How cold is too cold to shut the truck down for the night?
 

Just paid 60 bucks to a tow truck guy who had to boost me :( Ran the truck yesterday for half an hour - everything was good. Shut down for the night, used the bunk heater to sleep, and then the temp went down to -2F or -18 C all of sudden. The starter was barely turning. The tow truck stayed hooked to the battery for 30 minutes, and I was able to start the engine. Whew ... thank goodness I replaced all 4 batteries last winter. So now I'm running the engine till morning for sure. My question is: if I don't want to idle for heat, at what temperature I MUST idle anyway so that the truck can be usable in the morning? Thx!

I would suggest that you idle when the temperature gets below about 22F. If you want to be safer then if the temperature gets below freezing then you could idle.

You need new batteries imo. I used to only idle my truck if it got into the single digits F when I had my bunk heater. Most diesel engines will start in extremely cold weather if you have enough cranking amps. They are expensive as hell but get some yellow top optima batteries which are deep cycle and you will be fine for 3 years. It's a hellava investment but you will make it up by not idling your truck.

With 4 good batteries, perhaps a Canadian could be fearless to -15 Celsius/5 degrees Farenheit?
However, I'm betting you've got some bad batteries in the mix with your recent results.
Which way are the dogbones connected to your batteries?
Too many trucks are wired with the connections conveniently both at the same end of the row, instead of one at either end for balanced charging and loading. The battery furthest from the connections has a bad habit of being under charged, or the one closest gets overcharged, shortening their lives.
See the solar power folks for tips on getting the best from our battery investment?

It's sure looks that way. Or alternator problem....Otherwise, you shouldn't be getting any problem to -20C, assuming of course your fuel is good for that temperature. But just to be safe, i'd idle if park for extended period of time in a very single digits F.

I agree with the others concerning your batteries probably needing to be replaced.

I got them replaced last year. They are 1 year old. The old ones died in similar freeze. On top of the batteries it says "extreme" something... so these should be good batteries. One reason why I couldn't start the engine is because I spent 3 days in this place without moving the truck. The first 2 days it was only below freezing temp (-2C or 29-30 F). I idled the truck for half an hour each day to charge the battery a little bit but 30 minutes evidently is not enough when you use the sleeper heater half a night (each night for 3 nights) and hook up your computer to a power inverter. Would good batteries last 3 days in a mode like this?

RE: connections. The wires go across all batteries like a couple of 'x' letters. The nearest connectors to the outside are marked "negative".

If you were using other things which require battery power then they may have pulled the batteries down too low. Coupled with the cold may have been too much for the batteries. If they are only a year old they should still be good. It would be easy to have them checked. That is where I would start.

How about something like this?: Digital Volt Meter, Plugs Into Cigarette Lighter To Determine Battery Condition
When the voltage drops too much, start the truck and recharge.

you could get loads that go south, and then stay there. Lol