I'm a novice to the whole photovoltaic scene, I'm wondering about buying a deep-cycle battery for a solar panel, I remember reading that you shouldn't really take your deep cycle battery below 50% of it's capacity/charge to maintain good condition, so to get double the capacity of battery storage than you'll actually need.

My question is how do you monitor the charge/status of your deep-cycle battery? I know there are charge-controllers etc. that control the flow and stop your batteries from being damaged by being over-charged but are there devices that will show their current charge/status?