Ignoring the battery safety aspects of the problem there is a simple mechanism that could be used simply, reliably, and without the need for absolute precision.
First thing would to have 2 trailing cables from the "Tanker" plane. they should be far enough apart that the empty plane's propeller would not catch them. Also each cable would be a different polarity. i.e. One positive and one negative. At the end of each trailing cable there would be a metal cup shaped contact with a high strength magnet on the "Tanker" side of the cup. The rim or lip of the cup would be soldered to the trailing cable.
On the "Empty" plane there would be 2 probes fixed to the planes wings, (one each wing). Each probe would be for a single polarity and have a similar cup and magnet arrangement as fitted to the trailing cables from the "Tanker" plane. The magnets should be oriented such that the opposite terminal is repelled if held close. Ie north pole to North pole or south pole to south pole.
The probes could be as simple as a couple of BBQ skewers with the contact and magnet attached.
When the "Empty" plane approaches the "tanker" the probes would be magnetically attracted to the trailing wires and the metal ends would be the electrical contacts for the passing of the charge current.
Just a basic and simple approach!
Have fun!
Ah, but it doesn't support re-charging while inverted!
Seriously though, wouldn't a single point of contact for the planes be better in term of being able to engage receiver plane to tanker - like the probe-drogue approach? The different polarities could be separated by geometry in the single attachment point/cup/plug/drogue thingy. (note - also doesn't support re-charging while inverted)
DamoRC