How do I make a truly versatile solar powered cell phone charger?

I have many questions! I am trying to make a solar powered cell phone charger that has lots of versatility. When no phone is connected, it needs to charge 4, 2500 maH Ni-MH batteries. When a phone is present, it needs to charge from either the batteries or a 12 V, 1.5 W solar panel. I have found several voltage regulator circuits, but I don't understand how to calculate the proper capacitor values to stabalize the current using a PT5101 voltage regulator, or how to get 5V constantly if the voltage ranges from 2.4 V, from the AA's in series, or as high as my 12 V from my solar panel?

I don't want to buy a circuit, or salvage one, but create it from individual components. Any help?.

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I think you will need separate circuits for each type of power input your using. For anything lower then 5V you'll want a boost circuit that can take that low voltage and boost it up to the 5V you need. Something like a Minty Boost, which uses between 2V and 5V,  would be a good solution. You don't have to buy the kit. Its open source hardware so you can get all the design files from that web site. Then you can source all the parts yourself and build it on your own PCB layout.

Anything between 7V and your high of 12V (often these 12V panels can put out as much as 15V in full sun) you'll use a simple linear voltage regulator like an LM7805 or for better efficiency stick with that PT5101 package. Use a switch to toggle between the small battery and the solar panels and you have yourself a nice little charging system.

Only problem is you have that small gap between 5V and 7V that you won't be able to do much with. Other then that your pretty much covered.
Phaedrush (author)  mpilchfamily5 years ago
Thank you! This is exactly what I needed.
The PT5101 isn't a good choice if your supply is swinging across the desired output voltage.

You really need an inverting topology, since you don't care what "polarity" the input is relative to the output.
Phaedrush (author)  steveastrouk5 years ago
I actually was pricing the PT5101 and avoided it for the cost alone and opted for the LM8705 which I already had.

Can you explain why I would want an inverting topology? The solar panel and the batteries are polar, and I'm going to hook it up to a USB female plug so I know what polarity the output will be as well.
Only the output is polarised. You don't care what the polarity of the input is relative to the outputs. It is impossible to build a power supply that has an input voltage below and above the output voltage . If you use an inverting topology the input is always several volts different and can never straddle the output.
Phaedrush (author)  steveastrouk5 years ago
Ah, I see what you are saying about the inputs. I plan to include a switch to let the user decide between charging their USB device off either the batteries and the solar panel, or to charge the batteries at maximum voltage using the solar panel. It won't actually use both voltages at once. But I am going to research the inverting topology because I'm relatively new to electronics and don't know much about it.
If you're not careful with that approach, something is going to get zapped.
lemonie5 years ago
Just charge a 12V battery and run the charger(s) off that.

Phaedrush (author)  lemonie5 years ago
Thanks for your response, but that defeats the purpose of what I want to acheive.

For something that would actually work, I don't see how the purpose would be defeated.

Phaedrush (author)  lemonie5 years ago
Well, I don't want to add a 12V battery. I want to use AA batteries, to make it compact and portable. Going off the idea of creating something that would fit in an altoids size tin.

I am sure a 12V battery would work, but charging it with a 12V solar panel would never completely fill it.