The Bluetooth headset uses a 5V charger. If it says 3V on the headset, it should be its normal operating voltage, which has nothing to do with electric charging. Because the battery is 3.7V. If the charger is 3V, it will not be charged. The 5V charger is no-load voltage, plus the load (power management components) to charge the 3.7V battery just fine. The power management component of the headset also has a voltage range to protect it. Therefore, it is normal for 5V to charge a working device named 3V.
However, if you directly use the 5V charger to charge the 3.7V battery, it can be charged several times, but it will burn the battery protection board if it is charged too much.
It can be used. The 3 volts written on the headset is the battery voltage of the headset. Since the charging voltage must be higher than the battery voltage, a 5-volt charger is very suitable for a 3-volt rechargeable battery. Due to the small power capacity of the headset battery, the power consumption of the headset is also very small, and the use of a 5-volt slow charger does not take too long to fill the headset battery.