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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 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

simulate this circuit – Schematic created using CircuitLab

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.

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    \$\begingroup\$ Not without a boost converter. \$\endgroup\$ – Brian Drummond Jul 18 '18 at 9:44
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Your circuit won't work ! Current only flows to where having lower potential than the source. So, in your case, the battery will power the solar, rather than vice versa.

The diode in your circuit won't conduct since it is reverse-biased.

If you are willing to charge battery with such low power low voltage solar panel, consider to use boost converter and car battery charger instead.

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Think of voltage as water level in a tank. If you connect a tank siting on the ground that has 4.5 meters of water to one next to it, on the ground, with 12 meters, is the water going to flow from the lower to the higher, without help?

In the same way, the 4.5 V solar panel cannot "push" electricity uphill against the 12 V battery. It is possible to make a power converter to "pump" the 4.5 V up to 12 V, but that is well beyond the scope of your question. Three 4.5 V panels could be put in series to charge the car battery, but continual unregulated charging will dry out the battery, destroying it.

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That doesn't work. The 12V from the car battery will "pull up" the 4.5V from the solar panel. The Diode D1 will block the current from flowing into the 4.5V source (beside some leakage current), so nothing will hapen. If you'd like to use two voltage sources you might want to add a switch to select either one of these two.

schematic

simulate this circuit – Schematic created using CircuitLab

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