Hi, I hoping to build a solar powered usb charger, this incorperates 2 AAA batteries at 1.5v each; these would be steped up from 3v to 5v (for usb charging) The idea of the solar panel is to be permentaly "trickle" charging the batteries. I am new to electronics and was wondering how to work out how much "power" the solar panel would need to generate to produce a good supply to the battery, so it would not go dead charging either an ipod, phone ect.

Thanks is advanced..
PS Sorry if it is a bit basic and confusing lol

 

Typically the maximum trickle charge for a NiMH battery is 1/10 of it's ampere-hour rating so, for example, a 2AH battery could be trickle charged at up to 200mA. Thus you would want a solar panel that would put out a maximum of 200mA at about 3V in full sunlight.

Using such a solar panel would require about 15 hours of full direct sunlight to completely charge a set of discharged batteries or probably about two full days of normal sunlight if the panel is stationary and pointed at the noon position of the sun.

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Carl
Curmudgeon Elektroniker

 

With the solar panel being permanently on would it keep the batteries toped up if the usb charger would be drawing power such as ipod,phone.
Thanks

 

Depends completely on the device you're charging and it's voltage and capacity, it also depends on the efficiency and voltage tolerance of your boost converter and the amount of light you expect to be available as well as how often you use it. I'd use as big a set of NIMh as you can possible fit (D cells would be nice) as that gives you large over capacity from an energy storage perspective so you could pretty much charge anything you wanted from it weather or not there was sunlight if the cells were fully charged. The big thing you need to qualify exactly is, the availability of sunlight, the EXACT power requirements of the device(S) you're going to charge and how frequently you expect to use it. Without reliable numbers for those three things minimum there is no way to answer your question.

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ok so imagine you have 4 1.2v AA NI-MH 1800Mah batteries for the storage and you had a 6v 400Ma solar panel and you want to charge a nokia N95 with a capacity of 950Ma and needs 5v to start charging. you want to allways charge off the batteries so you get a steady flow and can go inside or out of the sun for a minute and not stop charging. You dont want to overcherge the batteries so you would want a voltage minitor to stop charging the batteries if they were full.How would you do this?

 

The Ni-MH battery might not last long if it is overcharged. Energizer recommends a trickle charge current of no more than 1/40 x C. 1800mAh is pretty old (2500 today is common) and 1/40th is a current of only 45mA.
Use a battery charger IC.

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Uncle $crooge

 

To prevent NiMHs from overcharging you might use a MAX712. It is (hardware) programmable to any number of cells between 1 and 16, provides trickle charge (C/16) and selectable fast charge for C/4 to 4C.

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Proper Planning Prevents Piss Poor Performance

 

So what if you