[Isaac Zachary]

If an alternator drags down the fuel economy by 10%, half of which is inefficiency, if you made it 99.9% efficient then you'd only increase overall efficiency by about 5%.

I do question whether alternator technology really hasn't improved over several decades. Are modern alternators really only 50% efficient?

As far as the battery goes the lead acid battery is tried and tested. You can make a lithium battery work. But now it needs thermal management, both cooling and heating, plus electrical management because they need to be balanced and if you leave your lights on you don't want a lithium ion battery discharged below it's threshold. Not to mention lithium ion can't be used in a 12V system, so you'd have to use another voltage and you couldn't use common 12V parts.