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Battery Charging States: What You Need to Know

Batteries are in a constant state of change. Even if you’re not using them, they’re slowly discharging. When you do use them, they discharge even faster.

However, charging them back up and maintaining their charge is crucial to keeping them healthy and long-lasting.

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 initial stage of the charging process for a battery. This stage is essential for ensuring that the battery reaches its maximum capacity. During this stage, the battery is charged at the highest safe pace that it can tolerate until it reaches a specific voltage.

To accomplish this, the charging current supplied by the battery charger is applied to the battery continuously until its voltage reaches near 80-90% of its full charge. As the battery is charged, its voltage increases gradually until it reaches its peak charge capacity.

During the bulk charging state, the charging current stays constant while the battery voltage increases. This constant current flow ensures that the battery’s internal charge capacity is replenished, allowing it to reach its maximum capacity.

Once the battery voltage reaches its peak or full charge, the charger switches to the absorption mode. This is the next stage of the charging process, which is crucial for ensuring that the battery is fully charged without overcharging, extending its lifespan and preventing damage.

In summary, the bulk charging state is the first and most important stage of the battery charging process. It delivers the maximum current to the battery, ensuring that it reaches an acceptable voltage level, which is necessary for efficient and safe operation. By monitoring the charging process and ensuring that the battery is in the correct charging state, you can help extend its lifespan and improve its overall performance.

Absorption (Regulation) Stage

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

To achieve a full charge, the battery charger applies a constant voltage to the batteries while the current decreases. During this stage, the battery voltage remains constant, and the charging current gradually decreases until it reaches a very low level, also known as a trickle charge.

Once the battery reaches full charge, the charger will switch to the float mode, where the battery is maintained at a constant voltage to prevent overcharging. During the absorption stage, it’s important to monitor the battery voltage and current to ensure that the battery is not overcharged, which can lead to reduced battery performance and lifespan.

By properly managing the absorption charge stage, you can help ensure that your batteries are fully charged and ready to use, without risking damage or reduced lifespan. Understanding the various stages of battery charging and how to manage them effectively can help you get the most out of your batteries and extend their lifespan.

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?

The temperature has a significant effect on charging. Higher temperatures increase the rate of chemical reactions inside the battery, which can lead to faster charging and higher capacity.

However, if the temperature gets too high, it can damage the battery. That’s why it’s important to use a charger that has temperature correction so that it can adjust the charging rate according to the battery temperature.

For example, at 20°C, a 12V battery should be float charged at 13.5V. If the temperature is increased to 30°C, the float voltage should be brought to 13.3V in order to compensate for the higher temperature.

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.