is it bad to drain a deep cycle battery
A deep-cycle battery is designed to discharge between 45% and 75% of its capacity, depending on the manufacturer and the construction of the battery. Although these batteries can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 45% discharge. There is an inverse correlation between the depth of discharge (DOD) of the battery, and the number of charge and discharge cycles it can perform.
Deep-cycle battery