Bobby is correct. You will want to make sure you install them in parallel configuration...or you will end up with a 24 or 36 volt system depending on how many batteries you decide to install.
Think of it like a small radio which requires six volts to operate and if you install the AA bateries according to the directions (pos + to neg -) you will total the six volts required to power that radio.
Same principle here. If you wire pos + of one battery to the neg - of the next and so on until you hook up to the electrical system, you will had added the output of each battery to the next. Thus being larger than the required 12 volt
dc. If you wire the bateries in parallel...pos + to pos+ and neg - to neg - ... you will have only increased the total "holding" capacity for providing 12 VDC to your system.