I run 2 16v batteries in the car. Both batteries are the AGM type. Recently I noticed the drivers side battery only has been discharging a white powder onto the trunk floor under the hold down rubber. Anyone I've talked to about this seems stumped. I had taken that battery out & looked for any holes or cracks, thought maybe it was rubbing against the threaded rod hold downs, but nothing was obvious, Then the other nite was charging them & the green fully charged lite came on as well as the red lite but that was blinking, meaning abort, Testing 4 voltage I got crazy readings between 12,3-16,5 back & forth. The batteries are old & I ordered new ones, but was curious as to the powder effect, thought AGM's wouldn't do that, any thoughts? Dave ss/ea #1355

My understanding is that the AGM batteries are a gel, but if there is a hole or crack they will leak. The issue you describe sounds like corrosion from a leak

AGM Batteries have a fiberglass cloth(Absorbed Glass Matt) like stuff in them that the acid is absorbed into, they should not be able to leak. The battery is either being overcharged or has gone bad and is venting cell pressure. AGM Batteries do gas like regular batteries it just all stays contained unless there is a problem that causes the valve to open.

Lots of people confuse them with gel batteries but they are not gel filled. Gel batteries are really not super common in the automotive world.