Alternative alternator cut out setup for STD 3 wire alternator.
 

Alternative alternator cut out setup with Alternator Battery Buffer for std 3 wire alternator connections.

1. Use smaller starting battery from motorcycle or ride on mower, say about 10-20% capacity of OEM battery and re route alternator circuit to this battery. This requires altering the supply connections for sensor/control wire and indicator lamp wire, the original B+ wire can stay on the alternator and a new one added to this pole for new battery. This circuit is always connected so alternator/regulator is safeguarded from potential voltage spikes from constant switching. New battery negative is just earthed as normal.

2. Use high current solenoid (100amp) in line on original B+ wire to connect or disconnect OEM battery & vehicle systems to alternator circuit. Having a 12v starter battery in both circuits should ensure no voltage spikes, when switched on they are just a parallel battery bank, if the OEM battery is down it will pull system voltage down and alternator will ramp up charging, when switched out the reverse will happen alternator will quickly top off small battery and reduce charging to absolute min maintenance as there is no current draw outside of it’s own control regulator & field.

3. Solar panel with charger is connected into OEM battery charger to keep vehicle system topped up, and additional batteries and home charge set up systems can be added depending on use and application.

Advantages of this setup is that alternator and vehicle electronics is safeguarded from voltage spikes, but is always ready to be switched in if required to top up OEM battery, the disadvantage is a bit of extra cost for additional battery & when switched in battery voltages will equalise, so when switched out again there is the penalty of topping up smaller battery again. This may work quite well though if the automated solenoid is just actuated by brake switch, so light pedal application (wire in a dash light to give positive feedback) will ramp up alternator and give maximum charging & hence deceleration, once brake pedal is released it just goes back to topping off little alternator battery.

I would most likely include manual control for permanent on or off of the system, ie a long night drive just connect up and let alternator do it’s job, but for shorter day trips, leave it off, on a long downhill decent, switch on for first portion and time the switch off for last section so small battery is fully charged by the time you reach the bottom (I don’t do neutral, clutch in or EOC), timing this would come with experience. You would likely want to include twin voltmeters to monitor both systems and feedback, fine tuning and to allow you to modify driving appropriately as you learn it’s in’s & out’s.

What do you think?

I think capturing the energy that is normally wasted during braking is a wonderful idea. However, discharging lead acid batteries will reduce their life, and I doubt the money saved in fuel would offset the cost of replacing batteries more frequently.

I'd be more inclined to run sans-alternator with a different battery technology, or with supercapacitors.

If ya have all that.
 

How about just setting it up to be hybrid drive with a powered switchable field wire? That is next logical step rite.
Ive always wondered what effect would be from running one of those 150 - 200 amp alternators as power assist only from a dead stop accel situation???

I see these types of ideas as transitional points, if I add additional storage batteries (possibly lithium?) and solar panels and see how much alternator really is required and if alternator braking really does provide any benefit. I agree one must be wary of heavy discharge on lead acid batteries, reduced cycle life can easily kill FE benefits.

Yes ideal, and I've thought about it, but haven't found an easy way to do conversion or adapt appropriate unit. Once you start to add mechanical components there's a whole lot of engineering and alignement issues to be dealt with as opposed to just re doing wiring.

Alternator cut outs by themselves really won't improve fuel economy. It takes a system that knows the current state of charge of the battery and awareness of the brake thermal efficiency of an engine based on current load to balance the charging at the right times to get a boost in economy.

Other than braking calculating this real time is beyond most people while driving. An alternator kill switch is good for a temporary boost in power however.

Ideally yes, but I think benefits can be gained without having all that data, particularly if supplimentary power (solar) is used to charge batteries and alternator is left disconnected during short commutes.