@aklein24 , I decided to reproduce your findings for documentation.
I can reproduce using a USB port from a Computer.
I can not reproduce using a 5V power supply with micro USB connector.
To Break (with USB cable/port):
Li-Po @ 3.81 V
USB cable/computer port (4.95V no load) to Boron USB connector, Boron begins charging Li-Po @ 440 mA, 4.05 V measured on Li-Po Pin.
Remove USB Port power source, Li-Po now powers Boron.
Ground EN pin causing a Shutdown.
Re-attach 4.95V USB port power source = nothing happens, no recharging.
Un-Ground the EN Pin, Boron Boots normally but will not resume Li-Po charging, no matter what.
The same test with a 5V power supply on the Boron’s USB connector works properly and will resume charging under the same test.
A year ago, Boron’s had no trouble recharging Li-Po’s during and after an EN Pin Shutdown (my Trial #1), but that was older hardware and firmware.
Since any PMIC calls in setup() are performed after an EN pin shutdown, something fishy is going on.
My first guess involves the D+/D- data lines, since that’s the major difference between my (2) quick tests. But that would point to the PMIC retaining some phantom power source (board capacitance ?) when the only voltage source is the Li-Po during a EN pin shutdown. That might explain why the PMIC never attempts to re-qualify the source after EN pin shutdown when Vusb wasn’t available. But if so, that should have also been the case a year ago when my Borons had no trouble.
This may also be related to how Boron’s on Utility Power could have a quick interruption of power and never again maintain the Li-Po.
@marekparticle, care to take a swing?