Nick,
Sorry, but I’ve worked with these for suckers decades and I can’t stop myself.
*** Warning: Technical Waffle follows***
Optima’s aren’t “dry”, no lead acid battery can be. They are basically two pieces of Lead immersed in Sulphuric Acid, generically known in the industry as a “Flooded Lead-Acid” battery. When you recharge then they give off gas - then you get two basic types of cases. Vented & Valve regulated. Vented basically let the gas go into the air like batteries of old. Valve Regulated have a pressure valve to keep the gas contained unless they are abused and case damaging pressures are reached, when they "vent". Some of the gases can be recycled by the chemistry. Harder to spill, Acid doesn’t “evaporate” so you probably don’t have to top up the acid for the life of the battery, some can be used laid on their side also contain the explosive Hydrogen which can cause issue under rare conditions (Boom!). We’ve had electronics in a cabinet badly corroded by hydrogen when the wrong sorts of cells have been charged at very high rates. Optima’s are valve regulated, and have different shaped lead electrodes so you get more surface area and hence higher capacity for a given amount of lead. Good batteries? Yep, definitely. Probably couldn’t get a higher battery to fit a given space.
So what is a dry battery? One when the electrolyte is a paste rather than a liquid.
And a Cell? – The basic chemical building block: put 2 pieces of lead in sulphuric acid. Charge it up and it gives back 2V. Connect several together and you have a Battery of Cells.
How big can they get? Typical car battery might be rated at 480 Ah (48Amps for 10 hours). We’ve got quite a few 3200 Ah - Each 2Vcell is about the size of an esky.
Blah blah blah.
Oh and the jokes on me now - I had a flat battery in the GT this morning

Of course the GT was parked in front of the Forester, boot-to-boot.
Get spanners and jumper cables.
Unbolt the Forester's battery.
Lug it around to the front of the GT.
Jump start the GT.
Park the GT out of the way.
Put the Forester's battery back.
Wash up.
Change business shirt with greasy cuffs.
Wake up wife.
Start Forester.
Get worried when Forester then had trouble starting
Tell wife to drive around for a while before stopping.
Get driven to the station.
Text Boss to let him know I'll be late.
Sit in train and fume.
I'll be be playing the "quality for price" game now.
Al.