It is essential to fuse the wiring between a solar panel set up and the battery bank for safety. If the current flowing in the circuit is higher than it can safely handle overheating occurs. An overload is most commonly caused by a short circuit. An appropriate fuse will cut the current flow if a short occurs in the wiring between the solar panel and the battery. The size of the fuse should be just a little over the maximum current provided by the solar panels - for example, if the highest current from a solar panel is 3A then a 4 or 5A fuse should be used. If a short occurs the fuse will burn out and the short circuit be over within hundredths of a second before serious damage can occur. A short circuit is extremely dangerous, and if not prevented will often lead to fire breaking out when dealing with 12V lead acid batteries. The fuse should be positioned on the positive side of the circuit as close to the battery as possible. Each battery should have its own fuse.
Automotive blade type fuses and fuse holders (pictured below) are excellent for this purpose as the fuses are easily accessible if they need changing, and it is clear if a fuse has blown. Blade fuses are available in many different sizes, each being a different colour.
http://www.reuk.co.uk/Fuse-and-Solar-Panels.htm
