Boron Solar Charging Through VUSB

I have a 6V 2W solar panel from Adafruit and am trying to Power/Charge a Boron with a cylindrical 3.7V LiON battery (also from Adafruit). Charging the battery from the micro USB port through the computer works fine with a solid orange charging light. Connecting the solar panel to the VUSB pin the orange charging light blinks about 2 times per second.

I’ve tried lowering the charge current by setting:

SystemPowerConfiguration conf;

    conf.powerSourceMaxCurrent(200)         // Default 900
      .powerSourceMinVoltage(3880)          // Default 3880 
      .batteryChargeCurrent(100)            // Default 896  
      .batteryChargeVoltage(4112);           // Default 4112    


This has had the same result, a slow orange blinking light. Voltage output from the solar panel is ~7.3V and when connecting a ~40ohm resistor across the leads there is some voltage drop and the resistor gets hot so I assume the solar panel is working properly.

Any thoughts on what the issue could be here?

The docs say there is a power management IC fault register. How do I go about reading this register and interpreting the fault code?

For a 6V panel, change this :

To this:

I appreciate the quick reply. Just tried changing source min voltage to 5080 and it had no effect. Disconnected from the computer I’ve also tried powering the Boron via the VSUB pin using a 1A variable power supply. Providing 4.5V, 5V, 6V and 7.5V to the VUSB pin directly I am able to charge the battery and get a solid orange light at each input level. So any hand shake between the computer and Boron was taken out of the equation.

Essentially the only thing unable to charge the battery right now is the solar panel… But I don’t have any indication it’s faulty either.

Can you measure the Panel’s Voltage while the Boron is blinking (attempting, but not charging)

@mdavey ,

Couple quick questions:

  1. Did you try putting a decoupling capacitor across the solar panel leads? I find a 470uF aluminum cap is helpful to smooth out the power fluctuations that are normal for panels and it might support a higher charging current. As you likely know, you need to balance power output and demand on a panel to prevent the voltage crashing. Low voltages can trigger the slow flashing of the amber led indicating a pmic fault.

  2. Can you hook up an oscilloscope or volt meter to measure the voltage and see if your panel can support the power demand while maintaining a voltage over 5.08V?

Hope this helps,



Had to wait for the sun to come back out but yup that was it. Adding a decoupling capacitor to the solar panel leads did the trick. With mid morning sun and minimal load from the Boron, VUSB voltage is staying around 6.5V.

Had a 1000uF cap readily available so used that. I’ll play around with the cap sizes and charge settings to dial it in but I appreciate everyone’s help!


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