n14 in century class . . .when to plug in block heater

This vehicle is a 2000 century class with a cummins n14. It has been converted into a toterhome, so I am not really as familiar with big diesels as most of you all. It is having some issues cranking in cold weather and I would like to know if this is normal and just requires more use of the block heater or if I should replace some batteries.

I got the truck last year and 3 of the 4 batteries it had were junk. I put 2 new ones in along with the one that was okay and that took care of things. I do remember that in the cold it would crank slower but it was a very mild winter last year and I don't think I ever cranked it with temps under 30F. Lately we have had temps around 25F and if I try to start it will turn over slowly a couple times and then it doesn't want to go anymore. The last time it did this I plugged it in the block heater for 30-40 min and it cranked over pretty good after that and fired up. I wish I was able to use the block heater more but it is at an outdoor storage unit. Then after the truck warms up it can be restarted with no problem, cranks over just fine. Obviously in addition to the engine being warm the batteries are also freshly juiced up from the trucks alternator. So basically what I would like to know is - is this a normal behavior or should I add another new battery to the system? Thanks

. . .I think of cold weather and diesels as an issue of not being able to get combustion mixture temps up enough to initiate combustion. I wouldn't think that the thick oil would make this big of a difference but like I said this is my first time dealing with an engine of this size.

 

If I do anything I am just going to check the three I have that they are all holding a charge good and then buy a fourth to drop in there. I was advised (I believe on this forum) that 4 batteries were not necessary and thus I asked this question. I hate to be a dick but I don't see how a battery is a cheap component to replace on this truck - with 4 good batteries your looking at $500 easy which is 10 times what I paid for road side assistance for the year that will jump me if the need arises. I know things add up on these but its a little different when your not driving for a living. Far as I am concerned if the truck won't start under 30F but starts fine after it is plugged into the block heater for a while that is really not much of a problem. Shouldn't subject it to that cold of a start anyways. I do like the solar powered battery tender idea and I agree that more batteries (so long as they are all at 12v) is better. Your getting more amps and less voltage drop when the starter cranks.