If you are using 66w, fast charge to 40w mobile phone charging, the mobile phone battery is not very good, if you are in urgent need of temporary, using someone else's charger, occasionally use this damage to the battery is more dangerous, but if you often use it, it will reduce the service life of the battery.
Because you use a 66w charger to provide 66w charging power, and the actual phone only accepts 40w power, it may cause the battery to overheat
A 40 watt fast charge is better for the battery than a 66 watt fast charge.
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Because under normal circumstances, the battery charging current is smaller and the effect is better, and the electrochemical reaction inside the battery is deeper and more thorough. Because the greater the power of fast charging, the faster the charging speed, but the more damage to the battery. The power is smaller when charging, and the charging speed is slower, but the damage to the battery is also small.
It doesn't hurt much, because it's okay to charge a 40w phone with a 66w charger. First of all, the mobile phone battery is lithium battery, the voltage is the same, but the output current of 66w is relatively large, the charging speed is relatively fast, occasionally a problem is not big, the mobile phone battery charging is best to use the original better, if a long time with high-power charger charging will certainly damage the mobile phone board