What kind of operation you place upon the batteries has a great deal to do with their life cycles.
True, but cycles don't run your stuff, amp hours do. The cycles are secondary to Ah delivered. The amp hours a given battery can potentially give over those life cycles the depth of discharge doesn't impact it as much as many believe. A battery cycled to 80% DOD over its' life has less than a 10% difference in delivered Ah than one discharged to only 50% DOD. It does mean you cross the finish line sooner but that comes down to a choice between twice as long or twice as fast. The "50% Rule" is mostly a myth since even if you're only going by cycle life, you would have to be cycling your battery nearly every day, for years, before you'd come close to using up the cycle life even to 80% DOD. Then, even if you managed to do that you just realized all the capacity the battery was able to give making it the most cost effective you can get. Since there's a calendar life consideration and numerous other factors like stratification, partial state of discharge and various other impacts to service life you'd be doing well to use cycle life and delivered Ah up before the other effects take their toll.
A wet cell or a gel cell should not be discharged below 50% of their amp/hour rating.
"Wet" implies several topologies. The one that wouldn't deal with cyclic use is an automotive starting battery ("SLI") that you referenced. Gel cells usually have pretty good cycle life but are more for standby service like UPS and emergency lighting applications. Pretty good for ham radio too, since most hams don't cycle batteries very hard or very often. Over the years I've always run the calendar out on the gel cells I end up with before I use them up.
A good heavy duty wet cell is the 6 volt golf batteries.
These are the best value in lead acid storage. There are "better" batteries but they cost 2 or more times as much, so dollar per Ah they are a good deal. Just need to pay attention to water levels but for the first few years they're pretty easy to keep up with.
Don’t forget to use a good battery maintenance charger. One the provides an initial charge, maintanace charge and a float charge.
Can't agree more. One with a thermal sensor is even better, but having the proper finish and float voltages are very important to fully recovering the charge and maintaining it. I have several here that I use but not at the same time I run the radios so at some point I'll have to see which ones are radio friendly.
Mark K5LXP
Albuquerque, NM