Charge a guard with an single charger. It does not only lights red or green! but also combinations are possible.
Herewith a summary of the explanation of the LED color (combinations)

The guard conditions in a basic operation ( without watch) are followings:

•  no LED lights: the guard isn't connected via charger or docking station with USB

• guard is connected wit USB:

*LED=red, if charged under 80%
*LED=green, if charged more as 80%  

Standard conditions:

        *LED=green: guard is ready for to use
        *LED=red:     guard is out of order

The LED conditions with watch are following:

•  if guard isn't connected with USB- no light's!

• if guard connected with USB and the watch (paired):

           *LED= red, when the watch and the guard are not ready for 24 hours
           *LED=green, when the watch and the guard are ready for 24 hours