You may have heard the term “deep-cycle battery” when you have been discussing or reading about solar power systems, and it’s true that this type of battery is the type that is recommended for such uses, as it will last much better and give better service than others.
The type of battery that you have in a car is called a lead-acid battery, because those are its chief constituents, and it is also termed a starting battery, because it is designed to allow a heavy starting current for the motor to be drawn, but apart from that is not subject to much fluctuation – your alternator recharges it, and keeps it topped up.
The battery type that you should have in your solar power system is also commonly a lead-acid battery, and it is termed a deep-cycle battery as it is better designed than a starting battery for being discharged continuously to a low (or deep) level, and then being cycled, or recharged, back to its capacity.
What makes a deep-cycle battery better than a starting battery for sustained discharge? It’s mainly in the lead plates, which are a heavier construction than for the other type. It’s sometimes difficult to tell what battery you have, as it’s not easy to see from inspection, so you are reliant on the description and the supplier. There are some batteries that are heavier, but not true deep-cycle – typically these are cheaper and may be used in golf carts. Oh yes, to buy real deep-cycle batteries is expensive, but you would not be satisfied with the performance of the automotive kind. Golf cart batteries are better than car batteries, and may give you acceptable service.
Deep-cycle batteries are made for frequent discharge to a deep level, and have ratings based on their performance for those levels. A common figure is the rated number of discharge cycles that a battery can stand, and it will usually be down to a mere 20% of its full capacity, although sometimes they can be rated for 10% and/or 50%. A discharge cycle is simply discharging the battery down to that level then charging it back up to full capacity.
Deep-cycle batteries are also used for RVs, because the way these are used may require the battery to be run down significantly before it is recharged. They are also used in forklift trucks and other industrial uses for the same reason, and are sometimes called traction batteries because of this.