Ohm (YC S15) is a smarter, lighter car battery that works with your existing car
(1) Those nasty things inside car batteries are easily recycled and in fact are recycled more often than not (like 95%+)(2) Car batteries do not last only 3-4 years as per the OP.  Car batteries can last decades.  There are many variables, but the car in my battery is far older than 4 years. (lol)(3) Replacing an easily-recyclable product with a less-recyclable one is no step forwards.(4) XX addressed by OP comment, see below XX 
This thing is a capacitor?  What's the voltage potential in there? Any device with internals significantly beyond that may be a serious fire risk in a crash..(5) Car batteries are not needlessly heavy, nor are they a stock size.  Manufacturers do care about performance.  If they thought the car could do with lighter battery they would use one.  Have some respect for the thought behind stock engineering before replacing it.(6) Shutting down the voltage supply can be a real pain.  A "dead" battery generally still has enough juice to keep electronics ticking over.  Cutting it completely is not like turning off a switch.  It often means resetting/rebooting stuff once the voltage is restored.  For some (BMW) it can even mean a trip to the dealership for a special new battery ritual.
http://www.bimmerforums.com/forum/showthread.php?1355106-Why...