How are “equalize”, “boost” and “fast charge” charging different?

Commonly understood meaning of the term:

Fast charge – Faster recharge of a discharged battery

All batteries, even those assembled into unitized blocks, are all built of individual battery cells connected in series to obtain the required DC voltage. Like all manufactured products, there is variation between the capacities of each cell in the battery. As the battery ages, this variation increases. Since the battery is a chain of cells that is only as strong as the weakest link some scheme is required to ensure that all cells stay at peak capacity.

A scheme called “equalizing” is commonly used in both lead-acid and nickel cadmium batteries. Equalizing temporarily elevates the charging voltage of the entire battery string above the normal “float” voltage. The elevated charging voltage allows all cells, including the weak ones, to accept more current from the charger than they would at float voltage. A consequence of the elevated equalize voltage is that all cells in the battery are overcharged. This is acceptable for short periods provided the battery has sufficient electrolyte.

What is “fast charging”?

“Fast charging” temporarily increases the charger’s output voltage to compensate for the battery’s internal resistance. This allows the battery to continue accepting maximum current from the charger for a longer time – instead of reducing its charge acceptance early as it would if charged at normal float voltage.