Category: Phone battery

 You should have an odd question – your smartphone seems to be charging relatively quickly until it’s about 80% charged? It takes longer to reach 100%, why? This is due to the way your device handles lithium batteries.

Three stages of lithium battery charging

Lithium batteries have three charging stages, each designed to protect the battery in its most vulnerable state. these are:

1. Constant current precharge, also known as “trickle charge”

2. Constant current regulation mode

3. Constant voltage regulation mode

precharge

The first stage applies when the battery is depleted or when the battery voltage is below 3.0 V. The battery must be reactivated slowly to prevent problems caused by the battery being left in a discharged state for extended periods of time. A battery’s passivation layer is a protective layer that forms as part of a normal chemical reaction and may need to recover, and low-voltage trickle charging allows this to happen.

This pre-charge phase typically takes place around 10% of the maximum charging speed. This explains the delay in powering a fully discharged smartphone. For example, a depleted iPhone will often display a depleted battery symbol for a few minutes before reaching the necessary voltage for normal startup.

During the precharge phase, the current remains constant (but at a lower rate than in the next charge phase), while the voltage gradually increases.

Constant current charging

Once the battery reaches 3.0 V, your phone will start charging gradually faster. During this phase of charging, the current is set to a constant high rate, while the voltage is increased over time. This is the amount of time your device will charge at its fastest speed, as well as using any available fast charge modes.

This phase charges the battery to approximately 80% of capacity in the shortest time battery safety allows.

Constant voltage charging

Once the battery reaches about 80% charge, charging switches to constant voltage regulation mode. At this point, the voltage is held constant to keep the battery at maximum state of charge, while the current is slowly reduced. This prevents overcharging and avoids damaging the battery. This also means that as the charge level approaches 100%, the charging speed will slow down.

The current will continue to drop until the battery is charged to near capacity, at which point charging will stop completely. At this point, you’d better take your smartphone out of the charger.

If you leave the phone connected, the battery will discharge slightly until it reaches around 3.9 to 4 V, at which point the top-up charge will take place. The device will continue to discharge and charge as long as you connect it to the charger.

How does this affect fast charging?

You may have noticed that some smartphone manufacturers advertise that fast charging can charge your device to “50%” or “80%” in a short amount of time. This is because fast charging is conditional. If your battery is already high, fast charging is unlikely to provide much improvement.

Fast charging can only be used during constant current regulation of charging. Once you reach constant voltage regulation mode or the battery capacity is around 80% or higher, safeguards are in place to protect the battery from damage.

Overcharging the battery is a bad idea. At best it can cause damage, affecting the battery’s ability to stay charged, but at worst it can cause the battery to gradually heat up, causing bodily harm. Fortunately, modern smartphones manage the charging process for you to prevent this from happening.

How a smartphone can further protect your battery

Lithium batteries are not susceptible to the dreaded “memory” effect that old rechargeable batteries suffer from, where they “forget” their charge capacity unless fully discharged first. Lithium batteries are not foolproof, and they do lose capacity with each charge cycle over time.

The charge cycle is not just from 0% to 100%, but represents the cumulative wear and tear of the battery. For example, charging from 50% to 100% on two consecutive days uses a full charge cycle. Some smartphone makers have taken steps to avoid premature battery aging through a feature called optimized charging.

Optimize charging work by charging your device to 80% and waiting. By learning about your habits and routines, the device will be in the final stages of charging to coincide with when you’re most likely to remove the charger, such as when you wake up in the morning.

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Thanks for reading, hope it helps!

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