No, for competition you can't use any batteries except the two that were in your 2003 kit. These are marked EX12-18. Any other battery in your robot will not pass inspection and will get your team DQ'd if an official sees it on the field.
I heard from one of the FIRST officials at St Louis that the intent of this rule is to prevent veteran teams from getting a power advantage over rookies by having more spares.
__________________
Richard Wallace
Mentor since 2011 for FRC 3620 Average Joes (St. Joseph, Michigan)
Mentor 2002-10 for FRC 931 Perpetual Chaos (St. Louis, Missouri)
since 2003
I believe in intuition and inspiration. Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution. It is, strictly speaking, a real factor in scientific research.
(Cosmic Religion : With Other Opinions and Aphorisms (1931) by Albert Einstein, p. 97)