Lithium-ion batteries are damaged when the voltage exceeds 4.5V. This can pose serious safety risks to the user. For this reason, all lithium-ion batteries have a protection chip like the one illustrated below. This circuit acts in two ways: it prevents the voltage from both exceeding 4.4V, a conservative value compared to 4.5V, and from dropping below 3V, thus avoiding that the battery deteriorates rapidly (the safety limit beyond which the battery should no longer be discharged is 2.7V).

In fact, when the display shows a charge of 0%, the actual battery charge value is of approx. 5%. This is to prevent the battery from reaching the discharge voltage limit and being damaged.

This protection circuit does not manage the battery charging but physically disconnects the battery if the voltage or current exceeds the safety parameters. Since the voltage set by the charge control circuit typically reaches 4.35V when the battery charge is at 100%, the protection circuit is not involved in any way. This means that the battery stays connected to its end-of-charge voltage.

Some charging circuits apply a Topping Charge while others apply a Trickle Charge.

These two strategies are only relevant at the end of the battery charging phase, which in both cases involves an initial phase at constant current and a final phase at constant voltage.

When the battery reaches 100%, a Topping Charge waits until the charge has dropped below a certain threshold before re-starting the charging process and bringing the battery back to 100%.

A Trickle Charge, instead, holds the battery at its end-of-charge voltage so that the charge stays at 100%, while the absorbed current reduces to almost zero (just a few µA) as shown in the graph.

"With iOS 13 and later versions, the iPhone uses machine learning to recognize your daily charging habits so that the Optimized Battery Charging option will only activate when the iPhone prediction is to be connected to a charger for an extended period of time". This is how Apple explains the new feature, which, however, still presents some critical issues.

Indeed, being a software level feature that is governed by machine learning, it is subject to many inaccuracies. If the prediction of the user's charging habits is not accurate, the device is kept at the end-of-charge voltage until it is manually disconnected by the user, or the charge is stopped before reaching 100%.

All circuits controlling the charge of smartphones, laptops, tablets and digital cameras, charge lithium-ion batteries up to the maximum voltage supported by the cell, which is typically 4.35V. This allows the consumer, who is looking for an optimal runtime, to benefit from the full potential of the battery charge.

However, charging the battery to its maximum voltage shortens the battery lifetime and may compromise its safety. The diagram shows the cycle count as a function of the charging voltage. At an end-of-charge voltage of 4.35V, the number of cycles of a normal lithium-ion battery is half of what it would be at  4.2V. Therefore, for a lithium battery to benefit from a longer lifespan, it should be charged at a voltage that is lower than the maximum allowable voltage. If it’s strictly necessary to charge the battery at the maximum allowable voltage, the battery should not be left in charge for an extended period of time. The end-of-charge
controller should disconnect the cell from the voltage to let the discharge cycle begin.

It's like relaxing your muscles after an intense workout.

We recommend using ultra-rapid charging only when necessary. A well-designed, ultra-rapid charger should provide the user with the possibility to select a specific charging time and choose the most suitable charge to the selected time. The figure compares the lifespan (number of cycles) of a typical lithium-ion battery when charged and discharged at 1C, 2C and 3C rates. The higher the charging rate (3C), the faster the discharge cycles will decrease. The battery lifespan can be extended up to 500 cycles by charging and discharging below 1C, with 0.8C being the recommended rate. Do not apply a quick charge when the battery is cold or hot. Charge only at moderate temperatures and do not charge an old battery too rapidly.