If you need to connect a bunch of batteries together to create a battery bank for a particular solar supply project, you need to know how to hook them up "in parallel." Whether the battery bank will supply power for a long time or be used as a backup, the batteries must be connected in the proper order -- positive to positive and negative to negative.
Put the two automotive or marine batteries side-by-side. These will now be called battery 1 and battery 2. The red positive tabs will be on one side, and the black negative tabs will be on the other.
With the red battery cable, connect the red positive terminal of battery 1 to the the red positive terminal of battery 2. DO NOT connect a positive terminal to a negative terminal!
With the black battery cable, connect the black negative terminla of battery 1 to the black negative terminal of battery 2. Be sure to connect everything using the same polarity -- red to red and black to black.
The batteries are connected: + to + to + and - to - to -. In other words, they are connected positive to positive to positive and negative to negative to negative. In this configuration, the total capacity is the sum of the capacities of the individual batteries, and the voltage is unchanged.
Now connect the item you want to power to one end of the battery bank, and connect the power source, e.g., solar power, wind power, etc.