Battery Charging States: What You Need to Know

Batteries undergo a continuous state of change and discharge slowly even when not in use. Usage accelerates the discharge rate. Charging and maintaining the battery's charge is essential to preserve its health and longevity.

There are four stages to battery charging, and each one is important to know. They are:

  1. Bulk Charge Stage,
  2. Absorption (Regulation) Stage,
  3. Float Charge,
  4. Equalization Charge.

Each state has a specific purpose and thus its own voltage, current, and temperature limits. Knowing these limits and how to properly charge your batteries will help you get the most out of them.

Here's a quick rundown of each state.

Bulk Charge

The bulk charging state is the first stage of the battery charging process, and it is critical for achieving the maximum capacity of the battery. The battery charger supplies the maximum safe current to the battery during this stage until it reaches a specific voltage.

The charging current flows continuously into the battery until the voltage reaches 80-90% of its maximum charge capacity. As the battery charges, the voltage gradually rises until it reaches its maximum capacity.

During the bulk charging state, the current supplied to the battery remains constant while the voltage increases, replenishing the battery's internal charge capacity to its maximum level.

The charger enters absorption mode, the second stage of the charging process, once the battery voltage reaches its peak or full charge. This stage ensures that the battery is fully charged without being overcharged, extending its life and preventing damage.

To summarize, the bulk charging state is the most critical stage of the battery charging process, providing the maximum current to the battery and allowing it to reach the required voltage level for efficient and safe operation.

It is possible to improve the battery's lifespan and performance by monitoring the charging process and ensuring that it is in the proper charging state.

Absorption (Regulation) Stage

The absorption charge is the second stage of the battery charging process, ensuring that the battery is fully charged. During this stage, the voltage reaches its maximum, while the current gradually decreases.

To achieve a full charge, the battery charger maintains a constant voltage on the batteries while decreasing the current.

The battery voltage stays constant during this stage, and the charging current gradually decreases until it reaches a low level known as a trickle charge.

When the battery is fully charged, the charger enters float mode, keeping the battery at a constant voltage to prevent overcharging.

Monitoring the battery voltage and current during the absorption stage is crucial to ensure that the battery is not overcharged, which can affect battery performance and lifespan.

Proper management of the absorption charge stage can help ensure that your batteries are fully charged and ready for use without damaging them or reducing their lifespan.

Understanding the various stages of battery charging and how to manage them effectively can help you maximize your battery's lifespan and performance.

Float Charge

The float charge is the third and final charging state where the voltage is reduced to a level that maintains the battery in a fully charged state. The purpose of this stage is to keep the battery at full charge without overcharging it.

The float stage is where the charger applies a constant voltage to the batteries, while the current decreases to a very low level (trickle charge). The battery voltage will remain constant during this stage, while the current decreases until it reaches a very low level.

Equalization Charge

The equalization charge is an optional fourth stage that is sometimes used to bring all the cells in a battery pack to the same voltage. This is done by applying a higher voltage to the batteries for a period of time.

The equalization stage is where the charger applies a higher voltage to the batteries for a period of time. This helps to bring all the cells in a battery pack to the same voltage.

Is it possible to charge a battery with a constant voltage?

Yes, it is possible to charge batteries with a constant voltage. Constant voltage charging charges batteries at a maximum voltage, often 13.8V to 15V, and a maximum current, depending on the charger specification.

When a “flat” battery has been charged for a period of time and the maximum voltage is approached, the charge current will decrease in proportion to the battery's internal resistance. The time taken to decrease from the maximum voltage to the “trickle charge” current is known as the “top-off time.”

Is it possible to charge a battery with a constant current?

Yes, it is possible, but the process requires a specially designed charger that keeps a constant current until a certain voltage is reached.

These types of chargers aren't very common. Because they can overcharge and overheat batteries if they are not carefully monitored and the battery temperature is not maintained.

What is the effect of temperature during charging?

Temperature plays a crucial role in the charging process, as higher temperatures increase the chemical reactions within the battery, resulting in faster charging and increased capacity.

However, excessively high temperatures can cause damage to the battery. Therefore, it is essential to use a charger that can correct the charging rate based on the battery's temperature.

For instance, a 12V battery should be float charged at 13.5V when the temperature is 20°C. If the temperature increases to 30°C, the float voltage should be lowered to 13.3V to compensate for the higher temperature.

By properly adjusting the charging voltage according to the temperature, the battery's lifespan can be extended, and its overall performance can be improved. It's crucial to monitor the battery's temperature during charging and take necessary measures to prevent overheating or overcharging.

What size of battery charger should you have?

The size of the battery charger you need depends on the type and size of the battery you are using. However, as a general guideline, you should have a charger that can produce at least 10% of the battery capacity, i.e. a 15A charger for a 120Ah battery.

Please note, that this is just a guideline, and you should always check the manufacturer's recommendations for the specific battery you are using.


Now that you know the different charging states of batteries, you can be sure that your batteries are properly charged. By using a charger with temperature compensation, you can ensure that your batteries are charged safely and efficiently.

And by using a charger that is the right size for your battery, you can be sure that your batteries will be properly charged without overcharging them.