smartphone charger voltage rating
The solar charger manufacturer will have made the charger with the intention of being USB charging compatible. 

That said - I have seen (and bought a sample of) Chinese designed and made equipment that output over 8V on the USB power leads!!!

Telling us the brand and model of the solar pack is a really good idea.

Measuring unloaded, lightly loaded and loaded output voltages as below is also essential if you are serious. 



It is quite likely that the unit does NOT produce 5.5V despite what the label says - and it is also possible to find chargers that DO make 5.5V out when their labels suggest otherwise.

I'd love to just say that it was safe, but there is a very very very small chance that if your charger did produce 5.5V that it MIGHT damage some phones. 

An IC manufacturer would have to be demented to design an IC that interfaced to USB and which had a 5,5 V abs max rating. Unfortunately, there are a very few demented IC makers about :-(. I have seen a USB power interface IC with a 5. 5VDC abs max voltage spec !!!. It's highly likely that such an IC would in fact survive 5.5 VDC OK, but I mention it for completeness. (The IC I encountered was being proposed for use in the open-hardware smartphone project - since abandoned in favour of commercial platforms. Not an IC that I would ever use!!!

SO - testing / fixing:

Operate your solar pack UNLOADED.


Measure the voltage at the output connector.
If 5.2V or less, all OK.  

If > 5.2V, load it with a 1000 ohm resistor. This is a very light load.
If Vout &lt;= 5.2V with 1k load all probably OK - see (i) below.

If Vout >  5.2V, loaded and otherwise then see (ii) below


(i) Decide if you are feeling lucky - the pack loads down OK with 5mMA load. It will almost certainly be OK in practice.
 Almost certainly :-). 

If you are not feeling lucky and do not want to make Murphy's day then either 


Return to sender / vendor or
Add 1k resistor or less if less is needed across output in some manner to reduce vout max to &lt;= 5.2V.

This could eg be across terminals on PCB inside, or across contacts in plug or cut lead and patch in or make a socket-plug adaptor or cable or ... .  

It may also be possible to adjust the output of the pack down ever so slightly. 


(ii) Load with a 47 ohm (or thereabouts) resistor (about100 mA load) and measure output voltage.
 If Vout &lt; 5.2V loaded like this then it is marginal and you may be able to decide some sort of workaround.

If still > 5.2V at this sort of loading then some more serious action is wanted.


Adjust down if possible. 
or add a 1A 20V Schottky diode in series wit the output with a modest electrolytic across the USB +/- downstream of the diode. I say 20V Schottky as you get about 0.1V less voltage drop at around 500 mA with a 20V compared to 30V diode, all else being equal. You can use a 30V or 40V if 20V not available. 

Check output voltages as above with diode added.


NB - if the USB connector &amp; port is also used for charging then the diode will block charging. In this case you will need to make a simple adaptor that connects from Solar controller to charger directly and from solar controller to phone via diode and capacitor.