But knowing that you've worked for Interstate, I'm inclined to listen to your advice on batteries 
Calcium is an inert substance (filler if you will) that is alloy'd into with the Lead when they pour it into the molds.
This is basically no different than cutting your product so you have more volume to spread it out longer.
One other thing...Calcium is used to help harden (alloy) the lead plates to keep them stiffer.
They do that to keep the plates from bending under heat and shorting out eventually.
How much to add is the chef's to decide.
There's only 3 major battery manufacturer's in North America.
Exide
Johnson Controls
East Penn Deka
Exide carries some unique group sizes like for the 1st generation Mustangs.
Johnson Controls is known as Interstate.
There are some smaller firms in America like Northstar making higher performance batteries.
They're using 100% uncut virgin lead. That's lead never been recycled before.
There may be Calcium but the pure lead makes it a denser weight of battery.
They weigh significantly more group size to any other competitors group size.
Very few AGM's in the market are using 100% virgin lead like the Northstar.
The competitors majorically are using recycled lead.
If you have a battery with denser and more pure lead, you'll reap the rewards of a combined higher cranking amps and amp hours. The balance of both is the key.
More plates in each cell makes more cranking amps.
Less cells creates more amp hours=reserve.
Boat batteries have super thick plates (like for trolling) and low cranking power for the most part.
The more cells, the less fluid, the hotter they get under extreme weather, and wear out sooner.
I'm running a group 34 Northstar on my Hemi and it is consistently on top for cranking and reserve for this behemoth engine cranking demands. It jumps back to 100% charged very quick once she's running for a few minutes.
Some claim Optima's are the best.
When I was selling batteries, we would get anywhere from 2-6 per week that were failed will still under warranty. Earlier Optima's had a definite better performance lifespan than the present one's now being made.
Some say they changed once they moved production to Mexico. Optima's claim to fame is the cinnamon roll AGM cell design. It's thought that you can stuff more lead in per cell. The cells are very thin and wrapped with an absorbent fiberglass mat soaked with Electrolyte.
On conventional wet cell batteries, I prefer East Penn Deka any day of the week over the other two competitors. Way too many times picking up cores: Interstate in one hand group 31. East Penn group 31 in the other; Night & day difference in the weight of each of them! Apples vs. Apples. East Penn's probably weigh 10-15 lbs more in that group size.
