we can only fit 100 vlaues in our array Voltage5[], is there a limit on hom many values that can be put into an array?
yes, size of memory!.

I take it this is not where the limit of 100 is shown
static unsigned char Voltage5[]={'1', '2', '3', '4', '5','6','7','8','9','0','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o'};

In the RD2 you have some (512?) bytes of "internal external" memory. The compiler manual will tell you how to direct variables there (XRAM in Keil) instead of IRAM (Keil name). NOTE: (SDCC who knows) with Keil you must modify startup.a51 to select the "internal external" memory.

Also does this array have to be writeable? Otherwise use 'code' (or whatever SDCC calls it)

Erik