"plug-in mild hybrid" upgrade
 

I have an idea for a device that will offload the alternator and therefore improve fuel economy. Basically, it will consist of some batteries along with a voltage regulator and control electronics. It will be wired to disable the alternator and take over maintaining charge and voltage. Once it is discharged, it will allow the alternator to take over as usual. It can be made easily removable for charging or a charging connector be integrated into the system.

A simple implementation can just be a battery and voltage regulator that connects to a 12v power outlet. Since the outlet cannot handle enough current to run some large loads like the headlights or A/C, it will only partially offload the alternator.

If the alternator is on a dedicated belt, the belt can even be removed to eliminate all the associated friction losses. Of course, automatic switchover will be impossible if that's done.

A battery charging another battery? Did I get that right? Doesn't sound advantageous to me.

How about using the alternator only when engine-braking?

if you were to use a cog belt instead of a v belt that would take care of alot of the friction issues, then it would just be spining weight and the air drag that some alternators have of their crude fan so a better fan might be in order as well.
you can get dc/dc converters that will drop voltage down from 24 or 48 or even 144 volts dc down to 12 or 13.2 volts dc for running lights in electric cars off the traction battery.
if the alternator is not hooked up to a load then it's field coils are off as well and it's just dead spinning weight.

The point is that the batteries in the upgrade are designed for deep cycle, so they substitute for the alternator. You just plug it in to recharge it.

Why not just run deep cycles and have an alternator cutoff switch?

Deep cycle batteries are not good for starting and the load voltage would be lower. With my idea, the normal starting battery is kept charged, keeping the 14.4v expected by all the loads. The device can also be easily moved from vehicle to vehicle. It can also be used as a portable power supply for other applications.

Then stop using your starter, most driveways and parking lots have enough slope to bump start.
or with the system I discribed if you had your normal battery for starting that acted as a buffer with the dc/dc converter your lights would never dim and you could still use your starter.

Rpm limit switch, micro switch on throttle. tap into brake light switch, and relay

Rpm > idle + no throttle= power to relay and connect alternator

Brakes on= power to relay and connect alternator.

I have a deep cycle in my car and it starts up way better than when I didn't have the deep cycle. It's an optima yellowtop, so that may have something to do with it.
If you're worried, keep the car plugged in when you start it, thus relying a bit less on the battery, then unplug quickly, get buckled in, and off you go.

I'm doing this on my car. I have everything set up, but still setting up my mpguino to get a good baseline without the alternator plug-in delete.
Since I only have a 20 minute commute, I'm just going to disconnect my belt and run off the battery. I just need to make some LED headlights designed for my new 12.5v instead of 14.5, or rig up a way to re-aim my lights and use my high beams when I'm using just battery.