car battery to provide power in the dark. 

I have read other questions on the StackExchange about placing differing voltage sources in parallel and understand that there would be a reverse current into the solar panel from the 12V car battery. But I could not find a good explanation of what would happen when using a diode in front of the lower voltage source. 

If I place a diode such as [1N5400](https://www.diodes.com/assets/Datasheets/ds28007.pdf) rated 50V 3A in series in front of the lower voltage source, will the diode cause the current to only flow away from the lower voltage source and the lower voltage source would either:

1. supplement power to the load?
2. charge the higher voltage source (car battery) if there was any extra power?
3. diode absorbs the reverse current forever since the car battery has a higher voltage? In this case would a zener diode make any difference?

In the diagram below V1 would be car battery and V2 would be the solar panel.

![schematic](https://i.stack.imgur.com/1k7vx.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2f1k7vx.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

If this is realistic design, are there any protection circuits I need to prevent overcharging with such a low voltage solar panel? Thanks for the help.