I've been alternator free for about a year now. I'm getting at least 10% better FE.
My current set up is a 12V costco deep cycle that I got for $49 at the end of season clearance and a used 6V trojan t-105 that I got free. I have them set up in series for 18V. Assuming a 10% improvement that battery was paid for after 10 $60 fill ups. The battery is only discharged about 10% for each trip I make and should last for over 1000 cycles. I already had an automatic battery charger but you can pick them up for $30 if you look for deals.
I am running at 18V to keep the spark nice and hot and the fuel pump at max pressure.
FE gains from alternator disconnect have been shown on this site by metro mpg and others to give 10% better mpg. That ranks up there as one of the biggest single gains in the ecomod list. And that gain was at 12V. They would probably get a bigger gain at 14.5+V.
A deep cycle battery that is only discharged 50% should last well over 1000 cycles.
Chart of Depth of cycles vs life.
Alternators are at best 40-50% efficient so for every watt produced the engine has to produce more than double the watts not including the belt friction.
Just to keep the average car running +/- 18A turns into about one HP when you do the math which is drawn at all engine speeds including idle. Idle happens to be the most inefficient engine speed. Remember too, you have to replace the energy lost to starting: 300+ amps at the best of times.
Once you learn how to figure out your amp draw you can estimate your battery life just like with gas. You can even get a battery gauge just like a fuel gauge showing % discharge. Or keep the original starting battery in the car as a spare. I have never run out of battery juice myself.
The best thing is I get to say I have a hybrid.
