As an engineer with an interest in batteries, I've read some technical publications on the subject (not just the popular press). I'd say Bob Austin of Thataway nailed it well in his recent post here.
Running low on water and discharging to a low voltage are two ways to really shorten the life of a lead acid battery. Deep cycle batteries suffer less than cranking batteries, but both are damaged each time you drain them very far.
When you buy a new car and the battery's been discharged on the dealer's lot a couple times, it's already got a reduced life span.
My CD22 with a 90 and a 9.9 has two cranking batteries and a 1/2/both switch. I run one battery or the other on any given day. Not both. The other's in reserve. When I replace them, it will be with marine deep cycle batteries for the durability. Cranking a fuel injected 90 horse engine that starts instantly is not likely to overtax the current supply ability of a deep cycle battery. Why not have the more durable construction of the deep cycle battery? If you're cranking a V-6 or V-8 and it cranks a while, maybe a starting battery is justified.
A really good gizmo is an integrating amp hour meter, like those made by Xantrex. West Marine and lots of sources sell them. There's a two channel version also. I have one in my sailboat. It constantly adds and subtracts the current being drawn from and charged into the battery and gives you a running total of the remaining amp hours in the battery. Plus it's a very accurate volt and amp meter. You can really monitor your batteries, lights and electronics with this device. It's what the serious cruising sailors use.
If you run your engine often and only drain the batteries a little once in a while, you don't need to monitor much. But if you're camping overnight and running the batteries down using lights, radios, refrigerators, etc. you really should have such a meter.
Starting batteries are optimized for short bursts of high current with less voltage drop during cranking. Deep cycle batteries are optimized to better resist damage when discharged somewhat deeply, like to the 50% or 30% level once in a while. Starting batteries don't hold up well when treated like that.
Jeff
