Download to Arduino

Hello all, I want to download my program to my arduino, so I could shove the arduino onto a boat without a computer running it. Right now I have an external battery providing power to the electronics, but not the board. When I disconnect the arduino from my computer, it doesn't work. How would I, in a sense, "download" a program to the arduino? Thank you all, Wilky

The program is already on the arduino or it wouldn't do anything when connected to the computer either. It sounds like all you need to do is provide power to the board.