OK great, thanks. I guess that will probably make a big impact. I'll change the strings and see what happens. Might take me a while to figure out how to do that 
The nested loops are there for the file name of the "split file". According to the sd library I'm using I think the file name has to be char. I am creating the file name one character at a time. Each new file name has to be "the previous file name +1". The only way I could figure out how to do that was to increase the ASCII character value by 1 each time, for each digit, in the correct order. Each digit starts at 48 (ASCII for "0"), and goes up in 1's to 57 (ASCII for 9), then resets to 48 and the one to the left goes up by one - going from right to left. So the first file is called "A0000001", the second is called "A0000002", the third is called "A0000003", and so on until I run out of battery. Hence the nested loops. (Using while rather than for, so that it is slightly easier to see where the loops end). I'm not used to using arrays and chars to make strings. The programming language I normally use just has string, double, bool and int, with automatic implicit conversion between them. There is probably a better way to increase the file name by 1 each time!