how to get correct voltage

Re: how to get correct voltage

You can place two panels in series for 8 volts @ 250 mAmps.

The 5 volt charger (assuming it is a linear charger or PWM, not buck mode switching) will drop the 8 volts to 5 volts (and waste it as heat).

Solar panels will not be damaged by running them below their rated voltage (you can short them out all day long just fine--however, never hook them up backwards to a battery; that will kill them).

You then have to look at the specifications for your 5 volt 2 amp charger.

There will be some maximum input voltage that you don't want to exceed (Voc -- open circuit voltage -- of a solar panel is ~1.3-2x the Vmp voltage). So, the charge controller needs to withstand ~11-16 volts when the batteries are fully charged. And handle the waste heat while charging the batteries (assuming linear regulator).

If you have the panel specification (Voc, Vmp) you can better understand your ratings (Voc is worst case high voltage when panels are cold--freezing or below; Vmp is worst case low voltage when the panels are very hot--full sun, no airflow, 100F+ day).

-Bill

Re: how to get correct voltage

The output voltage is dependent on load and solar panels are, more or less, current sources. Once the there is enough sunlight to reach operating voltage, the output current mostly proportional to the amount of sunlight hitting the cell.

Generally, those panels are used to charge 1 or 2 cell LED lamps with 1.5 to 3.0 volt NiCad or possibly NiMH.

Anyway--measure the open circuit voltage of each panel. And then measure the short circuit current of each panel.

You can parallel panels of the same voltage (and add the current).

And/or you can put panels with the same output current in series (and add the voltage).

The real issue tends to be the load itself. Many will be destroyed if the input voltage is too high. And solar panels tend to have a wide operational range of voltage (that ~2:1 range for Voc cold to Vmp hot). Most typical DC devices / adapters are not really designed to work properly over such a wide range.

Do you know anything about the charger/load device? You can certainly build your own voltage controllers (3-T regulator to control the voltage, or use diodes in series to clamp the output voltage) to protect your electronic load--but it is a bit of a pain and will probably waste energy.

The other way to charge batteries is -- typically, if you keep the current below C/10 (i.e., only charge a battery at 10% or less of its rated capacity in Amp*Hours or mAmps*hours)--you will probably not need a charge controller as there is not enough current/power to damage anything.

But, since I don't know what your load looks like (electrically)--I am not really able to give you specific instructions. And I would hate to tell you something that would damage/ruin your device.

-Bill