4 Stage Battery Chargers Explained

  • Aug 16th 2018

Good battery chargers or charge controllers/regulators will charge your batteries in stages depending on how much charge remains in the battery. Three or four stages are the most common, this article will explain how a four-stage battery charger works. We will also explain why it is important that you use the correct program settings on your charge controller.

How 4 stage charging works

  • Stage 1 (Bulk) – If the batteries are at a lower state-of-charge, usually less than 80% full, this is the first stage. When in the Bulk stage the solar panel or generator will put as much Ampere into the batteries as possible. This is the high-speed charging stage, the higher the Ampere the faster the charge.
  • Stage 2 (Absorb) – when the batteries reach the “Absorb Voltage” (this number differs depending on program settings but generally around 14.5 Volts for lead-acid batteries), the charger or regulator should go into the Absorb stage. In this stage, the batteries are kept at the voltage set by the program. The Ampere going into the batteries is lower in this stage and the batteries charge more slowly. The Absorb stage can end either after a set time has passed or when the number of Ampere going in drops below a threshold. This is all decided by the charge program settings.
  • Stage 3 (Float) – when the Absorb stage ends, the charge controller will drop the voltage to a program defined value and start the Float stage. This stage will kick in when the battery reaches 100% charge.
  • Stage 4 (Equalization) – this stage will actually overcharge the battery. But it is only turned on periodically to override the Float stage. This is optimal for lead-acid batteries and will extend the life and performance of the battery.

The importance of using the correct charge settings

If you use the correct program settings for your battery charger or charge regulator it will extend its lifespan and performance. To know what the right settings are you will have to check what kind of battery you got. It will of course also depend on the type of charger you are using. For example, a deep cycle battery charger can charge deep cycle type batteries. You can read more about the difference between a deep cycle battery and a normal car battery here. But deep cycle batteries got several sub types. So depending on which subtype you want to charge, you have to select the appropriate program for it. You should also note that there are battery types that should not even be charged in 4 stages, such as Lithium-ion batteries. Li-ion batteries should not be over charged, so in this case, it is even more important that you select the correct charge settings or the battery might get damaged. If you want to read more about charge controllers that will help you extend battery life you can read our article on The Best MPPT Charge Controllers