Has anyone done research to determine where the Diehard Platinum marine and automotive batteries are truly different on the inside? I know that you get more posts with the marine flavor, and internet wisdom suggests that marine/deep cycle batteries have larger plates to handle discharging better than a standard starting battery. But it seems like if this was true and the marine construction was different, then the marine battery should weigh more (bigger plates = heavier right?) and possibly be larger than it's non-marine equivalent, and have better ratings? For the group 34, all the other specs are the same - CCA, Ah, Reserve capacity between the 34 and 34M.
I've got a nearly 5 year old Diehard Platinum 34M that seems to be slowly fading (it's my auxillary battery in the 80). It's steady state after charging seems to be around 11.8 - 12V rather than 12.6 - 12.8V. Right now Sears has the DHP group 34 in sale for $175 and comes with a 4 year replacement + 100 month prorated warranty. The 34M is still at $219 and only has a 3 year warranty.
So I'm trying to reconcile if it's worth it for me to buy another Marine battery, or just run the group 34 as the better overall value.