Many people find that the charging process of lithium batteries is usually divided into two stages when looking at the charging curve:

First constant current (CC), then constant voltage (CV).

This mode is almost applicable to mobile phones, electric vehicles, and energy storage batteries. So here comes the question...:

Why not keep charging with constant current? Is it not possible to directly charge with constant voltage?

The answer is actually related to the movement pattern of lithium ions inside the battery.

1.What is constant current charging

Constant current charging, as the name suggests, refers to maintaining a constant current throughout the entire charging process.

For example:

1 Ah battery is charged at 1 A;

A 10 Ah battery is charged at 10 A.

At this stage, the charger will continuously input a fixed current to the battery.

As lithium ions continue to embed into the negative electrode, the battery voltage gradually increases.

2. Why use constant current first

Because the battery is currently in a 'low power' state.

There are still a large number of vacancies inside the negative electrode that can accommodate lithium ions.

let me put it another way:

Lithium ions are relatively easy to enter the negative electrode.

At this point, using a larger constant current can quickly replenish the battery.

In fact, about 70% to 90% of the capacity is filled during the constant current stage.

So:

The constant current stage is mainly responsible for "fast charging".

3. Why can't I keep charging at a constant current

As the battery level increases, the situation begins to change.

The number of lithium storage sites in the negative electrode is decreasing.

Continuing to maintain high current charging will result in:

Polarization increase

Increased battery heating

Rising risk of lithium analysis

Especially when approaching full charge, the rate at which lithium ions enter the negative electrode decreases significantly.

If a large current is still forcefully inputted, some lithium ions may not have time to embed into the negative electrode and directly deposit into metallic lithium.

This is lithium deposition.

Therefore, constant current charging must end before reaching the cut-off voltage.

4. What is being done during the constant pressure phase

When the battery voltage reaches the set upper limit, the system enters the constant voltage phase.

At this moment:

The voltage remains unchanged;

The current gradually decreases.

It can be understood as:

Give lithium ions more time to slowly enter the negative electrode.

As the battery approaches a fully charged state, the current will decrease.

Finally, the charging will end when it drops to the set value.

5. Why is the constant pressure stage so slow

Many people will find that:

The phone charges quickly from 20% to 80%;

But charging from 90% to 100% noticeably slows down.

That's why.

The goal of the constant pressure stage is not to pursue speed, but to ensure safety and longevity.

If a high current is maintained for the last 10% of the battery, the aging rate of the battery will significantly accelerate.

Therefore, later charging must be slowed down.

6. What would happen if there was only constant pressure

In theory, it can also be charged.

But in the initial stage, the current will be very large, which can easily exceed the battery's capacity.

Not only is it inefficient, but it may also damage the battery.

Therefore, in practical applications, constant voltage charging is rarely used alone.

7. Conclusion

Lithium batteries adopt a charging method of "constant current first, then constant voltage", which essentially seeks a balance between charging speed, safety, and cycle life.

The constant current stage is responsible for rapid energy replenishment;

The constant pressure stage is responsible for ensuring safe filling.

The seemingly simple two-stage charging curve is actually one of the most mature and reliable charging strategies for lithium batteries after years of development. Understanding this process also means understanding why the charging speed slows down as the battery approaches full charge.