@peteb , I have not performed much testing w/ the Argon, but here's a link to some Solar testing from a Xenon (which I "believe" uses the same charging circuitry, IIRC)
The following note might be of interest to you:
Note: the Xenon will not begin recharging the Li-Po until the voltage drops to 4.05V or below (can take days), which agrees with the Torex Datasheet
The Xenon's Charging cycle can be terminated by a quick voltage drop from a cloud. A new Cycle wont begin until 4.05V is reached.
Again, I appoligize if this doesn't pertain to the Argon, but I suspect it might.