Help with MYOG Steripen Freedom solar charger

Lots of things to consider.

The wallwart is rated for 1000mA = 1 Amp.
I doubt the solar panel puts out one fifth of that.
The Steripen may require more current than the panel provides.
You just have to try.

The diode may be duplicated in the Steripen.
A diode unnecessarily reduces the panel efficiency if not needed.
If you connect a 100 resistor across Usb pins 4 and 1 and you see 3 or more volts, then the Steripen does not have one and you need to add yours.
Another way to test is to try and light a headlamp from pins 1&4.
Try both polarities.
If it doesn't light, the Steripen already has a protection diode.

The diode has an arrow into a bar: (+) –>|– (-)
There may not be an arrow, just the bar.
One output of the panel is (+).
The diode (+) (arrow) end goes there.
The (-) (bar) end of the diode goes to Usb pin 1.
The (-) end of the solar cell goes to Usb pin 4
Usb pin 1 should be marked with '1' or a dot.

The Steripen may or may not require pins 2 and 3 to be held to 2.3v and 2.7v.
That requires four resistors to create those two voltages.
Try without first.

Thank you for your comments, they're much appreciated.

Do you know if will I need a regulator of some kind to protect the Steripen from damage?

-Connor

I doubt it. An extra volt shouldn't harm any lithium charger. I looked at several chips that might be susceptible, but all were at least 6V max rated. And if there is a diode in line, it drops the voltage 0.7V (for most diodes) so you've got only 5.3V at the Steripen. The USB spec is 5.25V max, close enough that there is no issue.

Put the diode in line and check the voltage.

The real issue may be that the Steripen charger only works with higher current, but again I doubt it. That is a problem for iPhone charging because the phone keeps bleeping in and out of charge mode when the current isn't strong enough or the panel passes through momentary shade. Then you need a buffer battery and circuitry to manage it.