MF vs Gel vs AGM - clarifications
As mentioned above, the "MF", or maintenance free lawn and garden battery is a flooded wet-cell "SLI", or Starter-Lights-Igniton battery. Draw no more than 10% of it's Ah capacity rating for longest cycle life.
Maintenance Free / MF
It *appears* to be a "sealed" battery, but it is not. Tightly sealed and a wet-flooded with no easily removable caps? Yes.
What it was designed for:
The idea was that since most consumers don't take regular care of their flooded wet-cell water levels, they put additional electrolyte in the cells, and tightly sealed them in the *hopes* that the consumer could nurse them without any care through 3 to 4 years.
Savvy consumers either bought regular flooded cells and maintained them well, or if presented with an MF battery, pulled the tops and maintened them even better than normal.
Gel vs AGM
To this day, confusion continues.
GEL: Truly sealed. Plates are separated with a jelly-like substance of electrolyte. 14.1v maximum charge voltage. 10% or 0.1C max charge current. Easily abused - more below...
AGM: Truly sealed. Plates are separated to the naked eye with what appears like a paper-towl saturated with electrolyte. Appears dry to the eye if opened. Closer plate spacing than gel provides lower internal resistance. 14.4 to 14.7v maximum charge voltage. 25% or 0.25C maximum charge current (unless specialty types like pure lead Optima or Odyssey).
Charger manufacturers:
Due to the decades of confusion between Gel and AGM, many charger manufacturers realize that the consumer doesn't know or care and wants convenience with a single charge level for both.
Hence, many of todays charger use a single voltage for BOTH gel and agm, using the higher voltage of agm to accommodate both types without having the consumer having to make a decision.
The problem is that a gel, if treated to an agm type charging voltage and current, will quickly degrade. Voids in the gel appear close to the plates reducing overall capacity. High amounts of gassing tends to bulge gels subjected to agm treatment.
Since the consumer values convenience over knowledge, gels are replaced more quickly since the "charger says it's ok for gels and agm's!" It is so bad that even many charger manufacturers don't know or don't care. Or perhaps they DO know.
TRASH:
There are a lot of videos showing diy'ers popping the tops off agm's and "watering them" since they appear dry to the eye. Unfortunately, the broken seal exposes the plates to oxygen, and then watering them changes them from a sealed agm to a poorly performing flooded!
Hence, what used to be a max charge rate of 0.25C for an agm has now dropped to the more typical 10% or 0.1C rate for a flooded. Not knowing this, the trash batteries immediately bubble over.