Originally Posted by Gizmo1137
I am measuring 12.8 volts DC and the voltage drops as soon as I start cranking.
Some voltage drop during cranking is normal, and expected. There are, of course, limits...
It does crank but sounds/seems like, but not for sure, that it is slower than usual.
With a 'weaker' battery, it will crank slower and you will be able to hear the difference.
When I crank it over the headlight goes out and the dash lights dim ever so slightly. I am pretty certain I hear the fuel pump engaging.
The headlight (and other electrics I don't know offhand) are switched off by a Load Shed relay. This diverts amperes not used in starting the engine, making more amperes available to turn the engine over, pump fuel and provide spark. Once the engine starts, the alternator can take over and the headlight can be switched back on. To paraphrase Star Trek, "Engineering, divert all power to the starting circuits."
The other thing I noticed after posting this issue and after the battery was on charge for quite a while and maintaining the green light, is after trying to start several more times than before it sounded a little weaker than the first 1 or 2 attempts.
The green light measures voltage, and voltage only. This is similar to measuring water level in a pond, which tells you how high the water is but does not tell you how deep the sand and sediment is that has accumulated on the bottom and reduced the pond's volume. Your battery is similar to that pond, and over time it will 'silt up' and lose capacity while appearing at the same level on the shoreline, and high temperature will accelerate the 'silt' build up in a battery. Also, like a pond, when amperes (water) are drained out of a battery the voltage (water level) will go down, and with a lower voltage (water level) the starter will turn slower.
You may be able to take it to a shop that has a tester that can measure its behavior under load (drawing heavy amperes) to see if it still has enough capacity, or if it has 'silted up' and needs to be replaced.