I saw that requirement in the Rotax installation manual and had in mind buying an EarthX 900 (15.6Ah/400 CCA). In all the Eurofox aircraft that I saw with a 912iS I saw the EarthX 680C (12.4Ah/320 CCA). I asked the engineer about this and he showed me an 'equivalent' chart of lithium v lead acid batteries, which put my mind at ease that the 680C would be sufficient. I can't recall the exact figures but the 680C had more than the equivalent of a 16Ah lead acid battery.
I think it all has to do with the fact that lithium batteries can discharge to 100% (in other words, chuck out 100% of their charge) whereas lead acid batteries can only throw out 80% of their charge and it is recommended not to discharge below 50% (to prolong the life of the battery).
680C for me :-)
