Not counting the belt/pulley losses, I think the alternator offers extremely little drag when the field is off. I would think the P/S pump and A/C compressor are much lower hanging fruit.

:) I have a switch that opens or closes an 80 amp contactor between the alt and main battery. I used to manually have to pull the 120 amp fuse, but now I can just flip the switch when my wife takes the car and its back to stock.

I have been using it stock but being in the competition I think I will throw the extra battery/inverter/charger back in there ;) I took it out for winter because the diesel pulls 60 amps (coolant glow plugs) for about 10 minutes after starting, basically until the water temp hits 120F. My second battery would drop below 10.5v with that kind of load for that long. Now I hit 120F in about 3-5 minutes even starting cold.

How hard of a start is it in the summer without the plugs (and maybe an EBH to assist)? I'm thinking back to my Diesel days and how it relied much less on batt. input and how you could take advantage of that. Could you essentially disconnect the cooling fans and hold on to extra heat if you needed to stop while running errands? ...or even an grid-based start to warm up the plugs with a DC transformer.

I just remember being able to drive hours on dead batteries in Diesel - that has to account for some benefit in design...

RH77

Are you saying your field draws that much? If not, why would you have to kill the connection between your alternator armature and the battery? Are some of your diodes are shorted?

I think it's the rating on the contactor - ie. it's rated to break an 80 amp current. Just a guess.

I like the idea, but my mind keeps coming back to the extra power that has to be spent to energize the coil in the contactor when you're running the alternator!

I learned recently that (a) I have no fuse to disconnect, (b) disconnecting the field sensing/ignition/idiot light wires doesn't disable the field, so I would also need to install a physical disconnect on the main battery wire (as well as the others) if I want to be able to "easily" isolate the alternator while leaving the belt attached.

I think I'd lean towards a simple knife switch.

Hey that?s the same thing my wife asked me ;)

Metro got it, I just use a big relay to open the connection between the alt and the battery. The relay is just rated to handle 80 amps, basically physically disconnecting the battery from the alt.

I wish I could disconnect the alt all together but when my wife drives to one of her jobs it is usually at night and 40+ miles one way and she doesn't/won't mess with my extra battery setup, and I am not sure it would hold up without a charge for 2 hours in winter. So I have to keep it easily switch able back to stock.

RH77, yes I can start and run at about 20F with no glow plugs or coolant plugs. It is just rough at first and takes forever to warm up and gets bad mileage until it warms up.

Basically the four 20 amp glow plugs only run right on startup for about 10 seconds or so depending on ambient temp. As soon as they switch off the three 20 amp coolant plugs kick on until the coolant is at about 120F. Again I could run without them but would take longer to warm up. I believe they are mainly on there for emission reasons, not really to heat up the car faster, well really both get you to the same point.

But yes I can run it pulling very low power, like 1 amp once all warmed up, just enough for the ECU to think about what it has to do.

why not use a latching relay? They only use power to change state and will stay at whatever state they are last set at. So you could switch them on and off all you want without needing a constant drain on the battery.

sample ebay auction here to see what they look like, but there are many other auctions and stores selling them.

edit- https://cgi.ebay.com/20-pcs-12V-coil-...QQcmdZViewItem is an auction for 20 of them for 50 bucks so if a bunch of us wanted them we could get them pretty cheap.

That's a good idea.

I have seen these mentioned elsewhere and I assumed that's how they worked, but I wasn't completely sure. Thanks for confirming. That counts as one new thing I learned today.

Generally contactors require little current to hold them closed. For example, my company uses a huge 3 phase 480V, 160A AC contactor on a piece of equipment we manufacture. It is open and closed by 24V DC.

The specs on it are that it takes 540 watts (22.5 amps) to close it, and only 8 watts (0.3 A) to hold it closed. And this is one big contactor, about the size of a small car battery. I would imagine the amount of current to hold an 80A 12V contactor closed would be much lower.

It still seems it would be easier to put a small switch between the regulator and the field. I know...it means opening the alternator and running a couple of new wires in there...