Have a look at this site, it explains quite well what happens to the current and voltage of a string of batteries depending on how they are wired.
Serial and parallel battery configurations
What charger you need depends on how the batteries are wired. If the batteries are 1.2v 1800mAH (1.8a) each, and there all wired in series then the pack would be 12v 1800mAH (1.8amps). If the same batteries are wired in parallel then the pack would be 1.2v 18000mAH (18amps)!
HTH