Getting the daft question in early.

[mick the builder]

When a battery is fully charged, where does the charge from the alternator go, and if you could easily disconect the belt, wouldn't you get more mpg with maybe a low battery warning light telling you to reconnect the belt. Thats it, i've done it. Hopefully it will be off the main page by this evening.

 

[Kwakerman]

When the battery is full it doesn't 'load' the alternator so much therefore the alternator doesn't require as much engine torque to turn it (so theoretically you use less fuel too). Don't forget alternator is also powering all the remaining electrics (lights, indicators, radio, ignition (on petrol engines)). If you disconnect the belt the battery will have to power all those to and could end up flat to the point the starter won't have the oompf to start the engine.

 

[mick the builder]

When the battery is full it doesn't 'load' the alternator so much therefore the alternator doesn't require as much engine torque to turn it (so theoretically you use less fuel too). Don't forget alternator is also powering all the remaining electrics (lights, indicators, radio, ignition (on petrol engines)). If you disconnect the belt the battery will have to power all those to and could end up flat to the point the starter won't have the oompf to start the engine.

Thanks for that. When i said remove the belt, i did mean from a diesel engine that that had no electrical accsesories on.

 

[chezvegas]

Thanks for that. When i said remove the belt, i did mean from a diesel engine that that had no electrical accsesories on.

It will still be pulling power for the fuel cut off solenoid and all of your dash instruments. A small load but don't see am advantage in shutting off the alt supply. With it not charging the warning light would be on constantly. Changing tyres and removing unnecessary weight will probably have a more effective gain in mpg

 

[timtheenchanteruk]

except your lights, indicators, heater fan, radio...

phone charger, sat nav, winch...

oo, forgot, fuel solenoid.

Plus, you'd have to change the drive belt for a shorter one, and modify the tensioners etc, the belt drives yer fan and water pump on the fender 200/300tdi (believe the TD5 too, but not 100%) and of course PS pump.
Under low/zero load conditions, the alternator is easy to spin by hand, so I wouldnt worry about it

 

[the_wolf]

200tdi has two separate belts, the alternator belt is connected to the power steering pump which itself is connected to the fan belt

 

[timtheenchanteruk]

I stand corrected

 

[sp00k]

200tdi has two separate belts, the alternator belt is connected to the power steering pump which itself is connected to the fan belt

In standard configuration. Kits are available to 'delete' the power steering pump from the loop, which I've done, so I just have a belt around the crank, waterpump and alternator.

OP, along similar lines, my old engine was from a fitted for radio which has an additional 24v generator. This is driven much as you've described by fitting a belt on demand (the crank pulley has two pulley wheels). I've never used it but as with a normal alternator it spins very freely when unloaded.