It was an $8 trustfire:
I estimate I charged a pair of 18650s around 60 times before it went R.I.P. I guess it took one too many 4 foot drops to the concrete. The power cord where it plugs into the unit has always been loose, leading to accidental drops to the floor when carrying it. Oh well.
Hopefully this $12 charger is more robust:
I hope I get lucky and receive it soon. I estimate that among all my semi charged batteries, I only have about 10 hours of run time if I only use one P7 at low power. :fingers crossed: