New j.d. Powers reliability study

[Seejay]

I don't claim to be an expert on batteries, but after doing some research I believe that AGM batteries are lead acid also. Here are a couple of quotes I found (at two different sites):

"AGM batteries differ from flooded lead acid batteries in that the electrolyte is held in the glass mats, as opposed to freely flooding the plates. Very thin glass fibers are woven into a mat to increase surface area enough to hold sufficient electrolyte on the cells for their lifetime."​

"AGM technology became popular in the early 1980s as a sealed lead acid battery for military aircraft, vehicles and UPS to reduce weight and improve reliability. The sulfuric acid is absorbed by a very fine fiberglass mat, making the battery spill-proof. This enables shipment without hazardous material restrictions. The plates can be made flat to resemble a standard flooded lead acid pack in a rectangular case; they can also be wound into a cylindrical cell."​

As mentioned in the above quote, non-AGM batteries are frequently called flooded cell batteries. Many of the newer flooded cell ones are also sealed and maintenance free, and therefore cannot have water added, but not sealed to the same extent as AGM. Also, due to the glass mats holding the electrolytes, AGM batteries can be used at any angle or even upside down.

What you said about the emergency relief valve on AGM batteries is correct.

Yes, AGM and Gel batteries are lead-acid. Sorry if I misled anybody here. "standard lead-acid" = "flooded cell"