The 4-Stages Of Charging

Bulk Charging 

This is the first stage of charging, where the charger delivers a high current to the battery to quickly bring it up to a state of charge of about 80%. The voltage of the charger is typically higher than the voltage of the battery during this stage. This is the stage in which the charger identifies the battery and its needs, then applies a high voltage/amperage charge to do the bulk of the charging. The bulk charge stage ends when the battery reaches about 80% of its capacity. This stage is fast and efficient, as the battery can accept a lot of current without much resistance. Bulk charge is also known as constant current charge, as the current remains constant while the voltage increases.  

The purpose of bulk charging is to quickly charge the battery to a usable level. The high current allows the battery to accept a large amount of charge in a short period of time.

Absorption Charging 

This is the second stage of charging, where the charger maintains a constant voltage while the current slowly declines. This allows the battery to fully absorb the charge and reach a state of charge of about 100%. Meaning, in this stage the charger reduces the current and maintains a constant voltage to allow the battery to absorb more charge. This stage is slower and less efficient, as the battery becomes more resistant to accepting current. The absorption charge stage ends when the battery reaches 100% of its capacity. The Absorption charge is also known as saturation charge or topping charge.

The purpose of absorption charging is to ensure that the battery is fully charged. The constant voltage allows the battery to accept the remaining charge slowly and evenly. 

Float Charging 

This is the third and final stage of charging, where the charger maintains a constant voltage at a lower level than the absorption stage. This prevents the battery from overcharging and keeps it at a fully charged state throughout the day. It applies a very low current or pulses of current to prevent overcharging and self-discharge. Float charge is also known as trickle or maintenance charge, and it can extend the battery life. Float charge is usually applied after the battery reaches 100% of its capacity in the absorption stage. 

The purpose of float charge is to prevent the self-discharge of the UPS power battery and increase the charging depth. 

Important Points To Remember

The amount of time each stage takes depends on the type of battery and the charger being used. For example, lead-acid batteries typically require a longer bulk charging time than lithium-ion batteries. 

A battery that has been deeply discharged needs an absorption time of several hours, whereas a battery that is only slightly discharged requires a much shorter absorption period.

Absorption charge voltage varies with battery type and temperature. For example, for a 12 volt AGM battery, the absorption charge voltage can be 14.6-14.8 volts, while for a gel battery, it can be 14.2-14.3 volts.

It is important to note that not all chargers have all three stages of charging. Some chargers may only have a bulk charging stage, while others may have a bulk and float stage. It is important to check the charger's specifications to see which stages it has.

Here is a table summarizing the three stages of charging:

Contrary to popular opinion; Bulk charge and Equalization charge are not necessarily the same. They are different charging methods that serve different purposes.

Equalization Charging

This is the fourth stage of charge in which the charger applies a higher voltage than normal to the battery for a short period of time. Meaning, it is a charging method that temporarily elevates the charging voltage of the entire battery string above the normal float voltage. This allows all cells, including the weak ones, to accept more current from the charger than they would at float voltage. This stage is slow and less efficient, as the battery becomes more resistant to accepting current. Equalization charge is also known as balancing charge, as it equalizes the voltage and specific gravity of individual cells within the battery and it can restore the battery capacity and prevent sulfation. 

The purpose of equalization is to extend battery life and ensure capacity.

Diagram below, depicts the 4-Stages of the Battery Charging Cycle