I have been wondering for awhile if you can use any other alternate battery (not lead acid) that are still legal in competitions. I know that Lithium ion batteries are not legal, but I was mostly wondering about nickel-lithium batteries. I have been looking everywhere but cant seem to find a definite answer.
Read the Battery Rules:
http://www.usfirst.org/sites/default/files/uploadedFiles/Robotics_Programs/FRC/Game_and_Season__Info/2015/2015GameManual0116.pdf
Specifically R20
I think it will answer your question.