How are “equalize”, “boost” and “fast charge” charging different?
Each of these terms describes the same function of the charger where the charger temporarily elevates the battery’s voltage above the float level. There are different uses for elevated charge voltage, as shown below:
Commonly understood meaning of the term
Equalize – Periodic “topping up” of battery capacity, and correct cell capacity differences
Boost – Can refer to “equalize,” “fast charge,” and sometimes both
Fast charge – Faster recharge of a discharged battery
What does “equalize” charging do, and why is it needed?
All batteries, even those assembled into unitized blocks, are 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 equalized voltage is that all cells in the battery are overcharged. This is acceptable for short periods provided the battery has sufficient electrolyte.
Overcharging greatly increases the rate at which the water in the battery electrolyte is electrolyzed into oxygen and hydrogen gas. Since low electrolyte levels will permanently damage the battery it is important to limit when, and for how long, the battery is charged at the equalized voltage.