A rotovator/lunavator with a 200 km long tether rotating in space above the lunar surface drops momentarily to swap a large battery pack (mass of 250 kg) for a depleted one to be charged.  Is there a simple and effective mechanism that would do this? 

If the operation is too fast for a release then a catch, maybe two "bays" could be used.  Instead of exchanging position on the tether on route, the base (where the exchange takes place) could coordinate which side or position is open when the exchange happens so that the spent battery can be placed at the correct location.  Some king of catch and release mechanism - hook and eye?  Ideally, an object in flight could also be grabbed.

Here is a clever rotovator catch and release mechanism, it is at 3:00 into the video:

 

Longer description of the lunavator power here:

http://www.scienceforums.net/topic/110962-how-to-cheaply-and-efficiently-power-a-moon-base/

 

A working version of the tether animation.  Can't seem to edit the original post...

 

Even before the mechanism for release and catch, I want to see how the tether is built.

A description for a mechanical engineer, you know: what material, what shape, how much stress at 1680m/s tip speed and what mass.

I remind that tethers of carbon nanotubes are science-fiction, and their performance essentially unknown. The best existing material is graphite fibre.

Then, you might provide details about residual speed and height.

1 hour ago, Enthalpy said:

Even before the mechanism for release and catch, I want to see how the tether is built.

A description for a mechanical engineer, you know: what material, what shape, how much stress at 1680m/s tip speed and what mass.

I remind that tethers of carbon nanotubes are science-fiction, and their performance essentially unknown. The best existing material is graphite fibre.

Then, you might provide details about residual speed and height.

Spectra® 2000 Hoytether™

http://www.tethers.com/papers/CislunarAIAAPaper.pdf

Residual speed and height both need to be zero for the landing site ideally, but the paper states 0 m/s and 1 km height (top of a lunar mountain).