What is the difference between these battery types

Sealed Lead-Acid Cells (SLACs) usually have the electrolyte impregnated in a fiber(glass) matt that separates the lead plates. Gel-cells are a little different, in that instead of a matt the electrolyte is a gel rather than a liquid. Both are "sealed" but will have pressure relief vents in case they're over-charged. If they ever do vent, chuck 'em.

Both of these types (GelCells and SLACs) are good for applications where the orientation isn't fixed. Flooded cells tend to spill. ;-)

Just for the sake of contrariness, I find myself impelled to mention the strange cell caps I saw once on a flooded lead-acid in an airplane. They were about four inches tall, and had a little snorkel-type arrangement where when you pulled negative gees or went inverted, the little ball would fall "up" the cap and block off the liquid path. :-)

Cheers! Rich

The crud that tends to scale off the plates wouldn't do so well back between the plates.

If you've got crud scaling off the plates, you've got no business trying to use it in an operational airplane, i.e., you need a new battery STAT! ;-)

Cheers! Rich

Ever see what's in the bottom of a car battery?

Being an aviation-related issue, they would have a way of dealing with that. I have no clue what it might be (probably some treatment that makes the battery more expensive, heavier for the capacity, less reliable overall but more visibly unreliable, etc.).

Aviation stuff is the same as car stuff in principle, but boy they do a lot of the details differently!

Cars typically don't pull negative gees, or fly inverted except for very short periods of time.

Hope This Helps! Rich

The chemistry is the same.

Drivers last words: "Hey Bubba! Watch this! Yee-ha!"

You forgot, ", hold my beer" after the "Hey Bubba".