No effect, but long charging is not recommended.
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Because at present, most of the smart phones we use are lithium batteries, and basically have their own PMU module, a simple translation is the power management unit, one of the functions of this unit is to manage the power, and the current is also its pipe, it can be plugged in after the user, automatically to detect whether the current battery power needs to be charged. Some mobile phones also support fast charging function, control what time fast charging, how long after entering slow charging, to prevent current overload and then cause high temperature mobile phone. After being fully charged, the PMU will also automatically give a power off signal to prevent the phone battery from continuing to charge and entering the standby state. If long charging does have an impact, mobile phone manufacturers will not invent wireless chargers, which are continuous charging.
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However, the reason why it is not recommended to charge for a long time is because of safety reasons, above we mentioned that charging has no effect on the battery, but because of the characteristics of lithium-ion batteries, its stability is poor, especially after high temperature or impact, or the kind of strong fire, so it is recommended to unplug the charger if it is fully charged. Prevent the risk of fire due to the continuous connection of the power supply with high temperature input. But the probability of such an increase is so low that it is almost negligible.
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To sum up, long time charging has little impact on the mobile phone battery, you can rest assured to use, try not to play while charging, in this case, the mobile phone is easy to heat, but the battery life is greater.