Minimum Current Required to Charge iPhone via Solar Charger: Will 5V 200mA Work?

Hello forum,

I have a basic questions about solar chargers and trickle charging a phone.

It seems that all commercially available solar chargers

http://gigaom.com/2011/01/12/7-ways-to-charge-your-iphone-with-solar/

have an internal battery that eventually charges the phone (the Lithium battery inside the phone).

An Iphone seems to need a 5V voltage and current of 500mA. That is a 25W power.
What if the voltage is 5V and the current is 200mA (less than 500mA).
Would the phone still charge or not?
Do we need to provide both 5V AND 500mA for the charging to take place?

I believe these solar chargers have a DC-DC booster to provide the phone with 5V. Once 5V are provided, doesn't the phone automatically draw the current it needs depending on its impedance or not necessarily? What am I missing?

surely power in = power out

Thanks,
Kavan

This is an area that many people get hung up on, let me make this statement. "A solar module's output voltage is dependent upon its load". A typical amorphous silicon module has an output voltage that ranges up to 23 volts on a 10 watt module. Yes, you are looking at a smaller module but if you spec out the modules you should be able to find some that suit your purpose that can exceed 5.5 volts. If you want to get efficient try to locate some that that put out maximum wattage at close to your target voltage. It then becomes a simple problem of limiting the power so that you don’t overcharge your batteries and open the circuit so it won’t discharge if you forget it lying outside and the sun goes down. A PWM charging circuit would be a low cost, simple way to do that. If you notice the website you linked to, they quote an 8-10 hour charge time to charge the batteries in the charging assembly and a charge time of 3 hours to transfer that power to the phone. I would suggest you do likewise because leaving your phone on a table in the sun for 8-10 hours isn't really a practical solution. I would say just buy their device but it seems overpriced.

A USB port's power lines are designed to deliver no more than 500ma (at 5 volts) -- that's just the upper limit. The actual amount of current is determined by the resistance (impedance) of the load (the thing that is connected to the USB port's power lines -- in your case, the cellphone charging circuit).

And that's the other piece of the puzzle: the phone has it's own charging circuit. So, simply supply 5 volts (and I would limit it to 5 volts, no more, no less [unless you can acquire a spec that says otherwise]) and the charging circuit will draw what it needs. Now, it's possible that it will attempt to draw more than whatever you connect to it can supply, so if your power source can only deliver a current that is less than 500ma, then there may be a problem--i.e. the charging circuit may not function properly, but it won't hurt to try, so go ahead and experiment.

I got a solar charger from "mr positive":http://www.mrpositive.co.nz/solar-powered-aa-and-aaa-battery-charger-and-usb-backup-power/ . Has ideal addition to any emergency kit. Works well.

before or in the earlier part of my career, i was also ask during the interview on what is the name of the charging for a UPS to maintain it's supply. honestly, i wasn't able to answer such question. this as the time goes-by, i learned the answer when we bought a brand new UPS. i was told during the discussion that to maintain the supply, a trickle charge was needed in order to maintain such power specially if the said UPS is not in used for quite some time.


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Rayovac's PS73 is both a USB portable charger and battery power pack for USB powered devices. With up to 7 hours of instant mobile power, it's a lifesaver when you can't charge your phone the traditional way.

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