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USB Solar Electrical Schematic Review Help

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May 1, 2011
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I need some help with a Electrical Schematic. This summer i am planing to backpack the Appalachian trail and wanted to build a solar powered USB charger to power my cell phone and the AA battery's for my computer. at this point i am planing on getting 4 6V 340mA solar panels in parallel with a diode on each one, then a set of 4 rechargeable AA batteries in series parallel to the solar panels. Then i have 2 USB ports in parallel, and each of them has a linear Regulators- standard 5V 1A pos Vol Reg. Attached is a rough version of a schematic, i know it is poor. Could you please look it over for me to see if you think it will work and that it will not fry my electronics. solar usb port.png

The schematic showed up clearly.

Looks as though you intend to remove the battery pack and connect it directly to the charger. THe solar cells output voltage should be a few volts higher than the battery pack. Twice as high will be good to charge even on cloudy days.

However I'm not sure you can power the USB ports to power the computer.

The only way it can work is if your computer allows a direct connection from the USB ports to its batteries when it's turned off.

Do you get power to a USB device when the computer is off? Does a thumb drive stay lit up?

Chances are USB is disconnected when you shut down the computer.

But even if it's still on...
USB ports are not designed to power the computer through USB. They might produce 500mA to external devices.

You're thinking of pushing more than that into them the reverse direction. This is liable to fry circuitry inside. THere's a device which regulates 5V output. It won't permit current coming from outside.
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Your design is pretty sound, from a first glance. Item #1, put a fuse between your panels and the rest of your circuit, and carry a bunch of spares (or get a resettable fuse). If something breaks, you don't want your cells helping to start a fire.

Next, you need to review what it takes to charge your AA batteries. What kind are they (NiCd, NiMH, Li-Ion, etc)?

Depending on the formulation, they will have different charging needs. A larger priority than charging is making sure you don't OVERCHARGE them. If you're hiking along, you really don't want a pack of batteries exploding in/on your backpack, while it's strapped to your backside! You may need something as simple as a current-limiting resistor in series with each battery, or something more complex, like a small PCB with a homebrew/kit/pre-built charge controller on it.

I think Brad is a little confused. I don't think you plan to power the laptop from the USB connectors, just your phone. If I read your OP correctly, you plan to charge up the AA cells, and those will be used to run the laptop.... correct?

You should also check out how your cellphone handles voltage inputs less than 5V. As your cells are exposed to varying levels of solar irradiation (lighting), the voltage can change quite dramatically. Depending on how your phone handles voltages lower than 5V, you may not need anything; or you might need to build up a small undervoltage cut-out circuit (comparator + P-channel MOSFET?), so when the panel voltage drops below X volts, it will cleanly shut off the 5V regulators. Just something else to think about.
I will admit to some confusion. I've never heard of a cellphone with one USB port. Let alone two.

However I just observed websites showing how to make a cable to charge a cellphone from your computer USB port.

I guess EJM must have clipped his cellphone's proprietary power cable and attached a USB plug.

But I wouldn't have thought he'd need two USB?

Nor have I heard of a laptop computer that uses AA batteries. Laptops I know about use massive 14V battery packs.

Netbooks are more recent. They don't need as much juice.

So EJM's computer must be a newer model than that. A step smaller than a netbook maybe?

It's unsettling not to be up to date.

If I understand the design, he is going to have two USB ports with +5V supplied to them by the array. He will plug a USB cable into the powered ports, then the other end will go to his cellphone, so that it will get a charge using the energy captured by the solar array.

As for the laptop, I haven't run into any that run on AA's, but I wouldn't doubt their existence. I've seen all manner of micro-sized PC chassis and multi-board machines for use in industrial settings (we had a couple Pentium-class units that were dataloggers/HUD generators for a project vehicle team i was involved with in college). For overseas, they even have a laptop that is powered by a handcrank generator built into the side of it.


One Laptop per Child program
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