There are several types of solar panel systems. Some require a battery back up (or bank) and others are directly wired from solar panels to the solar powered device. With regards to the solar powered systems that require or utilize a battery, there are a few different types:
When it comes to solar battery banks, each is designed for a specific charge or discharge level. Some are manufactured wet cells where are manufactured sealed or gel cells, each coming with their own set of requirements. For example, golf cart batteries come in a 6-volt wet cell with thick plates that are designed for hours of high volume discharge with a fast recharge and car batteries come with a thinner plate and their high volume discharge last only seconds and requires a longer period to recharge. When using a battery bank system, make sure to use the appropriate size and quality of battery for the amount of solar power it will be required to store. They should be a deep cycle battery unlike the car battery, which is a shallow cycle. Using too small of a battery can be very dangerous.
Battery banks are an expensive part of a residential solar power system; some people claim that using reconditioned old batteries is possible. Find out more here.
When solar powered systems require a way to store the energy produced the battery comes into play. In some situations, depending upon the power company in your area, you may be able to connect to the electrical grid of the power company and sell any excess power your solar panels generate. Mind you, in the majority of the cases the power company is happy to purchase your excess energy but at a much lower rate then what they are selling it to you.
When using batteries there is maintenance required and they do not last the 20 years like your solar panels can. They will need to be stored in a safe, enclosed location that is non-metallic and well-ventilated.
It is possible to make your own battery bank with materials you can obtain online or from local hardware store or electronics store, as well as some things you may already have in your possession such as an electric stove, glass jar, table salt, and tap water. Items needed from a can include stuff like a sheet of copper flashing, alligator clips, a micro-ammeter that reads the currents, sand paper, wire brush, drill, and sheet metal shears.
Is it wise to attempt to make a battery large enough to be used as a battery bank to store your excess energy? Possibly, if you have the right technology at hand and know what you are doing. Be forwarned: knowing how a solar battery works is one thing, but building one yourself is entirely another! A faulty battery is highly dangerous, so seek qualified help if you need it (or, you can just buy one).