Here ya go, only 12 lbs for this badboy 
Lithium ion battery:
http://www.suncoastparts.com/product/LITHIUM.html?Category_Code=997TURBO
Nice ideaā¦no problems with one li-ion equipped car i just pulled out of winter storage, and it goes for weeks without running sometimes, still shows 90+%. It can be semi-reasonably-priced, too:
http://www.jegs.com/i/Lithium-Pros/403/L925/10002/-1?parentProductId=
A 93 Ah battery is about 40 lbs, this is about 10 lbs.
Whatās the expected lifespan of these li-ion batteries?
It is an AMS he said
I have a built '05 STi. In 9 years of ownership, I have attempted to use LW batteries 3 different times. In my experience, the heat in the engine bay kills them. The longest service I have experienced is ~18 months.
Those that relocate the LW battery to the trunk or under the passenger seat have experienced significantly longer life.
The RS4 battery is in the trunk (and appears to be HUGE), so this may be a good candidate for an easy swap to a LW battery. Not sure where the S4 battery is?
S4 battery is between the false firewall and the real firewall, if that makes any sense
Gotcha - I googled some pictures, and I see where it is. Definitely exposed to engine heat. :-\
wonder if the RS4 (which has the same battery) gets better battery life than the S4.
Is there a reason why it would have a longer battery life?
Yes
Not in hot engine bay?
Battery in trunk