The voltage limit beyond which lithium-ion batteries become damaged, with serious safety risks for the user, is 4.5V. This is why all lithium-ion batteries have a protection circuit like the one shown. This circuit acts in two ways: it prevents the voltage from exceeding 4.4V, a conservative value compared to the 4.5V limit, and it prevents it from dropping to a voltage below 3V beyond which the battery can rapidly deteriorate (the safety limit beyond which the battery must no longer be discharged is 2.7V). In fact, when 0% charge is shown on the display, the remaining battery charge is approximately 5%, to prevent the battery from reaching the discharge limit voltage.
This protection circuit does not manage battery recharging but physically disconnects the battery if the voltages or currents exceed the safety parameters. As the voltage set by the charge control circuit typically reaches 4.35V when the battery has reached 100%, the protection circuit will not intervene in any way, leaving the battery connected to its end-of-charge voltage.
Topping & Trickle Charge
Some charging circuits apply the Topping Charge while others apply the Trickle Charge.
The two strategies take place at the end of the battery charging phase, which in each case involves an initial phase at constant current and a final phase at constant voltage.
The Topping Charge, after the battery has reached 100%, waits for the charge to fall below a threshold to start charging again and bring the battery back to 100%.
The Trickle Charge, as opposed to the Topping Charge, keeps the battery at the end-of-charge voltage so that the battery remains at 100%, while the current drawn becomes almost zero (a few µA) as shown in the graph.
Optimised Battery Charging
"With iOS 13 and later, iPhone uses machine learning to try to understand your daily charging habits so that Optimised Charging only turns on when iPhone expects to be connected to a charger for an extended period of time." This is how Apple explains the new feature, but it has its challenges.
Being software-based and governed by machine learning, it is subject to many inaccuracies. If you don't have fixed schedules or if you don't lead a hectic life, for example, the iPhone will never know when you unplug the phone from the power supply.
Voltage VS Discharge Cycles
All charge control circuits in smartphones, laptops, tablets and digital cameras charge lithium-ion batteries up to the maximum voltage supported by the cell, in most cases 4.35V. This allows the maximum permissible battery charge as the consumer wants nothing more than an optimal runtime.
However, charging the battery at maximum voltage will shorten the battery's service life and may affect its safety. The figure shows the cycle count as a function of charge voltage. At 4.35V, the number of cycles of a normal lithium-ion battery is halved compared to an end-of-charge voltage of 4.2V. A lithium battery should therefore be charged at a lower voltage than the maximum permitted voltage in order to significantly extend its life, and if it has to be charged at the maximum permitted voltage, it should not remain there for an extended period of time. The end-of-charge controller should disconnect the cell from the voltage causing it to begin its discharge cycle.
It's like relaxing your muscles after strenuous exercise.
We recommend applying ultra-rapid charging only when necessary. A well-designed ultrafast charger should have charging time selection to give the user the possibility to choose the least stressful charge for the allotted 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 speed, the faster the discharge cycles will decrease. Longevity can be extended up to 500 cycles by charging and discharging below 1C; 0.8C is the recommended rate. Do not apply fast charging when the battery is cold or hot. Only charge at moderate temperatures and avoid rapidly charging an old battery.
Battery saver apps
On the Google Play Store, you can find numerous apps that monitor the status of the battery but fail to disconnect it from the power when it is charged.
There is an app, whose programmers claim that it can disconnect the battery from the charging phase by setting a maximum charging threshold. However, in order to use this app, which has not been proven to work properly, the system settings imposed by the smartphone manufacturer have to be changed by Rooting, which will invalidate the warranty if the device cannot be returned to its factory settings and could lead to the device Bricking (not turning on).
Samsung has even set up a 'meter' to keep track of any tampering with the system, even if you have managed to return to factory settings.
Lithium, the oil of the future
The supply alarm has already begun to sound, even though vehicles capable of running without petrol or diesel are still a rarity on our roads. Numerous funds are hoarding the metal, and the result is that lithium and cobalt prices have more than tripled in a couple of years, from $20,000 to over $80,000 per tonne.
85% of the world's lithium is found in Argentina, Chile and Bolivia. In Bolivia, the poorest of South American countries, the main source is the Salar de Uyuni, an enormous salt flat covering more than ten thousand square kilometres. Underneath this surface there is lithium in large quantities, but so far hardly used.
The problem is extracting it: it costs a lot of money and requires great technical expertise. Mining requires huge amounts of water for extraction. Chile, for example, has started to take water from the sea to continue extraction, with the big problem of having to purify it of salt. In Bolivia, on the other hand, there is a risk that groundwater will be affected by the cleaning of the extracted material. In some cases, several hectares of virgin forest are cleared, and lagoons are drained to store 'cakes' - these are mountains of excavated and waste material.
Does the smartphone disconnect the battery?
To prove that our smartphones are not so smart, a lab test was carried out to analyse how a latest-generation smartphone handles battery charging.
In the video test, you can see how the smartphone keeps the battery at its end-of-charge voltage of 4.35V even though it already reached 100% some time earlier.
Later, it is shown that the device is only disconnected from the mains when fully charged thanks to Witty.