I have a solar system and I am looking for a 12v circuit diagram that will detect when my 12v battery is at a certain low voltage and in so doing it will then activate a relay which will connect a charger to the battery.

So in theory below is an example of what I need:

The charger output would be always connected to the battery but the input of the charger would be disconnected from the mains supply.
Whenever the battery voltage falls to 12.0vdc the circuit would activate a relay which in turn would connect the charger input to the mains supply/grid and then whenever the battery voltage rises to 12.2vdc the circuit would then deactivate the relay which in turn would cause the charger input to be disconnect from the mains supply/grid.
I just want the battery to get a partial charge from the grid when it reaches a low voltage level.

Please help me out with a circuit diagram, info, anything.

 

You're a bit off the exact voltages, but this is the Bible for this sort of question:

http://batteryuniversity.com/

 

Use the search feature to search on this website for "TL431" and my user name "MikeML". You will find several circuits that use this chip as either a voltage drop or voltage rise detector. Also look here.