Deep cycle batteries, also known as gel cell batteries, are commonly found in wheelchairs, radios, and motorcycles. They are considered "premium" batteries, since they do not deteriorate in extreme heat or coldness. They also have the unique property of not spilling or leaking. You will have to purchase a specially made gel cell battery charger to charge this type of battery. There are three cycles in charging a gel battery: bulk, absorption, and float.
Read your battery label for any details concerning charging. Find the amp-hour rating and voltage.
Look through the manual for your deep cycle charger. Every one is slightly different.
Plug in the charger and set the voltage to match your battery. If the battery has no voltage information on it or is hard to read, set it to 10.5 to 15 volts. This is the "bulk charge" stage.
Place your battery into the charger, or hook it up so that the positive cable goes into the positive terminal, and the negative cable connects to the negative terminal. If there is another dial to indicate a deep cycle battery, turn it on.
Wait for the charger to change over to the "absorption charge" cycle. Voltage is between 14.2 to 15.5.
Wait for the battery to change over to the "float charge" cycle, where the charge goes down to 12.8 to 13.2 volts. You have two options when you see the voltage go down. You can turn off the charger, unplug it, remove the battery and place it back into your device. Or, if you are not going to use the battery right away, you can let it continue charging indefinitely. This cycle will not end unless you turn the charger off.
Do not buy a new battery and let it sit for later use. It must be connected to a charger if not in use. The first charge of a new battery takes the longest---up to 20 hours.