I would appreciate some battery education...Why would you want to replace a 1000 CCA battery with a 750 CCA one?? What's the practical consequence of downsizing?? It seems it would not have as much capacity to turn the engine over?? Am I missing something??
And why do you think that the 925 CCA ones fail more often than the 750 CCA ones?? Should I infer from that that the 1000 CCA ones would fail even more often??
Durally, I've read many of your posts & you seem to know what you're talking out based on experience, so I would welcome your help...I don't profess to be a professional mechanic, let alone a diesel mechanic, but it sounds like you are...
I'm not a battery expert, and there are a lot of variables to consider. In my case of the same Exide 750 vs 925 , the short simple answer is the number of batteries that come back to the parts/service guys at the dealer. Not many 750s at all in comparison to the 925s. Both start the trucks just fine and most drivers never notice a difference anyways.
There are a lot of factors involved in failure, assuming both batteries are identical otherwise, there's only really one way to get more CCA's. More surface area, and since the space inside the case is the same and the battery technology is the same, more plates are needed. In order to fit them, they need to be thinner which can make them more susceptible to failure for a number of reasons. Being thin, the plates can expand more during cranking, which over time can make them more susceptible to a short earlier in life. The thinner plates can be more susceptible to corrosion, all batteries grids will corrode, its just a matter of time and various factors can reduce the effect, but it is never eliminated. Keep in mind, different manufacturers techniques and quality will vary some, but there's still only so much variation that you can have in the same size lead acid battery.
Aside from active material depletion, Sulfation and "soft shorts" are the other leading causes of poor battery performance. Sulfation can be prevented, and can often be reversed. A soft short is when a portion of the battery is shorted out for some reason (possibly contacting the fallout at the bottom). Soft shorts are not always easily found because the battery often works fine right after charging, but will die quickly due to discharging itself. Thats why it can be important to charge the battery fully, let it stand for 12 hours, then load test it. Carbon Pile is still the cheapest effective load testing. There are more advance systems as well. No one system can really test everything though. IIRC more than 50% of batteries returned as "bad" are actually just fine, meaning proper testing is getting scarce because it is time consuming.
The reason to replace a higher CCA battery with a lower one is due to a potentially longer life and lower upfront costs. Most batteries will not make it 4 years. Batteries are not hard to replace, so I prefer to buy the cheaper ones as long as they work decent enough, versus spending double without getting double the life. That statement doesn't really correlate with CCA necessarily. If you actually need the higher CCA then you are going to have to buy it, however in your situation you most likely dont. CCA is the amps the battery will deliver after 30 seconds in 0* weather and remain above 7.2v. Modern electronic engines usually need to stay above 9-10v or they cutout to protect the electronics. An OEM may spec a higher CCA to for a long cranking time without dropping below a certain voltage. That doesn't mean its always necessary. How often are you cranking your SSL for 30 seconds? Unless you just changed the fuel filter or you are having an issue, probably not too often. The demands of pre-heaters do play a role in this though as well.
The downside of a lower CCA battery is simply less cranking time. However, you have to decide if you really need the extra cranking time. You probably don't on that specific machine. I use the 750CCA's in everything around here. Our SSL's all start fine with them. Larger equipment with 6-9L engines have two of them in parallel (or series on the 24v machines) and they can crank forever (remember that CCA is doubled though). Semi's with 12-16L engines usually have three of them in parallel and can crank just fine for a very long time.
I don't have a lot of nitty gritty tech on it other than multiple battery places saying that on the same battery, the higher CCA ones tend to fail quicker and more often. Thus I say if the lower CCA works, use it and in my case it has always worked. I've used the 925s before as well, not a huge difference. You are really only going to notice the difference if you have to crank for an abnormally long amount of time and at that point the higher CCA may be the difference between needing a charger/jump but how often is that going to happen where the higher CCA would've prevented needing a jump? You have to decide that for yourself based on experience, if a properly functioning system with fresh good batteries cannot crank for an adequate amount of time, then you need a higher CCA setup. Sometimes it takes a bit of trial and error, but with $70 batteries its not a huge gamble for me, if I ever find something it doesn't work in there's many other pieces of equipment around here I can use that battery for. I havent found one where the 750's weren't adequate.
Your mileage may vary though.