Hi,
is there an onboard voltage-regulator/circuitry for charging the battery connected to Li+ PIN on the boron with external DC supply? I am guessing the USB port is connected internally and should be able to charge the battery connected to Li+ PIN (is there a pinout of the USB port instead of using an actual microUSB?
Yes. The Li+ pin is also connected to the JST-PH + line and works identically.
The PMIC on the Boron can either supply power to the device or charge the battery from either Micro USB or from VUSB.
Yes. However, the fuel gauge (MAX17043) only will produce reliable values for single cell 3.7V LiPo batteries. If you are using a different battery you need to be careful it must be compatible at least with the voltages required by the MAX17043 and the charging capabilities of the bq24195. For example, you cannot use a 6V AGM batteries on the Li+ pin.
Awesome, thanks!
Using a single cell Li-ion 3.7V 
The solar cell is rated for 2V @160mA.
What’s the minimum voltage to charge attached Li-ion 3.7V?
Is a dc-dc step-up converter necessary to step-up the 2V to something like 3.3V/3.7V? @rickkas7
There is no built-in step-up for charging. I’d recommend a 6V solar panel for the Boron, Even though VUSB is nominally 5V it can actually take up to 17V as long as you don’t have other feather accessories that require 5V.
If your panel provides less than 1A at 6V, you may also need to reduce the input charge current in the PMIC configuration. Otherwise, when the charger turns on the output from the solar panel will collapse and it charging will just constantly switch on and off instead of charging.
What’s the equivalent pin on the electron 3g for charging the connected battery via dc supply? (besides from microUSB port I assume?)
The Electron has the VIN pin, which takes 4.4V to 17V.
Using a 12V solar panel with the Electron and E Series is an option that expands the charging range.
Using a 12V panel with the Tracker One is the minimum because the minimum voltage on the Tracker One M8 power input is 6V and the maximum is 30V.
So the minimum voltage that’ll charge the attached battery via the VIN pin is 4.4v for the electron?
Correct. You can lower it a bit in software, but the default configuration requires 4.4V to charge on all of the bq24195 devices (Electron, E Series, Boron).
For very small solar panels you’ll likely need to reduce the maximum charge current as well.
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