The riddle of the charger and the battery … Why does it slow down when it’s almost full?

All smartphone users notice that the battery charge becomes slower when the battery is almost full. This may raise questions for users.

Usually, a smartphone can charge very quickly, from 0% to 80%. As for the percentage left to complete the shipment, it needs more time.

The reason is due to the mechanism of action of lithium-ion batteries in general. These are batteries that are used in smartphones, laptops, and even electric cars.

Slow battery charge

Lithium-ion batteries are charged in three successive stages. These stages are mainly intended to protect the batteries from any risk on the one hand, and on the other hand to preserve their lifespan.

The first stage is known as the pre-charge stage, or pre-charge. During this step, a current of 3 volts is delivered to the battery. This lower capacity refreshes the battery and prepares it to complete the charging process. This is because the battery reaches a more dormant state when left discharged for a long time.

This method also revives the protective shield of the battery, which is one of its main components. But the pre-charging process only depends on 10% of the charging capacity supported by both the phone and the charger. This explains why you need to leave the phone connected to the display for a few minutes before you can turn it on if it has a 0% charge.

Charging steps

At the end of this step, the second step begins. This is exactly what starts when the battery reaches 3 volts stably. The second stage is known as constant current. The phone then begins to charge slowly gradually.

At this point, the phone’s fast charge mode is activated, if available. This step continues to charge the phone as fast as possible until it reaches 80%.

After that, the battery charge enters its third stage. This stage is also known as constant voltage. At this point, the phone receives constant tension, which is why the scene suffers more slowly. However, the purpose of this technology is to prevent the phone from overcharging and to protect the battery in general.

We also know that the charge of a smartphone is measured in watts. It is the product of voltage and current.

At this point, the electric current continues to decrease continuously until the battery is fully charged, and in the later stages, the current is at its lowest levels, due to the effect of the stability of voltage.

Until the phone is connected to the charger at this point, it only receives 4V of power. The phone stays 100% full and once it drops to 99% this simple voltage brings it back to 100%.