Alternator versus plug-in 12v battery charging
 

A useful quote from an industry white paper entitled "Improving Alternator Efficiency Measurably Reduces Fuel Costs" by Mike Bradfield, p.3:

"With a typical engine efficiency of 40%, a belt efficiency of 98% and an alternator efficiency of 55%, this leads to an overall energy conversion efficiency of only 21%. Assuming a fuel cost of $4.00/gal, this leads to an on-board electrical power cost of $0.51/kWh, or roughly 4 times a typical household utility rate."

Four bucks a gallon is obviously a little too high, even here in Cali, in current market conditions. But the nice round number he uses creates easy spit-balling estimates... $4/gal is 4x as high as "typical" grid costs.

That's more like 5 or 6 times what I pay for power.

And maybe only three times my rate. I would have to double check. $0.17/kwh right now I think.

Also, this is the first time I recall a "reliable source" citing a specific belt efficiency. Ninety-eight percent is pretty efficient, which is what we thought. It has an electric clutch and just freewheels with the windings are not activated. So keeping my alt belt on has obviously not cost much and it leaves me the alt on demand with my dash-mounted switch.

Nice comparison.

Yeah, belt efficiency is very high. I've heard many say its the 'belt that sucks power', but really this is not anywhere close to true. The little tiny fan on the alternator is probably sucking as much power as the belt alone.

But the cost for grid power includes being conveniently hooked to the end of a power line. Portable power probably costs more. And the cost of your grid-charged car battery going a little beyond its range might be quite high...

All the losses in the alt are from hysteresis and drag from the fan.
I believe that the hysteresis losses are at least as much as windage drag.

And so, preventing the alt from powering up prevents these losses too (hysteresis), correct? After all, when off, the pulley is just freewheeling.

It's free wheeling and beating air around with its built in fan.

On my GM 12si serries alternator that I put on my kubota tractor there is a noticeable change to the engine exhaust noise when the alternator is deenergized versus powered up only making a few amps on the little 2 cylinder diesel.

Yes. I feel that change too when I flip the switch on my dash to turn on the alternator. Idle changes if I am at idle. There is a little lurch like going into DFCO if I am driving.

There are MANY other losses:

Bearing friction
Rotor current
Diode losses
Regulator losses
Stator losses (resistive)
Rotor losses (resistive)
Magnetic losses

If the fan is such a big loss, don't spin it. Alternator delete vs disconnect. Obviously NOT a huge loss from the fan or everyone would be deleting instead of just disconnecting. The fan would not be required if the resistive and conversion losses were not so bad.