Parallel or series wiring is a requirement determined by the application itself.
Wiring batteries in parallel maintains the same voltage as each identical battery but adds the current rating of each battery (i.e. (2) -12Vdc/100Ah batteries in parallel = 12Vdc/200Ah).
Wiring batteries in series adds the voltages of each identical battery and maintains the same current rating of each battery (i.e. (2) – 12Vdc/100Ah batteries in series = 24Vdc/100Ah). Therefore, your application’s voltage and current requirements will determine the most optimum wiring configuration for your batteries.
Note: This could mean a combination of series and parallel wiring based on your voltage and current requirements.