Papaindigo has nailed all the high points for you.
For me personally - Get the Costco battery. Save that smaller one for the summer in the other Jeep. Installing a battery is EASY, you don't need to argue with a parts store droid about doing it for you or "the manufacturer states ONLY this battery in my book" b/c that is complete hogwash. If you can get a group 34 battery, it will fit perfectly. Look at the size of any group 34 battery, they are all the same physical dimensions. They don't have one you like? Compare those dimensions to what you do like, then look at ratings. Find one with the most amperage... And go for it. The height matters, and the outer dimensions matter a bit if you don't want to rotate the thing 90 degrees to face the engine (long side parallel to the fender). If you are willing to get creative, over 1000 CCA batteries can fit in there, but I wouldn't do that in the winter.
Here's what I would add to Papa's thoughts:
Our CRD does require a bit more "grunt" than most other similar size engines. FYI: The rating of the starter is somewhere in the vicinity of 525 amps. This is the same as a Ford V8 gasser! Your battery is above that level, sure... But not much. What this means is that you can get potentially 30 seconds of cranking (in a perfect world) out of that 600 amp rating... ONCE. Then the battery would be too low to deliver that rating again until it was charged.
Thats in a perfect world. The real world means you lose amps (and conversely need more from the battery) because of resistance in the cable, corrosion resistance in the ground points, cold oil making the engine tougher to turn, Usage of the glow plugs... You get the idea. If your engine fires in less than 30 seconds of cranking, you might STILL get away with that battery... Except for the cold.
That rating of CCA is at 0 degrees F, but a battery loses a LOT of capacity in the cold, and it isn't a linear change or an exact science. Go a few degrees below zero, and you might lose another 20 amps... You might lose 40 or more. Add in the other real-world problems... And you stay parked.
Now, to address your questions: More amperage rating? That will NOT HURT ANYTHING, as it is only a representation of the battery's total capacity. The battery will still deliver ~13.6 volts when fully charged. A lesser capacity rating won't hurt the CRD in the singular case of: Your CRD is able to start before that battery is depleted. This is the same for any battery. The symptoms are familiar to all of us - slow cranking, clicking starter, dim / blinking lights... No vroom. There isn't really any damage to the computers, they just won't run. But when the cranking speed slows due to reduced voltage from the low battery, the starter is actually trying to draw MORE amperage per volt to make up for the low voltage. This can overheat the wiring in the starter, and causes the battery to deplete even faster. It also damages the plates within the battery, reducing its lifespan permanently. When your battery starts to suffer and the cranking speed slows... STOP TRYING.
Good luck in the winter!
_________________ Proud supporting vendor of LOST Jeeps TRAVELING CRD TECH. I come to you! Need help? Just ask! I've taken it apart more than most. Email jeep [at] maincomputer [dot] com - BOARD MESSAGING IS BROKEN Over 225 CRDs currently driving with my valves, timing belt, rockers, or ARP Studs. Bad noises = REALLY bad things.
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