Whether the lithium battery can be directly connected to the USB cable for charging, there are two situations:
1. The lithium battery has a protection plate
If the lithium battery has a protection plate, it can be used directly to charge the USB cable.
The function of the lithium battery protection board is to prevent the lithium battery from overcharging and overdischarging. When the battery voltage reaches 4.2V, the protection board cuts off the current input.
2. The lithium battery does not have a protection plate
If the lithium battery does not have a protection plate, it cannot be charged directly with a USB cable.
The charging cut-off voltage of lithium battery is 4.2V. In other words, when the internal voltage of the battery reaches 4.2V, charging should be stopped, otherwise the lithium battery cells will bedamaged. The output voltage through the USB cable is generally 5V, which is higher than the cut-off voltage of the lithium battery. Therefore, if the battery does not have a protective plate, it will cause the battery to overcharge.
Ordinary lithium batteries usually have protective plates. But some also integrate the protection circuit inside the charger as needed.