# In need of some help (some questions relating to making a solar powered charger)

#### nyarlathotep

Joined Jul 15, 2009
3
I am very keen to start a project to make a solar recharger that i can use to recharge my PDA whilst out hiking, i have done some research but have a few questions i need answered, hopefully someone will be able to help me out, as though i am very keen to get in and make it myself, my knowledge is (at the moment) quite limited in this field.

Here goes:

My device uses a battery that is 3.7v 1350mAh (PA16A)

While i understand that the voltage of the solar panel must be higher than that of the battery i am charging, what is considered to be a safe voltage to go over when considering this? (for charging a PDA through a USB cable)

When looking at the specs of a solar panel, is the mA and mAh rating considered to be one and the same (i understand that mAh = mA/hour?) and what relation will the mA (and/or) mAh rating of a given solar panel have to do with the project? does more current mean a faster charge?

does wattage come into the equation at any point?

How do i tell if my Li-ion polymer batteries have cut off to prevent overcharging ? (aside from the obvious destructive answer of 'burst into flames') or is this feature built into the device not the battery

Would using more than one solar panel mean quicker charging, taking into consideration that there is more surface to capture sunlight (even on days of less light)

And with this in mind, i am guessing that i would probably then need to reduce the output voltage of the solar panels (if they are bringing over 5v to the circuit) somehow to suit the 5v input required for the PDA (a LM2937?)

And if this is the case, then how would (if they would) the inclusion of more panels make it more effective (is then just the increase of the mA and/or mAh) and if so, is there a maximum/optimum figure of current that i should be looking at attaining?

should i consider looking at putting something into the circuit to prevent the circuit from draining the battery in times of low light, and is this something i only need to consider if i am charging the device with the battery in it (instead of just charging the battery standalone)

Joined Jul 7, 2009
1,583
I would imagine that your PDA's circuitry simply needs 5 volts delivered to it for recharging; this is the typical USB voltage. The current question is a bit harder in that it's not clear whether the PDA will charge well at the 5 V but with a lower current than what's typically available in a USB connection.

The safest route would be to measure the current the PDA draws when connected to the USB bus for charging, then duplicate this current with your solar panels. However, depending on the current, you might not want to carry a solar panel large enough to meet this need. Then you need to experiment using a DC power supply capable of constant current control to see how well the PDA charges at lower current values. One would expect the time integral of current (i.e., the total charge delivered in e.g. mA*hr) to be the important number, but this might be naive with a poor PDA power supply design.

I would imagine you would need a regulator, as you have stated. You might want to put a diode in series with the PDA, as I've seen a solar cell that was intended to charge a car battery actually cause the battery to go dead (i.e., you don't want to take two steps back for every step forward).

#### nyarlathotep

Joined Jul 15, 2009
3
Would i be correct in saying that a standard USB plug provides 500mA of current?

i checked the wall charger and it outputs at 1A, and some 'rapid' car chargers for this model of PDA are touted to charge with a similar output, but as i understand it, it gets only half this current when i charge it through the USB cable.

#### Bernard

Joined Aug 7, 2008
5,784
I would pick a solar panel with at least 9V open circuit & 300 to 500 mA short circuit. Maximum power out is when panel is loaded down to about 70% of open ckt. V. A