Hi Donald,
Its also worth mentioning that a regulator wont give the best efficiency from a solar panel, a dedicated solar IC should be used with Maximum-Power-Point-Tracking (MPPT).
On the Sparkfun page there is a good explanation of why MPPT is best if you intend to use a solar panel - Sunny Buddy Solar Charger V13 Hookup Guide - SparkFun Learn
It depends on the charging IC used, on the UM FeatherS3 the charging IC used is the
TP4065, all it wants is a constant power - which can be maximised through the MPPT IC,
I didn’t get far enough through the datasheet to find out if/how the chip would behave when there is not enough power to charge the battery.
Maybe? See above, though the 5V port will be more than sufficient to charge a battery, its just if the LiPo charge IC would enter some fail-safe state, where removing the battery/second input releases it, allowing the battery to be charged.
I believe the Sparkfun and Sunflower boards handle this quite well.
Liam
