Is USB Hand Warmer Safe?

The safety performance of USB hand warmer is higher than that of rechargeable hot water bag. Generally, the USB warm-hand treasure can be charged with a 5V powered USB interface. The charging with the same voltage can be universal. Therefore, ordinary mobile phone chargers and the USB interface of computers can charge the USB warm-hand treasure. However, it should be noted that some mobile phones with fast charging function are 9 or even 12V chargers, which cannot be universal and may burn the equipment.