What is the difference between alkali, Ni-cad and sealed lead acid batteries?

More than one cell constitutes a battery. For that reason, a D cell is just that, a cell. But a 9v battery actually consists of six 1½V cells.

Various combinations of substances (metals, electrolytic solution, etc) provide different voltages. That's why carbon-zinc cells produce 1.5v, while NiCd only produces 1.2v. And lead-sulphur cells produce 2.3v.

Alkaline batteries are basically the longtime carbon-zinc cells that use a more efficient (and a little more expensive) alkaline paste for the electrolytic paste. Commonly, they are not rechargeable.

NiCd, or Ni-cad, are nickel-cadmium cells, and are rechargeable. Due to the buildup of cadmium crystals during usage, heavy-duty NiCd's often required complete discharge and recharge to break down the crystals to undo "voltage memory". Rarely was it a problem with light-duty cells such as used in consumer products. Still, NiCd's have been replaced with the more efficient NiMh (nickel-metal hydride) cells that are not susceptible to such problems. Contrary to popular belief, deep discharge and recharge are NOT good for the overall life cycle of rechargeable batteries.

And lastly, lead acid batteries (or lead-sulphur) are another rechargeable type of battery. This is truly called a battery as it consists of six 2.3V cells to make up your car's 12V battery (13.8V in an unloaded condition). Again, deep discharge is not good for the extended life of your car's battery. There are certain, specially-designed Deep Cycle batteries for use in boating equipment, or for electric golf carts, etc that can survive the constant depletion and recharge better.