Warren,
Not to worry--you don't really need a amp hour meter. Battery charging and discharging is dependent on chemical reactions. There is a surface charge on the plates, and it may cause the voltage by a volt meter to read high just after a charge, or low after a discharge. The battery voltage needs to be read at a "steady state" of 3 hours after any charge or discharge of the battery. For practical purposes, the measured voltage is a good measure of state of charge--but the amp meter method is the most accurate.
Dave.
I am sure that you realize that all flooded lead acid batteries are not equal--for example. There are different thicknesses of plates, surface area, and separators etc.
You cannot compare a Rolls battery (high quality) with an Exide (mid quality). Nor can you compare a Traction battery by SBS with a golf cart by carried by Sam's club.
From the link I posted above (and available many other places on the web or in technical bulletins) are typical battery life spans:
Starting: 3-12 months
Marine: 1-6 years
Golf cart: 2-7 years
AGM deep cycle: 4-7 years
Gelled deep cycle: 2-5 years
Deep cycle (L-16 type etc): 4-8 years
Rolls-Surrette premium deep cycle: 7-15 years
Industrial deep cycle (Crown and Rolls 4KS series): 10-20+ years
Telephone (float): 2-20 years.
NiFe (alkaline): 5-35 years
NiCad: 1-20 years
If you look at battery life vs depth of discharge, the comparable AGM and Flooded Lead acid batteries will both have similar life spans--and the life span of comparable batteries are dependent of depth of discharge. A normal "Cycle" is from fully charged to 20% discharged and back again.
This is for a quality lead acid (either AGM or Flooded lead acid):
10% DOD (depth of discharge) =5000 cyc. 20% DOD= 1800 cyc, 30%=1800 Cyc
40% = 1400 cyc, 50% = 1000 cyc, 70% =760 cyc and 80% = 500 cyc
SLA is just an abbreviation for "Sealed Lead Acid".
Again, there are some differences in battery design, but because the way the battery is used, and the construction it may have more cycles than some other batteries. However technically an AGM is a form of SLA battery.
The small SLA's such as are used in our rechargeable boat searchlights, or the computer back up power supply, the SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging would cause gassing and water depletion. Consequently, these batteries can never be charged to their full potential. To reduce dry-out, sealed lead-acid batteries use lead-calcium instead of the lead-antimony. Thus these batteries are not designed to allow deep cycling. I am not familiar with SLA, as you visualize them used in boats--except as I noted above, and perhaps you can provide me some examples.