On my 300SD I measure 11.5 or so volts at the glow plugs.

I think the losses here are not from the battery not having enough to put out but rather by the wiring.

If you want your glowplugs to get more V...then up the gauge of the wire feeding them.

Chances are you would only pick up a small amount of V if you paralleled another battery.

50amps vs 2 times 25-30 amps isn't that much difference for a good solid battery.

Now after the starter engages, that is another story...drawing 200-300amps to power the starter will cause a pretty decent vdrop and thus the glow plugs will see less v...

But unless you are going to be cranking for 10+ seconds, they aren't going to cool enough to care.

BTW for those interested, just go get a 100amp or larger 12v solenoid from your local parts place. Don't use a glowplug relay or something weird...just a plain old solenoid works fine. And they are pretty cheap.

If you find an old one off the firewall of a ford or something, go for it...just make sure it won't crap out on you

Ford Starter Solenoid:

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Here's a thought I had concerning a second battery. Place it into the trunk with a starter solenoid in series under the hood. Connect a toggle switch to the solenoid and place under the dash. To connect the battery, you could use car amplifier wiring, or if you have the wiring already in place, tee into it with the solenoid in the trunk. Although I wouldn't try starting the car with this battery, it would be good to glowplug with, or give yourself a jump if the main battery gets low. Just let it sit for 5 or 10 minutes. Also would save you if you left headlights on, etc.

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'79 240D 4 speed manual 105k miles

I'm certainly not an expert, but I have some experience with cold weather. I live in a climate that has is cold (-11C daily avg Nov-Mar, Extreem cold - -55C). I also have an uncle with a 2005 Ford PS in Tuktoyuktuk, NWT (find it on a map - you will like it) with the same practices. In both cases, without an electric block heater or a fuel-fired coolant heater, you are pretty much sunk. If you have ever tried to start a vehicle which has sat out for a length of time in the cold without being plugged in, if it does start, it isn't happy and unlikely that it is good for the motor.
My 7.3l (with the batteries the PO had in it 5 yrs ago) has dual batteries of differing ampacity with no fancy doo dads inbetween just a regular parrallel connection.
When the winter season decends upon us, I ensure the following - do the regular maintenance items (adjust valves, asess starter condition, change to lighter oil (or synthetic if you will), etc.) I also throw a very good set of booster cables in the trunk. Most northern places have plug in equipped parking spots, and I don't think I know anyone who has ever installed 2 block heaters. It usually only takes 3 hrs to warm up enough to start reasonably, unless it is very cold - at -40, I plug it in as soon as I shut it off. If you can't plug in, I would try to start and warm the car every 4 hrs as suggested by another poster, this is very helpful. I also have a cover over the rad to prevent wind from blowing through the rad and over the engine which will cool it much faster. I have also switched to synthetic transmission fluid and differential gear oil with no ill effects, as the transmission and differential aren't heated by the block heater.
I think that dual batteries is a good idea, if you find a good spot to install it. I don't think the isolator is a bad idea, I just don't know enough about it to comment.
I have been actively looking for a fuel fired coolant heater. If you watch e-bay long enough you should find one for around $500us. There are smaller ones that are better suited to our vehicles. This takes the pressure off of finding parking with electricity, and heats much faster than the electric model.
I think that the generator in the trunk might be a bit overkill, and possibly dangerous (fumes and so on).
Just my 2 cents.
Corey

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1982 300sd
1980 300cd (sold)

The 65 Amp alternator might have been big power back in their day but they are not these days, especially when there are so many of us converting to vegetable oil. Believe me, with a VO system, and all of the oil-heating power you need, more amps is a necessity. Don't know if you plan to convert to oil or not, but "Alternator & Starter" of Wichita, Kansas can build you a two-hundred amp alternator and match it to your alternator brackets. Thats what they just did for me. Give them a call at: (316) 265-8990. The owner's name is Nenad Dakovic. To keep his name simple, he goes by Ned. Tell him that Alfred Odom (owner of the Black 83 Mercedes) told you about him.

Peace.

Any problem with belts slipping or heat with that alternator? I guess you upgraded the wiring to match it.

...and the obvious question: How much $$$ did it cost?

Just get one of the kero fired jet engine type heaters to carry inna trunk. That way you can just warm up the whole rig in one swoop. Just be carefull of melting anything you don't want melted, or burn yourself when loading it all back up into the trunk .

Regards

RUN-EM
HOWDY, from warmer than you, TEXAS, U.S.A.

My Suburban with the 6.5 had dual batteries from the factory and that thing started beautifully in the sub zero temps after sitting in the parking lot all day.
Most of the school buses that I've been dealing with have 2 or 3 batteries hard wired in parallel. The key is to get the same exact batteries and off the same pallet if possible with the same build date. Do yourself or the next owner a favor and punch out the dates on the new battery so someone has an idea of how old they are. I think dual batteries are a great idea. I also like the 115 amp alternator upgrade.

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Currently driving a very clean 1985 300SD from the West Coast.