The Solar Charge Controller takes the ~20V from the solar panel and boosts it up to ~50V in order to charge the battery.  It should use a CC/CV (constant current/ constant voltage) charging approach to charge up the battery. And it is uses MPPT (maximum power point tracking) to get the optimal power out of the solar panel.

A common problem with this controller is that the fuse blows out. One theory is that this happens when the panel is under full sun when the battery is connected.  The fuse is inside of the controller and can only be replaced by first removing the controller from the ELF and then opening it up.

Another issue with this controller is that the target battery voltage cannot be adjusted by an owner. Genasun can customize this voltage at the factory when it is ordered.  An owner might want to adjust the target voltage to extend the life of the battery by charging it lower than the maximum voltage of the battery pack.

Also this controller is expensive, to get one with the correct target voltage for the ELF battery costs $250 (as of May 2018).

A possible alternative charge controller is:

Here is a very nice video review: https://www.youtube.com/watch?v=omAhsQqwWSo  And a follow up video https://www.youtube.com/watch?v=tkDRNbAWZo0

This controller is much cheaper: $35 (as of May 2018 on ebay)

The target voltage can be adjusted, and it also provides a read out of panel voltage and battery voltage.

The fuse is external, so it would be easy to change.

It does not appear very water resistant as Adam points out in the video.