I want to input a battery system with 2 parallel strings of 12 2V batteries in series , ie 24 2 V batteries with a system voltage of 48V.  If I input 24 quantity of batteries, 12 batteries per string and 2 strings it gives me 24V system voltage – it should be 48V ?  To get 48V I have to input 24 batteries per string.  What am I doing wrong?

A string is a set of batteries connected in series.  In a system containing multiple strings, the strings are connected in parallel.  So the “system voltage” or “bus voltage” is the same as the string voltage.  Whether your system contains one string or two strings or eight strings, if each string contains twelve 2V batteries then the bus voltage will be 24V because the strings are connected in parallel.  If you want a 48V bus voltage you’ll have to connect 24 of your 2V batteries in a single string.