Modern smartphones mostly use lithium-ion batteries, which have a different charging strategy than older nickel-cadmium batteries or nickel-metal hydride batteries. In the case of lithium-ion batteries, there is no need for a long initial charge and there is no need to worry about the "memory effect". Here are some recommendations for the first charge of a new smartphone battery:
When using it for the first time: A new phone usually comes with a certain amount of power and can be used straight away. You don't have to wait for the battery to drain completely before charging.
Time to first charge: There is no need for a deliberate first charge of 12 hours or more. Just charge it according to your normal usage habits, even if the battery level is not very low.
Avoid a complete drain of power: Lithium-ion batteries do not need to be fully discharged to 0% and then recharged. In fact, a complete discharge is detrimental to battery life. It is recommended to start charging when the battery level drops to around 20%.
Avoid overcharging: Although most modern phones have protections against overcharging, continuous charging for long periods of time, such as overnight, can still have a negative impact on battery life. Overnight charging can be avoided by using a smart socket or timer switch.
Short-cycle charging: Short cycle of charging (e.g. from 30% to 80%) is beneficial for keeping lithium-ion batteries healthy.
In order to guarantee safe and efficient charging, use the charger and cable that came with it or are officially certified for charging.
Lithium-ion batteries will suffer from performance when charged in an overheated or cold environment, so try to charge them at room temperature.
While no special first-time charging procedure is required, it's still a good practice to observe how a new phone performs during the first few charge and discharge cycles. This helps in the early detection of possible battery problems.
To sum up, for a new phone, you just need to charge it according to your normal usage habits. At the same time, adopting the good charging habits recommended above can help extend the life of the battery.