Solar Powered cell phone charger

[JerryJ]

I just built a solar powered cell phone charger for my Android phone (Samsung in this case)

I tried a few different methods using non-conventional thinking. Number one was to let the cell phone charge drop to about 20%. That way I could watch any steady progress.
Most consumer solar panels deliver 12VDC on the outputs. Some commercial grade panels can deliver much higher OP voltage.
I just wanted to keep my project short and simple. My inspiration was for camping and backpacking. Obviously most of us have access to electrical sites when we pull our TT into its spot. But this was only a fun project.

I started off with a smaller 2.5 Watt Coleman panel and hooked it up directly to my cigar USB adapter. There was too much demand from the regulator to provide anything substantial to the cell phone. The panel is only 2.5 watts.

Next I hooked up the same 2.5 watt solar panel to in parallel to a 12 volt, 7.2 Ah SLA battery. Then I used the USB 12volt cigarette adapters after (post) the SLA battery and then to the cell phone. It worked great. The cell phone was charging.

Ok, for my needs, the SLA battery was way too big and too heavy and the USB adapter to awkward to deal with.

First downsize the battery to 10x 12 AA rechargeable batteries (an 8 pack and a 2 pack)
I still fed the solar to the AA pack in parallel
Then I wired a positive lead the Vi of the 12VDC to 5VDC voltage regulator. The Vo of the regulator goes to the positive pin of a panel mount USB female port.
The center pin of the regulator goes to the ground of the battery circuit AND to the ground of the USB female connector. I also installed a toggle switch between ground of the powering and ground of the USB connector.

I allowed the 2.5 watt panel to fully charge the 10 AA batteries to about 13.8 volts. For 1.25 volt batteries that's more than required (Don't use alkaline batteries, they're not designed to take charges in this sense)

After the AAs were fully charged and keeping my volt meter constantly monitoring input voltage. I then plugged in my cell phone and had it on the battery charge mode so I could watch the progress. Again, everything worked 100%. In case anything went wrong I could toggle the circuit OFF.

I soon realized that the 2.5 watt solar panel did not have enough energy to keep up with the constantly draining AA pack. The voltage regulator got extremely hot (especially when touched) Ya I know...
I could not get the cell phone charge passed 89%. It turns out that too much energy was lost due to the regulator itself. I also added a proper heat sink to the regulator to protect things. and my stupid fingers !!!
I then borrowed my son's 40 watt solar panel.

The cell phone charged at a great rate, near to that of my wall charger.

Now, on the common sense side the 40watt solar panel is too big to take back packing but is ok for camping. even though my TT has more than enough 120vac outlets

So, your question would be...what's the sense?

I built this because I could and it was fun as a I learned a few things along the way.

I also know there are new foldup solar panels for back packing and they have USB ports for cell phones. Even with full sun there's only so much energy that can be captured. It's a great project to show the kids and grandkids to see how solar can work for them.
As an FYI while we know that these cell phones only take a +5vdc charge and so does an Apple iPhone. I heard it said that the center 2 pins on the USB have to be hooked to the positive using a 100 ohm resistor for each port. Do your own research because I only heard that from 2 youtube channels, I have no idea how true or untrue that hookup is.

[JerryJ]

BTW if anyone wants a simple schematic (block diagram) I can draft one up and post it here