Well how can my solar controller know when that battery is 'full' i.e. goes from bulk into float but my battery monitor can't?
Ahhh Grashopper, so we wish to delve into the mysteries of the universe?
Who'd a thunk that something as "simple" as a plain ol' lead acid battery could be so complicated, and generate so much confusion, discussion, and debate... Actually LA wet cells while "old" technology, are still fairly sophisticated and dependent on a huge number of factors including temperature, SOC (state of charge), physical construction, discharge rates (Peukert's law),
electrolyte stratification, and so on... All of which have been discussed elsewhere on the forum (endlessly).
Anyway, the short answer to your question is that your average "battery monitor" determines battery SOC via voltage. For a 12V LA battery average values are (about) 100% SOC = 12.7V, 50%SOC = 12.2V, 0%SOC = 11.8V These values may (will) vary depending on physical construction of the battery, and composition of the plate materials.
Your "Solar Controller" on the other hand, being a fairly recent innovation, will probably contain some sort of micro-processor and base its transition points on the current
acceptance curve of the battery rather than straight voltage measurement.
So what happens is, on a partially discharged battery as the charger
kicks on, both voltage and accepted input current will increase to a point where the voltage stabilizes at 12.7 V (your battery monitor now reads 100% SOC (Surface Charge saturation) and the accepted current drops to a "flat rate". Your Solar Controller will continue to supply charging current to the battery until the acceptance current is (usually) around 3% of the rated current. (During this phase, the battery is doing its chemical thing deeper and deeper within the plates and through out the electrolite.) At that point it reduces the voltage to the battery to the "float" level. At this lower voltage level the battery will only accept a minimal amount of charge to make up for self discharge losses. The larger your house bank, the longer the discrepancy between the battery monitor saying 100% and the solar controller still supplying charging current.
Are we confused yet?
In my experience the best way to determine the SOC of a lead-acid battery is the old fashion battery hydrometer
By measuring the specific gravity of the electrolyte, you know the true SOC. Chemistry don't lie.... Electronics
might. Of course, we are still dependent on temperature, electrolyte stratification etc...
Another good read on battery charging principles
Charging Information For Lead Acid Batteries ‚Äď Battery University
(Sorry for all the "smilies" but just in that kinda mood)