Explain the voltage drop curve of a set of lead acid batteries

Living on a boat and using batteries to power things over night, I've noticed that the battery discharge pattern seems to be a bit like a pyramid or the point of a sharpened pencil.


The batteries quickly fall back from 13.2 -> 12.7, and 12.7 -> 12.6 is faster than 12.6 -> 12.5 and by the time you get to 12.5 you seem to have hit the meat of the battery and further voltage drop is much slower, much like a pencil without a sharp point doesn't degrade as quickly as a freshly sharpened pencil.


We've never gone below 12.2, but I'm assuming at some point voltage starts falling faster, or maybe it doesn't.


99% of the load on these batteries comes from 2 refrigerators and 1 freezer and can range from 50Watts to 200Watts depending on temperature and how many are running simultaneously.


I'm curious about WHY the battery bank displays this voltage drop that isn't very uniform. What is going on inside the cells that cause this kind of voltage drop. What causes the battery to begin shouldering the load with less voltage drop around 12.5v? I'm sure there is some chemistry behind the reason, but I don't know what that is.


Charging voltage runs as high as 14.4 during the day when the solar panels are refilling them batteries, and we are adding 3.3-5.2KWh/day to the batteries, depending on sun and clouds.


