The new battery is out: less weight, more starting power, faster charging,
 the same size as the regular battery; 
 and the best, from 2012 the battery is compliant with the Rotax MAX Challenge regulations.

 Ask your local dealer for Rotax item 265030 "Rotax Battery Lithium type 12V-2,3Ah"

 http://www.kart-rotax.com/desktopdefault.aspx/tabid-151/