Is this solar setup flawed?

I have a 12V 10W solar panel that I'd like to use to charge a smartphone and/or a battery pack (5000mAh-15000mAh). I realise it's not a large panel but smartphones aren't that greedy and my use would be moderate.

I was hoping that I could just attach the panel I have to a car lighter receptacle and use a double USB plug that I already have. Is this a sensible arrangement?

The advantages, to me, are that all I'd need is a $5-$10 receptacle and I'd have a very simple setup (no inverter or charge controller, well, I suppose the USB plug would be a controller, perhaps).

Would this system work? Even if it did work would it be an efficient set-up? Would there be any danger of damage to smartphone or panel?

Also: will I need to earth anything or are the devices I charge capable of earthing? Would I need a diode to stop charge flowing back to the panel when there was no sun?

Thanks for reading, I'd really appreciate any input on these questions.

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Should work fine.

faintingcabbage (author)  mpilchfamily2 years ago

It'd better.

iceng2 years ago

I'm concerned if your solar panel has a built in inverse diode.

The diode is necessary to prevent uour battery discharging when a cloud blocks the sun.

Solar panels are just resistors if they are not generating more voltage then the battery !

faintingcabbage (author)  iceng2 years ago

There's a junction box so it'd be in there I suppose. Bit awkward to open it but I'd better...

I would not bother....

A junction box surely means there is an inverse diode inside the system..

faintingcabbage (author)  iceng2 years ago

Phew! That makes life easier. That's the whole point of my cunning scheme of course. Panel+receptacle+plug=entire system.

I was concerned that there'd be diodes and earths everywhere.

iceng2 years ago

Inverters ( switchers ) are not necessary to step-down voltage but they are much more efficient then regulators when used with a 12V 830ma solar panel which may deliver 1.9 amp at 5V.

faintingcabbage (author)  iceng2 years ago

I think all the devices that I would be wanting to charge are happy with DC. A lamp, smartphone and small battery pack.

So I think an inverter isn't needed, hopefully a charge controller isn't either.

Off topic slightly, I have a panel for hiking that is 8W Max Amperage 1.33A, is there a calculation to work out its voltage?

Off topic re your hiking panel:
Watts = Volts x Amps or Volts = Watts / Amps
8W = 6V x 1.33A or 6V = 8W / 1.33A

faintingcabbage (author)  rocketsurgery2 years ago

Thank you.


Keep in mind that using the LM7805 regulator you can only deliver the 830 ma of the solar panel.

The USB plug you have probably is made for voltages ranging between 6 and 14.6V, whereas a typical 12V solar panel, in full sun could be around the 20V mark (this drops with a load and with a length of wire). Test your USB plug output (some cheap ones' output will be relative to your input voltage). When loaded what is the end voltage of your solar panel? Basically, if you want it fool proof then put in a simple regulator (lots of simple designs out there), this will keep your solar panel healthy and keep the working voltage near the 14.6V mark.

Your USB plug may well be able to handle higher voltages and you can certainly "regulate" the solar panel voltage by using an appropriate length of wire (though this is not an efficient use of your energy)... really, just test what you have with your multimeter, add a simple regulator circuit if you want that peace of mind.

You ask about earthing things... voltage is all relative, so make sure your USB plug is working correctly (I would imagine that the voltage between the -ve terminal on the 5V side and the -ve terminal of the 12V side should be zero)... All the best.

You don't think the adapter will present enough of a load to cause the voltage to drop below 12V under full sun?

faintingcabbage (author)  mpilchfamily2 years ago

The USB plug can handle up to 24V, or so it says, you think it'll work, I now think it'll work, anyway, what've I to lose?

I'll just make sure I test it with things that aren't smartphones as a precaution.

Worst case scenario here... 1. The adapter burns out. 2. Not enough power reaches the phones.

faintingcabbage (author)  rocketsurgery2 years ago

The USB plug is of decent quality, it states: Input DC 12-24V Output DC 5V/2.1A.

My two concerns are that I might kill a smartphone or that it simply won't produce the power for my needs due to some inefficiency in my setup.