calcium is added to lead plates to reduce the self-discharge. There's lots of other stuff, too. Antimony is added to increase deep-cycle capacity. Other things are added to reduce the amount of Ca and Sb added, like tin, arsenic, selenium and cadmium. Control over plate chemistry is the big difference between good lead-acid batteries and not-so-good ones. It's also the difference between a battery suitable for deep-cycle or SLA use.
In my experience, the big advantage of jump packs are ease of use, storage, and charging. You can throw one in the back seat or back of the truck, and it will be fine. Hard to throw a car battery, and you do have to be more careful with them. Capacity is not as good, of course, but most of the time, it's not a real problem, you are jumping a car becuase the battery is dead or bad, not because of a mechanical problem. If you regularly need to start big diesels with dead glow plugs, you need something else.
