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What are the Solar Controller Battery Charging Stages?

Solar charge controllers put batteries through 4 charging stages:

  1. Bulk
  2. Absorption
  3. Float
  4. Equalize
Figure 3: Multi-Stage Battery Charging
Battery charging stages diagram

For lead-acid batteries, the initial bulk charging stage delivers the maximum allowable current into the battery to bring it up to a state of charge of approximately 80 to 90%. During bulk charging, the battery’s voltage increases to about 14.5 volts for a nominal 12-volt battery.

Then when the battery reaches the Absorption voltage set-point, constant-voltage regulation is applied but the current is reduced as the batteries approach a full state of charge. This prevents heating and excessive battery gassing.

When absorption charging is complete, the battery has about 98% state of charge. Then, the charging current is reduced further so the battery voltage drops down to the Float voltage.

The Float stage keeps the battery at maximum capacity throughout the day. For flooded open vent batteries, an equalization charge is applied once every 2 to 4 weeks to maintain consistent specific gravities among individual battery cells. The more deeply a battery is discharged on a daily basis, the more often equalization charging is required. Equalization is for flooded, not for sealed, GEL, or valve-regulated batteries which can be damaged by equalization.

Although lead-acid batteries are the most common type of battery regulated by solar charge controllers, lithium batteries are starting to gain traction. Morningstar launched an Energy Storage Partner program that involves work with many lithium iron phosphate battery manufacturers to maintain the highest state of charge for their batteries and to help maximize battery life.

The integration guides you can download provide custom solar charge controller voltage and time settings for absorption and float charging, and other information that you will need to charge your batteries safely and to increase their longevity. In addition to lead-acid and lithium, Morningstar solar charge controllers can also charge nickel, aqueous hybrid ion, and flow or redox flow batteries.